What is Light? Light is a form of energy that radiates from its source in waves. Waves have both a frequency (the number of times the wave repeats in a unit of time) and a length. The chart below shows the visible spectrum of light verses other types of waves. The Sun is the main source of light on this planet. Humans have long produced artificial light to supplement the sun’s light. This is done by transforming an energy source, such as gas, oil or electricity into light.
Transcript
What is Light?
Light is a form of energy that radiates from its source in waves. Waves have both a frequency (the number of times the wave repeats in a unit of time) and a length.
The chart below shows the visible spectrum of light verses other types of waves.
The Sun is the main source of light on this planet. Humans have long produced artificial light to supplement the sun’s light. This is done by transforming an energy source, such as gas, oil or electricity into light.
Types of Artificial Light Sources
Incandescent:
An incandescent bulb consists of a filament that glows when electricity is passed through it.
Incandescent Bulb
A halogen lamp is a type of incandescent lamp where high-pressure halogen gas is inside the bulb allowing the filament to burn hotter and longer.
Halogen Lamp
Gaseous Discharge
This technology passes electricity through a gas, which excites the gas and causes it to glow. Fluorescent, high-intensity discharge lights (HID) and low-pressure sodium lights use this technology. HID lamps use different gases to produce light:
Mercury Vapor Metal Halide High Pressure Sodium
Metal Halide Fluorescent Tube Compact Fluorescent Light
These types of lights require a ballast, which is a device that starts the lamp and regulates its operation.
A Ballast
LED
LED’s are the newest type of artificial light and were born out of the electronics and computer industry. An LED is a tiny electronic device that emits light.
With the numerous advances in lighting technology, it can be hard to chose the right light source unless you understand the basics of how light is described and measured.
Lighting measurements deal with:
1. The quantity of light – How much light comes out of the light and how much hits the surface you want to illuminate, such as a desk or countertop.
2. The quality of light – What is the color of the light itself and how does it affect what colors I actually see. Are the blues really blue or are they washed out? Can I distinguish one item from another on a dark night with this light?
3. The fixture efficiency – How much light actually leaves the fixture and is directed towards the area or surface that needs to be illuminated? How does that affect my cost?
The Quantity of Light
Luminous Flux or Light Output is the quantity of light that leaves the lamp. This is measured in lumens.
Most people think of measuring light as how much light comes out of a traditional incandescent bulb. Everyone knows that a 100 watt bulb will pleasantly light a bedroom and that a 15 watt bulb is a good night light.
As new, more efficient lighting sources are developed, the general public will need to start learning how to measure light in lumens. For example, a 100 watt incandescent bulb is about 1700 lumens. An equivalent CFL (Compact Fluorescent Light) would be a 28 watt bulb at 1600 lumens. A 15 watt LED can output as much as 1620 lumens.
Illuminance is the amount of light measured on the work surface, such as a desk or counter in the lighted space. This is measured in lux (metric) or footcandles (English). A lux is one lumen per square meter. A footcandle is one lumen of light density per square foot. One footcandle equals approximately 10 lux.
The chart below shows the illuminance one can expect to see outdoors in different conditions.
Footcandles Lux
Sunny Day 10,000 100,000
Overcast Day 1000 10,000
Very Dark Day 100 1,000
Twilight 1 10
Deep Twilight .1 1
Full Moon .01 .1
Quarter Moon .001 .01
Starlight .0001 .001
When determining acceptable light levels, it is usually more important to think of Lux or Footcandles rather than lumens as this more accurately measures the amount of light the human eye will see. Often lumens are “wasted” by throwing light into areas that are not perceived, such as the interior of the fixture or straight up into the night sky.
When buying a fixture, you can often get a spec sheet from the manufacturer that has a chart showing the lux levels (or footcandles) at various lengths away from the light source. The mounting height of the light source will affect these levels, so that is often specified on the spec sheet.
A Lux/Footcandle meter is a useful tool in understanding acceptable light levels. A simple meter can be purchased for less than $100 and is quite useful if you need to spec out or purchase a major lighting installation.
Lights can vary widely in their illuminance and descriptions of “Bright” or “Super-Bright” can be very misleading. A survey of several types of undercabinet lighting from a discount home improvement store revealed most had lux levels of less than 10% of the recommended levels. With the help of a lighting expert and a simple light meter, you can be assured to get the light output you really need.
The following light levels are recommended:
Footcandles Lux
Ambient Light(Home, Office, Classrooms)
15-30 150-300
Office Work, Reading 50 500
Supermarkets, Stores 75 750
Detailed Task Lighting 100 1000
Very Detailed Task Lighting 150-200 1500-2000
Poorly Lit Area Well Lit Area
Outdoor Lighting
The Lighting Research Center notes that in the early part of the twentieth century, when electric street lighting was beginning to be installed in many areas of the United States, moonlight levels were commonly used as a standard or reference point for outdoor lighting. In many instances, the visual quality of a street lighting design was measured against
moonlight. When considering the needs of an outdoor lighting installation, it is still helpful to think in terms of moonlight levels.
In rural areas, moonlight, with an Illuminance of approximately .1 lux on the ground, often provides enough lighting for people’s basic needs such as walking or finding a house or a car.
Outdoor lighting is measured here as the Illuminance on the ground.
Lux Footcandle
Parking Lots 1 10 =100 full moons
Sidewalk, Home Driveways .05 .5 =5 full moons
The human eye is able to adjust to a wide range of light levels.
Color of Light
Correlated Color Temperature (CCT)
Light has color, whether it emanates from the moon, the sun or your favorite reading light.
It might tend toward: orange - parking lot lightsyellow - common reading lights white - daylight and office light
The Correlated Color Temperature indicates the color of the light. The CCT is based on a Kelvin scale, which is actually a temperature measurement, but in this usage has no bearing on the actual temperature of the light or the heat it generates. The Kelvin scale is like the Celsius and Fahrenheit Scale, but starts at 0 to denote absolute zero – which is as cold as we think it can possibly get.
You may ask why a temperature scale is used to measure light color. Here’s why:
Many years ago it was noticed that when a piece of iron burns (called technically a “black body”), it changes color as it gets hotter, starting from an orange color, then yellow color and getting to a blue -white hot color. The temperature of iron burning is measured on the Kelvin scale. This observation was then used to describe the color of light.
Strangely though, the higher the “temperature”, the cooler the light is perceived and the lower the “temperature”, the warmer the light is perceived.
A CCT below 3200 K is considered a “warm” light, with more of a yellow tint and a higher CCT, above 4000 K, is considered a “cool” white, with more of a blue tint.
High Pressure Sodium – common highway light 1900 K
Warm Compact Fluorescent 2700 K
Incandescent 2700 K
Halogen Light - common spotlight 3200 K
Cool White Linear Fluorescent - typical office light 4200 K
When you purchase a light it is important to know the Kelvin color of the light. For a home environment, most people want a lower Kelvin, about 3000K or less. For an office environment, around 4500K is acceptable. Many bulb manufacturers now indicate the Kelvin color on the packaging of the bulb or on the bulb itself. This would be shown as “4000K” on the base of the bulb. Sometimes the packaging indicates simply that the bulb is a warm or a cool white.
The Department of Energy is pushing manufacturers to label their products with lumens and Kelvin color so consumers can accurately select the correct light for their application.
The color of light is also important in an outdoor application. A warm or yellow light is often used for landscape lighting to give a warm glow around a home. While many outdoor lights used for general illumination, such as street lighting or parking lot lighting are still quite warm due to the widespread use of High Pressure Sodium Lighting, there is a push to use higher Kelvin lighting such as Metal Halide lights as studies have shown that the eye can see better at night with this type of lighting.
How the Light Illuminates the Environment
The CCT designation gives a good idea of the light’s general appearance, but does not always tell you how it will illuminate different colors. Two lights with the same CCT may be dramatically different in how they show color due to their inherent radiant characteristics.
To easily describe and compare these characteristics simply the Color Rendering Index (CRI) was developed. The CRI is a numeric representation of a light bulb's ability to show colors "realistically," compared to a reference light source of the same color. The CRI scale ranges from 0 to 100—the higher the number, the more natural the colors will look to you. For good color quality, look for lights with CRIs of 70 or above.
Since the CRI rating for any given lamp is an average of eight test results, it can give no particular insight into the effect of the appearance of any one color. In the example below, the two light sources illuminating the object have a CRI of 70, however the light source on the right renders blue more naturally than the one on the left.
-Renders Blue less naturally -Renders Blue more naturally
The Department of Energy acknowledges the limitations of the CRI, especially when applied to bright white LED’s. They have the following recommendations:
"A long-term research and development process is underway to develop a revised color quality metric that would be applicable to all white light sources. In the meantime, CRI can be considered as one data point in evaluating white LED products and systems. It should not be used to make product selections in the absence of in-person and on-site evaluations." For more information on this topic, see the DOE site here.
The Color Spectrum of Light
Each light source will have a unique color spectrum. This spectrum is of a flashlight that uses a high-powered Cree white LED. This spectrum shows only the wavelengths that are visible to the eye. The spectrum determines the appearance of colors to the human eye. This spectrum has a large amount of blue, but low red color. This LED would render blues intensely, but wash out the red colors.
The Color of Objects
Color is produced by the absorption of selected wavelengths of light by an object. Objects can be thought of as absorbing all colors except the colors of their appearance, which are reflected as illustrated below. A blue object illuminated by white light absorbs most of the wavelengths except those corresponding to blue light. These blue wavelengths are reflected by the object.
Most light sources radiate in a 360-degree pattern.
The fixture often uses reflectors and lenses to “throw” more light where it is needed and wanted. The design and use of these devices is called Optics.
Optics is defined as the science that deals with the properties of light; in this case specifically dealing with the way light changes directions when it is either refracted and dispersed by a lens or reflected from a mirror.
LED’s are quite different in this respect as the light is radiated in about a 90-degree pattern and is noted as being “directional” or going only in one direction. This makes LED’s more efficient in task lighting, street lighting and other types of lighting that require light to be directional. However, with the use of Optics and other devices, an LED fixture can be made to be 360 degrees.
When comparing light fixtures, be sure to notice how efficient they are at putting light in the areas you need and want light. To make a full comparison of light fixtures you need to consider:
The overall energy usage of the fixture including ballasts, drivers and bulb The cost of the fixture The life of the light source and replacement costs over the lifetime of the fixture The amount, color and CRI of the light reaching the surfaces you wish to illuminate
LED - A New Form of a Light
What is an LED?
LED stands for Light Emitting Diode. A Diode is an electrical device that restricts current flow chiefly to one direction. Diodes are used in many electrical devices such as computers and audio equipment. A Light Emitting Diode is a special type of Diode that emits light when electricity is passed through it.
What makes an LED work?
Inside each LED is a small bit of chemicals. When electrons are passed through it, it emits light.
By changing this chemical compound, you can in effect change the wavelength emitted – infrared, red/green/blue (RGB), near ultraviolet, etc. By combining red, green and blue (RGB) LED’s you actually see a white light. Hence the method is called multi-colored white LEDs or RGB LEDs.
RGB LED White LED
Another method to produce white light involves coating an LED of one color (usually blue) with phosphor of different colors to produce white light. The resultant LEDs are called phosphor-based white LEDs.
How is an LED different from a typical light source?
An LED is a diode, meaning that it is polarized (one end is positive and one end is negative). By convention, current can only go from the anode (positive end) to the cathode (ground, or negative end). Therefore the LED does not “like” and alternating current that alternates directions, but “likes” a nice steady current source. This means that using an LED is not as simple as changing a bulb. It requires electronics to change the normal AC (Alternating Current) into a steady direct current (DC) with a low voltage.
LEDs can be said to be the digital version of light as opposed to traditional light sources, which could be said to be analog. Thus LED’s are much easier to manipulate, can change colors and intensity with an electronic controller and are adaptable to many, many different applications.
What is so special about an LED?
1. LEDs require lower power than traditional lighting sources to emit light. For example it takes about 10 watts to power an LED that is equal to a 60-watt incandescent bulb. The same light from a fluorescent takes 40 watts. The chart below shows a comparison of 5 different light source for lumens (a measure of light output) per watt (a measure of energy consumption).
2. LED’s are directional – only putting light where it is directed, resulting in higher efficiency.3. LED’s have an exceptionally long life. An incandescent bulb may last about 1000 hours or
less. A fluorescent tube should last 20,000 hours. A properly engineered LED will last 50,000 hours or more.
Basic advantages of LED Light
1. Energy efficient - LED’s are now capable of outputting 135 lumens/watt2. Long Lifetime - 50,000 hours or more if properly engineered3. Rugged - LED’s are also called “Solid State Lighting (SSL) as they are made of solid material
with no filament or tube or bulb to break4. No warm-up period - LED’s light instantly – in nanoseconds5. Not affected by cold temperatures - LED’s “like” low temperatures and will startup even in
subzero weather6. Directional - With LED’s you can direct the light where you want it, thus no light is wasted7. Excellent Color Rendering - LED’s do not wash out colors like other light sources such as
fluorescents, making them perfect for displays and retail applications8. Environmentally friendly - LED’s contain no mercury or other hazardous substances9. Controllable - LED’s can be controlled for brightness and color
LEDs are ideal for use in applications that are subject to frequent on-off cycling, unlike fluorescent lamps that burn out more quickly when cycled frequently, or HID lamps that require a long time before restarting.
LEDs can very easily be dimmed or strobed LEDs light up very quickly. A typical red indicator LED will achieve full brightness in
microseconds. LEDs mostly fail by dimming over time, rather than the abrupt burn-out of incandescent bulbs LEDs, being solid state components, are difficult to damage with external shock, unlike
fluorescent and incandescent bulbs which are fragile. LEDs can be very small and are easily populated onto printed circuit boards. LEDs do not contain mercury, unlike compact fluorescent lamps
Disadvantages and challenges in using LEDs
LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than more conventional lighting technologies. However, when considering the total cost of ownership (including energy and maintenance costs), LEDs far surpass incandescent or halogen sources and begin to threaten compact fluorescent lamps.
The Chart Below compares different light sources based upon the life of the bulb and the electrical cost at 10 cents per kWh (kilowatt hour). Note: fixture costs and installation costs are not included.
LED performance largely depends on correctly engineering the fixture to manage the heat generated by the LED, which causes deterioration of the LED chip itself. Over-driving the LED or not engineering the product to manage heat in high ambient temperatures may result in overheating of the LED package, eventually leading to device failure. Adequate heat-sinking is required to maintain long life. The most common design of a heat sink is a metal device with many fins, which conducts the heat away from the LED. For more information on this, refer to the Thermal Management tab.
LEDs must be supplied with the correct voltage and current at a constant flow. This requires some electronics expertise to design the electronic drivers.
LED’s can shift color due to age and temperature. Also two different white LED will have two different color characteristics, which affect how the light is perceived.
LEDs were first used for signal lighting, such as in a dashboard and later in tail lamps. In the past few years several companies have developed high power LEDs which are extremely bright and can now be used in applications that require a high light output, such as street lighting and task lighting. These are often referred to as "lighting class LEDs."
The Future of LEDs
LEDs continue to get brighter, more efficient and cheaper. Some predict a 2 or 3 times improvement in efficiency and brightness before the decade is over with significant price decreases. Whether these predictions are true or not remains to be seen, but what is certain is that millions of dollars are being invested in this technology every year ensuring a bright future for LED technology.
When sunlight is directed through a prism and onto a screen, it is separated into a rainbow pattern on the screen. The colors are arranged in this rainbow pattern beginning with red, followed by orange, yellow, yellow-green, green-blue, blue, blue-violet and finally violet. This pattern is called the optical spectrum and represents the part of the electromagnetic spectrum visible to human eyes. Other parts of the spectrum invisible to human eyes include the infrared part of the spectrum which we can feel as warmth on skin, and the ultraviolet part of the spectrum which can be recorded on photographic film. These parts of the spectrum are utilized in a variety of ways. The portion of the optical spectrum which human beings perceive as brightest is yellow-green light with a frequency of 555nm.
The intensity of light is perceived as brightness or darkness and serves as the basis for the brain's judgment of the light level. (Refer to the Standard Comparative Visual Sensitivity Chart)
Understanding Artificial Light
Artificial light sources are light sources which artificially combine just the necessary components of the optical spectrum. The way in which these components are combined determines the color rendering of the light source and thereby affects the way in which objects appear when illuminated with the light source
The Three Primary Colors of Light
Artificial light can be created by combining red (R), green (G) and blue (B) components as shown in the illustration. Changes in the RGB ratios change the characteristics of the light and therefore the mixture ratios of RGB fluorescent substances is an important point to be taken into consideration when creating artificial lights
Three Band Fluorescent Lamps
HG XHG X lamps are three band fluorescent lamps which apply the three primary colors of light. These bright lamps were developed to provide exceptional color rendering characteristics by increasing the yellow-green component which emphasizes brightness perception as compared with conventional fluorescent lamps and more carefully balancing the R, G and B components of the light
Why are Tunnels Colorless Worlds?
The orange colored low-pressure sodium vapor lights often used in tunnels provide monochromatic orange light Therefore your eyes perceive brightness but are unable to discriminate between colors
Although the human eye is not capable of distinguishing between different types of light, it has a remarkable ability to perceive slight differences in color between illuminated objects. However, the color rendering properties of light has a significant effect upon human psychology because it can cause color perception to change in a variety of ways. Therefore the selection of appropriate light sources is a very important point to be considered when designing lighting.
gentle and warm atmosphere
calm relaxing
Color Effects of a Variety of Light Sources
Type of Light SourceRadiant Color of
LampColor Change in Natural
ObjectsEffect on
AtmosphereColor
EmphasizedColor Perceived
as Dark
Incandescent LampYellow Red White
Red tinge Warm Red Orange Blue
Daylight Fluorescent Lamp Blue White Slight bluish tinge Cool Green Blue Red Orange
Cool White Fluorescent Lamp
Cool White Slight yellowish tinge Moderate warmth Orange Blue Red
high-color rendering Fluorescent Lamp
Cool White - Moderate warmth All colors No change
Mercury Lamp Green White White tinged with greenCoolness with a green tinge
Yellow Green Blue
Red Orange
Metal Halide Lamp White - Moderate warmthYellow Green Blue
Red
Sodium Vapor Lamp Orange Yellowish tinge Dark tinged with Yellow All colors but
Type of Light SourceRadiant Color of
LampColor Change in Natural
ObjectsEffect on
AtmosphereColor
EmphasizedColor Perceived
as Dark
yellow yellow
It is essential that we have an appropriate lighting plan which fits our individual lifestyles when we are relaxing or engaged in recreational activities or hobbies. Below, we provide several actual examples of lighting plans to serve as a guide to selecting appropriate light sources for your lifestyle. Please use these examples as checkpoints when purchasing lamps.
Lighting Advice for Different Lifestyles
Living and dining rooms are parts of the house where the most people gather. The uses to which these rooms are put varies greatly from household to household. In order to determine the type of lighting that is most suitable in a given household, the uses to which the rooms are put should be classified by mutual attributes as indicated below.
(Relaxation)
Sense of Refinement Emanating from Calm and Tranquil Spaces
Method :
The main lighting is provided by a core of indirect incandescent lights and overall lighting is moderate. Mobile light stands can be used for reading, etc. Brackets and stands are used to keep the position of light sources low.
(Happy time)
Lively Scene in a Relaxed Atmosphere
Method :
Set the level of illumination high and make the entire room bright. Use diffusion light such as fluorescent lamps so that peoples faces can be seen clearly.
(Dining A)
Dining with Many People in a Hearty Brightness
Method :
Set the level of illumination high. Using a lamp with good color rendering (such as EX-HG) gives an impression of cleanliness.
(Dining B)
Relaxing, Quiet Dining
Method :
Imaging the fancying atmosphere of a restaurant.Keep peripheral lighting moderate.Concentrate lighting around the table and illuminate nearby decorative objects with small spot lights in order to give an impression of a spacious room.
(Social gathering)
Create a Brilliant Atmosphere
Method :
Increase the illumination of the entire room and mix fluorescent and incandescent lighting to create a pleasant and cheerful atmosphere.
We at NEC provide a variety of lamps for use in peoples' daily lives. Each type of lamp has its own particular characteristics. We have prepared the graph below to serve as an aid in the selection of the ideal lamp for a particular purpose or application. This graph plots the color temperature of each type of lamp versus its average color rendering index (Ra). The horizontal axis of the graph indicates the average color rendering index of each lamp, the characteristic which affects the way in which the color of objects illuminated by the lamp are perceived.
The more right value indicated the better the color rendering, and the closer the color rendering approaches that of a standard light source of the same color temperature. The vertical axis indicates the color temperature of the light radiated by each lamp. The higher the value appears on the graph, the higher the color temperature becomes. The perceived color characteristics are also noted for the various color temperatures indicated.
In a fluorescent lamp the ultraviolet radiation, efficiently produced by an electric discharge in a mixture of low pressure mercury vapor and low pressure inert gas such as argon, krypton or neon, stimulates the phosphor material coated on the inside of the lamp tube to emit visible light. Therefore phosphors could be called converters for changing invisible ultraviolet light into visible light.
When a fluorescent lamp is switched on, first a heating current passes through the cathodes in order to increase the temperature of the cathodes and an arc discharge (a current flow through a gas) is produced by applying a high voltage between the cathodes. Once a discharge is produced, the temperature of the cathodes can be maintained by the collision of electrons without the need for any other cathode heating, and in addition the mercury vapor pressure increases due to vaporization by the discharge, and the current increases. Fluorescent lamps are designed to incorporate ballast and therefore provide continuous discharge in a balanced state at an optimum value.
The fast electrons emitted by the cathode collide with mercury atoms and a transfer of energy occurs between the two. As a result of this transfer of energy the mercury atom emits an ultra-violet radiation with a wavelength of 253.7nm. Another energy transfer occurs when this ultra violet radiation collides with the phosphor and visible light is radiated as a result.
Issues of Artificial IlluminationArtificial lighting[ edit | edit source]
Artificial lighting is not a natural component of the environment. Its production is aimed to replace natural lighting when it’s
not enough, such as in buildings with insufficient number of windows or during night. The sources of artificial light are
commonly lamps, which are devices that convert electrical energy into electromagnetic radiation.
Despite significant technological progress, no artificial lighting can copy the dynamics of natural light and its spectral
composition. It is therefor less favorable for humans.
Although artificial light cannot be used to replace natural light in a long period of time, it can be used for temporary
replacement with or without combination with day light when needed. The use of artificial light consumes a lot of energy,
which is another reason for capturing as much daylight as possible in building.
Measurement or artificial light
Artificial light can be measured in absolute units (lux) because it doesn’t vary throughout the day like day light. The daylight
must be completely excluded at the time of measurement. Evenness of artificial lighting (R) expresses the ratio between
minimum and average of E values:
R = E min / E avg
An artificial lighting map is created by isolines (called isoluxes) of illumination. The lines connect places with identical E
A lamp is an energy converter. Although it may carry out secondary functions, its prime purpose is the transformation of electrical energy into visible electromagnetic radiation. There are many ways to create light. The standard method for creating general lighting is the conversion of electrical energy into light.
Types of Light
Incandescence
When solids and liquids are heated, they emit visible radiation at temperatures above 1,000 K; this is known as incandescence.
Such heating is the basis of light generation in filament lamps: an electrical current passes through a thin tungsten wire, whose temperature rises to around 2,500 to 3,200 K, depending upon the type of lamp and its application.
There is a limit to this method, which is described by Planck’s Law for the performance of a black body radiator, according to which the spectral distribution of energy radiated increases with temperature. At about 3,600 K and above, there is a marked gain in emission of visible radiation, and the wavelength of maximum power shifts into the visible band. This temperature is close to the melting point of tungsten, which is used for the filament, so the practical temperature limit is around 2,700 K, above which filament evaporation becomes excessive. One result of these spectral shifts is that a large part of the radiation emitted is not given off as light but as heat in the infrared region. Filament lamps can thus be effective heating devices and are used in lamps designed for print drying, food preparation and animal rearing.
Electrical discharge is a technique used in modern light sources for commerce and industry because of the more efficient production of light. Some lamp types combine the electrical discharge with photoluminescence.
An electric current passed through a gas will excite the atoms and molecules to emit radiation of a spectrum which is characteristic of the elements present. Two metals are commonly used, sodium and mercury, because their characteristics give useful radiations within the visible spectrum. Neither metal emits a continuous spectrum, and discharge lamps have selective spectra. Their colour rendering will never be identical to continuous spectra. Discharge lamps are often classed as high pressure or low pressure, although these terms are only relative, and a high-pressure sodium lamp operates at below one atmosphere.
Types of Luminescence
Photoluminescence occurs when radiation is absorbed by a solid and is then re-emitted at a different wavelength. When the re-emitted radiation is within the visible spectrum the process is called fluorescence or phosphorescence.
Electroluminescence occurs when light is generated by an electric current passed through certain solids, such as phosphor materials. It is used for self-illuminated signs and instrument panels but has not proved to be a practical light source for the lighting of buildings or exteriors.
Evolution of Electric Lamps
Although technological progress has enabled different lamps to be produced, the main factors influencing their development have been external market forces. For example, the production of filament lamps in use at the start of this century was possible only after the availability of good vacuum pumps and the drawing of tungsten wire. However, it was the large-scale generation and distribution of electricity to meet the demand for electric lighting that determined market growth. Electric lighting offered many advantages over gas- or oil-generated light, such as steady light that requires infrequent maintenance as well as the increased safety of having no exposed flame, and no local by-products of combustion.
During the period of recovery after the Second World War, the emphasis was on productivity. The fluorescent tubular lamp became the dominant light source because it made possible the shadow-free and comparatively heat-free lighting of factories and offices, allowing maximum use of the space. The light output and wattage requirements for a typical 1,500 mm fluorescent tubular lamp is given in table 46.1 .
Table 46.1 Improved light output and wattage requirements of some typical 1,500 mm fluorescent tube lamps
Rating (W) Diameter (mm) Gas fill Light output (lumens)
80 38 argon 4,800
65 38 argon 4,900
58 25 krypton 5,100
50 25 argon 5,100 (high frequency gear)
By the 1970s oil prices rose and energy costs became a significant part of operating costs. Fluorescent lamps that produce the same amount of light with less electrical consumption were demanded by the market. Lamp design was refined in several ways. As the century closes there is a growing awareness of global environment issues. Better use of declining raw materials, recycling or safe disposal of products and the continuing concern over energy consumption (particularly energy generated from fossil fuels) are impacting on current lamp designs.
Performance Criteria
Performance criteria vary by application. In general, there is no particular hierarchy of importance of these criteria.
Light output: The lumen output of a lamp will determine its suitability in relation to the scale of the installation and the quantity of illumination required.
Colour appearance and colour rendering: Separate scales and numerical values apply to colour appearance and colour rendering. It is important to remember that the figures provide guidance only, and some are only approximations. Whenever possible, assessments of suitability should be made with actual lamps and with the colours or materials that apply to the situation.
Lamp life: Most lamps will require replacement several times during the life of the lighting installation, and designers should minimize the inconvenience to the occupants of odd failures and maintenance. Lamps are used in a wide variety of applications. The anticipated average life is often a compromise between cost and performance. For example, the lamp for a slide projector will have a life of a few hundred hours because the maximum light output is important to the quality of the image. By contrast, some roadway lighting lamps may be changed every two years, and this represents some 8,000 burning hours.
Further, lamp life is affected by operating conditions, and thus there is no simple figure that will apply in all conditions. Also, the effective lamp life may be determined by different failure modes. Physical failure such as filament or lamp rupture may be preceded by reduction in light output or changes in colour appearance.
Lamp life is affected by external environmental conditions such as temperature, vibration, frequency of starting, supply voltage fluctuations, orientation and so on.
It should be noted that the average life quoted for a lamp type is the time for 50% failures from a batch of test lamps. This definition of life is not likely to be applicable to many commercial or industrial installations; thus practical lamp life is usually less than published values, which should be used for comparison only.
Efficiency: As a general rule the efficiency of a given type of lamp improves as the power rating increases, because most lamps have some fixed loss. However, different types of lamps have marked variation in efficiency. Lamps of the highest efficiency should be used, provided that the criteria of size, colour and lifetime are also met. Energy savings should not be at the expense of the visual comfort or the performance ability of the occupants. Some typical efficacies are given in table 46.2 .
Table 46.2 Typical lamp efficacies
Lamp efficacies
100W filament lamp 14 lumens/watt
58W fluorescent tube 89 lumens/watt
400W high-pressure sodium 125 lumens/watt
131W low-pressure sodium 198 lumens/watt
Main lamp types
Over the years, several nomenclature systems have been developed by national and international standards and registers.
In 1993, the International Electrotechnical Commission (IEC) published a new International Lamp Coding System (ILCOS) intended to replace existing national and regional coding systems. A list of some ILCOS short form codes for various lamps is given in table 46.3 .
Table 46.3 International Lamp Coding System (ILCOS) short form coding system for some lamp types
Type (code) Common ratings (watts) Colour rendering Colour temperature (K)
Compact fluorescent lamps (FS) 5–55 good 2,700–5,000
Metal halide lamps (M-) 35–2,000 good to excellent 3,000–5,000
Tubular fluorescent lamps (FD) 4–100 fair to good 2,700–6,500
Tungsten halogen lamps (HS) 100–2,000 good 3,000
Incandescent lamps
These lamps use a tungsten filament in an inert gas or vacuum with a glass envelope. The inert gas suppresses tungsten evaporation and lessens the envelope blackening. There is a large variety of lamp shapes, which are largely decorative in appearance. The construction of a typical General Lighting Service (GLS) lamp is given in figure 46.1 .
Figure 46.1 Construction of a GLS lamp
Incandescent lamps are also available with a wide range of colours and finishes. The ILCOS codes and some typical shapes include those shown in table 46.4 .
Table 46.4 Common colours and shapes of incandescent lamps, with their ILCOS codes
Colour/Shape Code
Clear /C
Frosted /F
White /W
Red /R
Blue /B
Green /G
Yellow /Y
Pear shaped (GLS) IA
Candle IB
Conical IC
Globular IG
Mushroom IM
Incandescent lamps are still popular for domestic lighting because of their low cost and compact size. However, for commercial and industrial lighting the low efficacy generates very high operating costs, so discharge lamps are the normal choice. A 100 W lamp has a typical efficacy of 14 lumens/watt compared with 96 lumens/watt for a 36 W fluorescent lamp.
Incandescent lamps are simple to dim by reducing the supply voltage, and are still used where dimming is a desired control feature.
The tungsten filament is a compact light source, easily focused by reflectors or lenses. Incandescent lamps are useful for display lighting where directional control is needed.
Tungsten halogen lamps
These are similar to incandescent lamps and produce light in the same manner from a tungsten filament. However the bulb contains halogen gas (bromine or iodine) which is active in controlling tungsten evaporation. See figure 46.2 .
Fundamental to the halogen cycle is a minimum bulb wall temperature of 250 °C to ensure that the tungsten halide remains in a gaseous state and does not condense on the bulb wall. This temperature means bulbs made from quartz in place of glass. With quartz it is possible to reduce the bulb size.
Most tungsten halogen lamps have an improved life over incandescent equivalents and the filament is at a higher temperature, creating more light and whiter colour.
Tungsten halogen lamps have become popular where small size and high performance are the main requirement. Typical examples are stage lighting, including film and TV, where directional control and dimming are common requirements.
Low-voltage tungsten halogen lamps
These were originally designed for slide and film projectors. At 12 V the filament for the same wattage as 230 V becomes smaller and thicker. This can be more efficiently focused, and the larger filament mass allows a higher operating temperature, increasing light output. The thick filament is more robust. These benefits were realized as being useful for the commercial display market, and even though it is necessary to have a step-down transformer, these lamps now dominate shop-window lighting. See figure 46.3.
Although users of film projectors want as much light as possible, too much heat damages the transparency medium. A special type of reflector has been developed, which reflects only the visible radiation, allowing infrared radiation (heat) to pass through the back of the lamp. This feature is now part of many low-voltage reflector lamps for display lighting as well as projector equipment.
Voltage sensitivity: All filament lamps are sensitive to voltage variation, and light output and life are affected. The move to “harmonize” the supply voltage throughout Europe at 230 V is being achieved by widening the tolerances to which the generating authorities can operate. The move is towards ±10%, which is a voltage range of 207 to 253 V. Incandescent and tungsten halogen lamps cannot be operated sensibly over this range, so it will be necessary to match actual supply voltage to lamp ratings. See figure 46.4 .
Discharge lamps will also be affected by this wide voltage variation, so the correct specification of control gear becomes important.
Tubular fluorescent lamps
These are low pressure mercury lamps and are available as “hot cathode” and “cold cathode” versions. The former is the conventional fluorescent tube for offices and factories; “hot cathode” relates to the starting of the lamp by pre-heating the electrodes to create sufficient ionization of the gas and mercury vapour to establish the discharge.
Cold cathode lamps are mainly used for signage and advertising. See figure 46.5 .
Fluorescent lamps require external control gear for starting and to control the lamp current. In addition to the small amount of mercury vapour, there is a starting gas (argon or krypton).
The low pressure of mercury generates a discharge of pale blue light. The major part of the radiation is in the UV region at 254 nm, a characteristic radiation frequency for mercury. Inside of the tube wall is a thin phosphor coating, which absorbs the UV and radiates the energy as visible light. The colour quality of the light is determined by the phosphor coating. A range of phosphors are available of varying colour appearance and colour rendering.
During the 1950s phosphors available offered a choice of reasonable efficacy (60 lumens/watt) with light deficient in reds and blues, or improved colour rendering from “deluxe” phosphors of lower efficiency (40 lumens/watt).
By the 1970s new, narrow-band phosphors had been developed. These separately radiated red, blue and green light but, combined, produced white light. Adjusting the proportions gave a range of different colour appearances, all with similar excellent colour rendering. These tri-phosphors are more efficient than the earlier types and represent the best economic lighting solution, even though the lamps are more expensive. Improved efficacy reduces operating and installation costs.
The tri-phosphor principle has been extended by multi-phosphor lamps where critical colour rendering is necessary, such as for art galleries and industrial colour matching.
The modern narrow-band phosphors are more durable, have better lumen maintenance, and increase lamp life.
Compact fluorescent lamps
The fluorescent tube is not a practical replacement for the incandescent lamp because of its linear shape. Small, narrow-bore tubes can be configured to approximately the same size as the incandescent lamp, but this imposes a much higher electrical loading on the phosphor material. The use of tri-phosphors is essential to achieve acceptable lamp life. See figure 46.6 .
Figure 46.6 Four-leg compact fluorescent
All compact fluorescent lamps use tri-phosphors, so, when they are used together with linear fluorescent lamps, the latter should also be tri-phosphor to ensure colour consistency.
Some compact lamps include the operating control gear to form retro-fit devices for incandescent lamps. The range is increasing and enables easy upgrading of existing installations to more energy-efficient lighting. These integral units are not suitable for dimming where that was part of the original controls.
High-frequency electronic control gear: If the normal supply frequency of 50 or 60 Hz is increased to 30 kHz, there is a 10% gain in efficacy of fluorescent tubes. Electronic circuits can operate individual lamps at such frequencies. The electronic circuit is designed to provide the same light output as wire-wound control gear, from reduced lamp power. This offers compatibility of lumen package with the advantage that reduced lamp loading will increase lamp life significantly. Electronic control gear is capable of operating over a range of supply voltages.
There is no common standard for electronic control gear, and lamp performance may differ from the published information issued by the lamp makers.
The use of high-frequency electronic gear removes the normal problem of flicker, to which some occupants may be sensitive.
Induction lamps
Lamps using the principle of induction have recently appeared on the market. They are low-pressure mercury lamps with tri-phosphor coating and as light producers are similar to fluorescent lamps. The energy is transferred to the lamp by high-frequency radiation, at approximately 2.5 MHz from an antenna positioned centrally within the lamp. There is no physical connection between the lamp bulb and the coil. Without electrodes or other wire connections the construction of the discharge vessel is simpler and more durable. Lamp life is mainly determined by the reliability of the electronic components and the lumen maintenance of the phosphor coating.
High-pressure mercury lamps
High-pressure discharges are more compact and have higher electrical loads; therefore, they require quartz arc tubes to withstand the pressure and temperature. The arc tube is contained in an outer glass envelope with a nitrogen or argon-nitrogen atmosphere to reduce oxidation and arcing. The bulb effectively filters the UV radiation from the arc tube. See figure 46.7 .
Figure 46.7 Mercury lamp construction
At high pressure, the mercury discharge is mainly blue and green radiation. To improve the colour a phosphor coating of the outer bulb adds red light. There are
deluxe versions with an increased red content, which give higher light output and improved colour rendering.
All high-pressure discharge lamps take time to reach full output. The initial discharge is via the conducting gas fill, and the metal evaporates as the lamp temperature increases.
At the stable pressure the lamp will not immediately restart without special control gear. There is a delay while the lamp cools sufficiently and the pressure reduces, so that the normal supply voltage or ignitor circuit is adequate to re-establish the arc.
Discharge lamps have a negative resistance characteristic, and so the external control gear is necessary to control the current. There are losses due to these control gear components so the user should consider total watts when considering operating costs and electrical installation. There is an exception for high-pressure mercury lamps, and one type contains a tungsten filament which both acts as the current limiting device and adds warm colours to the blue/green discharge. This enables the direct replacement of incandescent lamps.
Although mercury lamps have a long life of about 20,000 hours, the light output will fall to about 55% of the initial output at the end of this period, and therefore the economic life can be shorter.
Metal halide lamps
The colour and light output of mercury discharge lamps can be improved by adding different metals to the mercury arc. For each lamp the dose is small, and for accurate application it is more convenient to handle the metals in powder form as halides. This breaks down as the lamp warms up and releases the metal.
A metal halide lamp can use a number of different metals, each of which give off a specific characteristic colour. These include:
· dysprosium—broad blue-green
· indium—narrow blue
· lithium—narrow red
· scandium—broad blue-green
· sodium—narrow yellow
· thallium—narrow green
· tin—broad orange-red
There is no standard mixture of metals, so metal halide lamps from different manufacturers may not be compatible in appearance or operating performance. For lamps with the lower wattage ratings, 35 to 150 W, there is closer physical and electrical compatibility with a common standard.
Metal halide lamps require control gear, but the lack of compatibility means that it is necessary to match each combination of lamp and gear to ensure correct starting and running conditions.
Low-pressure sodium lamps
The arc tube is similar in size to the fluorescent tube but is made of special ply glass with an inner sodium resistant coating. The arc tube is formed in a narrow “U” shape and is contained in an outer vacuum jacket to ensure thermal stability. During starting, the lamps have a strong red glow from the neon gas fill.
The characteristic radiation from low-pressure sodium vapour is a monochromatic yellow. This is close to the peak sensitivity of the human eye, and low-pressure sodium lamps are the most efficient lamps available at nearly 200 lumens/watt. However the applications are limited to where colour discrimination is of no visual importance, such as trunk roads and underpasses, and residential streets.
In many situations these lamps are being replaced by high-pressure sodium lamps. Their smaller size offers better optical control, particularly for roadway lighting where there is growing concern over excessive sky glow.
High-pressure sodium lamps
These lamps are similar to high-pressure mercury lamps but offer better efficacy (over 100 lumens/watt) and excellent lumen maintenance. The reactive nature of sodium requires the arc tube to be manufactured from translucent polycrystalline alumina, as glass or quartz are unsuitable. The outer glass bulb contains a vacuum to prevent arcing and oxidation. There is no UV radiation from the sodium discharge so phosphor coatings are of no value. Some bulbs are frosted or coated to diffuse the light source. See figure 46.8 .
Figure 46.8 High-pressure sodium lamp construction
As the sodium pressure is increased, the radiation becomes a broad band around the yellow peak, and the appearance is golden white. However, as the pressure increases, the efficiency decreases. There are currently three separate types of high-pressure sodium lamps available, as shown in table 46.5 .
Table 46.5 Types of high-pressure sodium lamp
Lamp type (code) Colour (K) Efficacy (lumens/watt) Life (hours)
Standard 2,000 110 24,000
Deluxe 2,200 80 14,000
White (SON) 2,500 50
Generally the standard lamps are used for exterior lighting, deluxe lamps for industrial interiors, and White SON for commercial/display applications.
Dimming of Discharge Lamps
The high-pressure lamps cannot be satisfactorily dimmed, as changing the lamp power changes the pressure and thus the fundamental characteristics of the lamp.
Fluorescent lamps can be dimmed using high-frequency supplies generated typically within the electronic control gear. The colour appearance remains very constant. In addition, the light output is approximately proportional to the lamp power, with consequent saving in electrical power when the light output is reduced. By integrating the light output from the lamp with the prevailing level of natural daylight, a near constant level of illuminance can be provided in an interior.
Human beings possess an extraordinary capacity to adapt to their environment and to their immediate surroundings. Of all the types of energy that humans can utilize, light is the most important. Light is a key element in our capacity to see, and it is necessary to appreciate the form, the colour and the perspective of the objects that surround us in our daily lives. Most of the information we obtain through our senses we obtain through sight—close to 80%. Very often, and because we are so used to having it available, we take it for granted. We should not fail to keep in mind, however, that aspects of human welfare, like our state of mind or our level of fatigue, are affected by illumination and the colour of the things that surround us. From the point of view of safety at work, visual capacity and visual comfort are extraordinarily important. This is because many accidents are due to, among other reasons, illumination deficiencies or errors made by the worker because he or she finds it hard to identify objects or the risks associated with machinery, conveyances, dangerous containers and so on.
Visual disorders associated with deficiencies in the illumination system are common in the workplace. Due to the ability of sight to adapt to situations with deficient lighting, these aspects are sometimes not considered as seriously as they should be.
The correct design of an illumination system should offer the optimal conditions for visual comfort. For the attainment of this goal an early line of collaboration between architects, lighting designers and those responsible for hygiene at the worksite should be established. This collaboration should precede the beginning of the project, to avoid errors that would be difficult to correct once the project is completed. Among the most important aspects that should be kept in mind are the type of lamp that will be used and the lighting system that will be installed, the distribution of luminance, illumination efficiencies and the spectral composition of light.
The fact that light and colour affect the productivity and the psycho-physiological well-being of the worker should encourage the initiatives of illumination technicians, physiologists and ergonomists, to study and determine the most favourable conditions of light and colour at each work station. The combination of illumination, the contrast of luminances, the colour of light, the reproduction of colour or the selection of colours are the elements that determine colour climate and visual comfort.
Factors that Determine Visual Comfort
The prerequisites that an illumination system must fulfil in order to provide the conditions necessary for visual comfort are the following:
· uniform illumination
· optimal luminance
· no glare
· adequate contrast conditions
· correct colours
· absence of stroboscopic effect or intermittent light.
It is important to consider light in the workplace not only by quantitative criteria, but also by qualitative criteria. The first step is to study the work station, the precision required of the tasks performed, the amount of work, the mobility of the worker and so on. Light should include components both of diffuse and of direct radiation. The result of the combination will produce shadows of greater or lesser intensity that will allow the worker to perceive the form and position of objects at the work station. Annoying reflections, which make it harder to perceive details, should be eliminated, as well as excessive glare or deep shadows.
The periodic maintenance of the lighting installation is very important. The goal is to prevent the ageing of lamps and the accumulation of dust on the luminaries that will result in a constant loss of light. For this reason it is important to select lamps and systems that are easy to maintain. An incandescent light bulb maintains its efficiency until the moments before failure, but this is not the case with fluorescent tubes, which may lower their output down to 75% after a thousand hours of use.
Levels of illumination
Each activity requires a specific level of illumination in the area where the activity takes place. In general, the higher the difficulty for visual perception, the higher the average level of illumination should be as well. Guidelines for minimal levels of illumination associated with different tasks exist in various publications. Concretely, those listed in figure 46.9 have been gleaned from European norms CENTC 169, and are based more on experience than on scientific knowledge.
Figure 46.9 Levels of illumination as a function of tasks performed
The level of illumination is measured with a luxometer that converts luminous energy into an electrical signal, which is then amplified and offers an easy reading on a calibrated scale of lux. When selecting a certain level of illumination for a particular work station the following points should be studied:
· the nature of the work
· reflectance of the object and of the immediate surroundings
· differences with natural light and the need for daytime illumination
· the worker’s age.
Units and magnitudes of illumination
Several magnitudes are commonly used in the field of illumination. The basic ones are:
Luminous flux: Luminous energy emitted per unit of time by a light source. Unit: lumen (lm).
Luminous intensity: Luminous flux emitted in a given direction by a light that is not equally distributed. Unit: candela (cd).
Level of illumination: Level of illumination of a surface of one square metre when it receives a luminous flux of one lumen. Unit: lux = lm/m2.
Luminance or photometric brilliance: Is defined for a surface in a particular direction, and is the relation between luminous intensity and the surface seen by an observer situated in the same direction (apparent surface). Unit: cd/m2.
Contrast: Difference in luminance between an object and its surroundings or between different parts of an object.
Reflectance: Proportion of light that is reflected by a surface. It is a non-dimensional quantity. Its value ranges between 0 and 1.
Factors that affect the visibility of objects
The degree of safety with which a task is executed depends, in large part, on the quality of illumination and on visual capacities. The visibility of an object can be altered in many ways. One of the most important is the contrast of luminances due to reflection factors, to shadows, or to colours of the object itself, and to the reflection factors of colour. What the eye really perceives are the differences of luminance between an object and its surroundings, or between different parts of the same object. Table 46.6 lists the contrasts between colours in descending order.
The luminance of an object, of its surroundings, and of the work area influence the ease with which an object is seen. It is therefore of key importance that the area where the visual task is performed, and its surroundings, be carefully analysed.
The size of the object that must be observed, which may be adequate or not depending on the distance and the angle of vision of the observer, is another factor. These last two factors determine the arrangement of the work station, classifying different zones according to their ease of vision. We can establish five zones in the work area (see figure 46.10).
Figure 46.10 Distribution of visual zones in the work station
Another factor is the time frame during which vision occurs. The time of exposure will be greater or smaller depending on whether the object and the observer are static, or whether one or both of them are moving. The adaptive capacity of the eye to adjust automatically to the different illuminations of objects can also have considerable influence on visibility.
Light distribution; glare
Key factors in the conditions that affect vision are the distribution of light and the contrast of luminances. In so far as the distribution of light is concerned, it is preferable to have good general illumination instead of localized illumination in order to avoid glare. For this reason, electrical accessories should be distributed as uniformly as possible in order to avoid differences in luminous intensity. Constant shuttling through zones that are not uniformly illuminated causes eye fatigue, and with time this can lead to reduced visual output.
Glare is produced when a brilliant source of light is present in the visual field; the result is a diminution in the capacity to distinguish objects. Workers who suffer the effects of glare constantly and successively can suffer from eye strain as well as from functional disorders, even though in many cases they are not aware of it.
Glare can be direct when its origin is bright sources of light directly in the line of vision, or by reflection when light is reflected on surfaces with high reflectance. The factors involved in glare are:
1. Luminance of the source of light: The maximum tolerable lumi nance by direct observation is 7,500 cd/m2. Figure 46.11 shows some of the approximate values of luminance for several sources of light.
2. Location of the source of light: This kind of glare occurs when the source of light is within a 45-degree angle of the observer’s line of sight, and will be minimized to the degree that the source of light is placed beyond that angle. Ways and methods of avoiding direct and reflective glare can be seen in the following figures (see figure 46.12).
Figure 46.12 Factors that affect glare
In general, there is more glare when sources of light are mounted at lower elevations or when installed in large rooms, because sources of light in large rooms or sources of light that are too low can easily fall within the angle of vision that produces glare.
3. Distribution of luminance among different objects and surfaces: The greater the differences in luminance are among the objects within the field of vision, the greater
will be the glare created and the greater will be the deterioration in the capacity to see due to the effects on the adaptive processes of sight. The maximum recommended luminance disparities are:
· visual task—work surface: 3:1
· visual task—surroundings: 10:1
4. Time frame of the exposure: Even light sources with a low luminance can cause glare if the length of the exposure is prolonged too much.
Avoiding glare is a relatively simple proposition and can be achieved in different ways. One way, for example, is by placing grilles under the sources of illumination, or by using enveloping diffusers or parabolic reflectors that can direct light properly, or by installing the sources of light in such a way that they will not interfere with the angle of vision. When designing the work site, the correct distribution of luminance is as important as the illumination itself, but it is also important to consider that a distribution of luminance that is too uniform makes the three-dimensional and spatial perception of objects more difficult.
Lighting Systems
The interest in natural illumination has increased recently. This is due less to the quality of illumination it affords than to the well-being that it provides. But since the level of illumination from natural sources is not uniform, an artificial lighting system is required.
The most common lighting systems used are the following:
General uniform illumination
In this system light sources are spread out evenly without regard to the location of the work stations. The average level of illumination should be equal to the level of illumination required for the task that will be carried out. These systems are used mainly in workplaces where work stations are not fixed.
It should conform to three fundamental characteristics: The first is to be equipped with anti-glare devices (grilles, diffusers, reflectors and so on). The second is that it should distribute a fraction of the light toward the ceiling and the upper part of the walls. And the third is that the light sources should be installed as high as possible, to minimize glare and achieve illumination that is as homogeneous as possible. (See figure 46.13)
This system tries to reinforce the general illumination scheme by placing lamps close to the work surfaces. These types of lamps often produce glare, and reflectors should be placed in such a way that they block the source of light from the direct sight of the worker. The use of localized illumination is recommended for those applications where visual demands are very critical, such as levels of illumination of 1,000 lux or greater. Generally, visual capacity deteriorates with the age of the worker, which makes it necessary to increase the level of general illumination or to second it with localized illumination. This phenomenon can be clearly appreciated in figure 46.14 .
Figure 46.14 Loss of visual acuity with age
General localized illumination
This type of illumination consists of ceiling sources distributed with two things in mind—the illumination characteristics of the equipment and the illumination needs of each work station. This type of illumination is indicated for those spaces or work areas that will require a high level of illumination, and it requires knowing the future location of each work station in advance of the design stage.
Colour: Basic Concepts
Selecting an adequate colour for a worksite contributes a great deal to the efficiency, safety and general well-being of the employees. In the same way, the finish of the surfaces and of the equipment found in the work environment contributes to creating pleasant visual conditions and a pleasant work environment.
Ordinary light consists of electromagnetic radiations of different wavelengths that correspond to each of the bands of the visible spectrum. By mixing red, yellow and blue light we can obtain most of the visible colours, including white. Our perception of the colour of an object depends on the colour of the light with which it is illuminated and on the way the object itself reflects light.
Lamps can be classified into three categories depending on the appearance of the light they emit:
· colour with a warm appearance: a white, reddish light recommended for residential use
· colour with intermediate appearance: a white light recommended for worksites
· colour with a cold appearance: a white, bluish light recommended for tasks that require a high level of illumination or for hot climates.
Colours may also be classified as warm or cold according to their tonality (see figure 46.15).
Figure 46.15 Tonality of "warm" and "cold" colours
Contrast and temperature of different colours
Colour contrasts are influenced by the colour of the light selected, and for that reason the quality of illumination will depend on the colour of the light chosen for an application. The selection of the colour of light to be used should be made based on the task that will be carried out under it. If the colour is close to white, the rendition of
colour and the diffusion of light will be better. The more light approaches the red end of the spectrum the worse the reproduction of colour will be, but the environment will be warmer and more inviting.
The colour appearance of illumination depends not only on the colour of light, but also on the level of luminous intensity. A colour temperature is associated with the different forms of illumination. The sensation of satisfaction with the illumination of a given environment depends on this colour temperature. In this way, for example, a 100 W incandescent filament light bulb has a colour temperature of 2,800 K, a fluorescent tube has a colour temperature of 4,000 K and an overcast sky has a colour temperature of 10,000 K.
Kruithof defined, through empirical observations, a diagram of well-being for different levels of illumination and colour temperatures in a given environment (see figure 46.16). In this way, he demonstrated that it is possible to feel comfortable in certain environments with low levels of illumination if the colour temperature is also low—if the level of illumination is one candle, for example, with a colour temperature of 1,750 K.
Figure 46.16 Comfort diagram as a function of illumination and colour temperatures
The colours of electric lamps can be subdivided into three groups related to their colour temperatures:
The selection of colours is very relevant when we consider it together with those functions where identifying the objects that must be manipulated is important. It is also relevant when delimiting avenues of communication and in those tasks that require sharp contrast.
The selection of tonality is not as important a question as the selection of the proper reflective qualities of a surface. There are several recommendations that apply to this aspect of work surfaces:
Ceilings: The surface of a ceiling should be as white as possible (with a reflection factor of 75%), because light will then reflect from it in a diffuse way, dissipating darkness and reducing the glare from other surfaces. This will also mean a savings in artificial lighting.
Walls and floors: The surfaces of walls at eye level can produce glare. Pale colours with reflective factors of 50 to 75% tend to be adequate for walls. While glossy paints tend to last longer than matte colours, they are more reflective. Walls should therefore have a matte or semi-gloss finish.
Floors should be finished in slightly darker colours than walls and ceilings to avoid glare. The reflective factor of floors should be between 20 and 25%.
Equipment: Work surfaces, machinery and tables should have reflective factors of between 20 and 40%. Equipment should have a lasting finish of pure colour—light browns or greys—and the material should not be shiny.
The proper use of colours in the work environment facilitates well-being, increases productivity and can have a positive impact on quality. It can also contribute to better organization and the prevention of accidents.
There is a generalized belief that whitening the walls and ceilings and supplying adequate levels of illumination is all that can possibly be done as far as the visual comfort of employees is concerned. But these comfort factors can be improved by combining white with other colours, thus avoiding the fatigue and the boredom that characterize monochromatic environments. Colours also have an effect on a person’s level of stimulation; warm colours tend to activate and relax, while cold colours are used to induce the individual to release or liberate his or her energy.
The colour of light, its distribution, and the colours used in a given space are, among others, key factors that influence the sensations a person feels. Given the many colours and comfort factors that exist, it is impossible to set precise guidelines, especially considering that all these factors must be combined according to the characteristics and the requirements of a particular work station. A number of basic and general practical rules can be listed, however, that can help create a liveable environment:
· Bright colours produce comfortable, stimulating and serene feelings, while dark colours tend to have a depressing effect.
· Sources of warm-coloured light help reproduce warm colours well. Warm-coloured objects are more pleasing to the eye in warm light than in cold light.
· Clear and dull colours (like pastels) are very appropriate as background colours, while objects should have rich and saturated colours.
· Warm colours excite the nervous system and give the sensation that temperature is rising.
· Cold colours are preferable for objects. They have a calming effect and can be used to produce the effect of curvature. Cold colours help create the sensation that temperature is dropping.
· The sensation of colour of an object depends on the background colour and on the effect of the light source on its surface.
· Environments that are physically cold or hot can be tempered by using warm or cold lighting, respectively.
· The intensity of a colour will be inversely proportional to the part of the normal visual field that it occupies.
· The spatial appearance of a room can be influenced by colour. A room will seem to have a lower ceiling if its walls are painted a bright colour and the floor and ceiling are darker, and it will seem to have a higher ceiling if the walls are darker and the ceiling is bright.
Identifying objects through colour
The selection of colours can influence the effectiveness of lighting systems by influencing the fraction of light that is reflected. But colour also plays a key role when it comes to identifying objects. We can use brilliant and eye-catching colours or colour contrasts to highlight situations or objects that require special attention. Table 46.7 lists some of the factors of reflection for different colours and materials.
Lime-green, pale gray, pink, orange, blue-gray 50–55
Blond wood, blue sky 40–45
Oak, dry concrete 30–35
Deep red, leaf-green, olive-green, meadow-green 20–25
Dark blue, purple 10–15
Black 0
In any case, identification by colour should be employed only when it is truly necessary, since identification by colour will work properly only if there are not too many objects that are highlighted by colour. The following are some recommendations for identifying different elements by colour:
· Fire and safety equipment: It is advisable to identify this equipment by placing a recognizable graphic on the nearest wall so that it can be found quickly.
· Machinery: The colouring of stop or emergency devices with bright colours on all machinery is critical. It is also advisable to mark with colour the areas that need lubrication or periodic maintenance, which can add ease and functionality to these procedures.
· Tubing and pipes: If they are important or carry dangerous substances the best advice is to colour them completely. In some cases it may be enough to colour only a line along their length.
· Stairways: In order to make descent easier, one band for every step is preferable to several.
· Risks: Colour should be used to identify a risk only when the risk cannot be eliminated. Identification will be much more effective if it is carried out according to a predetermined colour code.
GENERAL LIGHTING CONDITIONS
N. Alan Smith
Lighting is provided within interiors in order to satisfy the following requirements:
· to assist in providing a safe working environment
· to assist in the performance of visual tasks
· to develop an appropriate visual environment.
The provision of a safe working environment has to be at the top of the list of priorities, and, in general, safety is increased by making hazards clearly visible. The order of priority of the other two requirements will depend to a large extent upon the use to which the interior is put. Task performance can be improved by ensuring that task detail is easier to see, while appropriate visual environments are developed by varying the lighting emphasis given to objects and surfaces within an interior.
Our general feeling of well-being, including morale and fatigue, is influenced by light and colour. Under low lighting levels, objects would have little or no colour or shape and there would be a loss in perspective. Conversely an excess of light may be just as unwanted as too little light.
In general, people prefer a room with daylight to a room which is windowless. Furthermore, contact with the outside world is considered to aid the feeling of well-being. The introduction of automatic lighting controls, together with high-frequency dimming of fluorescent lamps, has made it possible to provide interiors with a controlled combination of daylight and artificial light. This has the added benefit of saving on energy costs.
Perception of the character of an interior is influenced by both the brightness and colour of visible surfaces, both interior and exterior. The general lighting conditions within an interior can be achieved by using daylight or artificial lighting, or more likely by a combination of both.
Evaluation of Lighting
General requirements
Lighting systems used in commercial interiors can be sub-divided into three major categories—general lighting, localized lighting and local lighting.
General lighting installations typically provide an approximately uniform illuminance over the whole of the working plane. Such systems are often based upon the lumen method of design, where an average illuminance is:
Average illuminance (lux) =
Localized lighting systems provide illuminance on general work areas with a simultaneous reduced level of illuminance in adjacent areas.
Local lighting systems provide illuminance for relatively small areas incorporating visual tasks. Such systems are normally complemented by a specified level of general lighting. Figure 46.17 illustrates the typical differences between the systems described.
Figure 46.17 Lighting systems
Where visual tasks are to be performed it is essential to achieve a demanded level of illuminance and to consider the circumstances that influence its quality.
The use of daylight to illuminate tasks has both merits and limitations. Windows admitting daylight into an interior provide good three-dimensional modelling, and though the spectral distribution of daylight varies throughout the day, its colour rendering is generally considered to be excellent.
However, a constant illuminance on a task cannot be provided by natural daylight only, due to its wide variability, and if the task is within the same field of view as a bright sky, then disabling glare is likely to occur, thereby impairing task performance. The use of daylight for task illuminance has only partial success, and artificial lighting, over which greater control can be exercised, has a major role to play.
Since the human eye will perceive surfaces and objects only through light which is reflected from them, it follows that surface characteristics and reflectance values together with the quantity and quality of light will influence the appearance of the environment.
When considering the lighting of an interior it is essential to determine the illuminance level and to compare it with recommended levels for different tasks (see table 46.8).
Table 46.8 Typical recommended levels of maintained illuminance for different locations or visual tasks
Location/Task Typical recommended level of maintained illuminance (lux)
General offices 500
Computer workstations 500
Factory assembly areas
Rough work 300
Medium work 500
Fine work 750
Very fine work
Instrument assembly 1,000
Jewellery assembly/repairs 1,500
Hospital operating theatres 50,000
Lighting for visual tasks
The ability of the eye to discern detail—visual acuity—is significantly influenced by task size, contrast and the viewer’s visual performance. Increase in the quantity and quality of lighting will also significantly improve visual performance. The effect of
lighting on task performance is influenced by the size of the critical details of the task and upon the contrast between task and surrounding background. Figure 46.18 shows the effects of illuminance upon visual activity. When considering visual task lighting it is important to consider the ability of the eye to carry out the visual task with both speed and accuracy. This combination is known as visual performance. Figure 46.19 gives typical effects of illuminance on the visual performance of a given task.
Figure 46.18 Typical relationship between visual acuity and illuminance
Figure 46.19 Typical relationship between visual performance and illuminance
The prediction of illuminance reaching a working surface is of prime importance in lighting design. However, the human visual system responds to the distribution of luminance within the field of view. The scene within a visual field is interpreted by differentiating between surface colour, reflectance and illumination. Luminance depends upon both the illuminance on, and reflectance of, a surface. Both illuminance and luminance are objective quantities. The response to brightness, however, is subjective.
In order to produce an environment which provides visual satisfaction, comfort and performance, luminances within the field of view need to be balanced. Ideally the luminances surrounding a task should decrease gradually, thereby avoiding harsh contrasts. Suggested variation in luminance across a task is shown in figure 46.20 .
The lumen method of lighting design leads to an average horizontal plane illuminance on the working plane, and it is possible to use the method to establish average illuminance values on the walls and ceilings within an interior. It is possible to convert average illuminance values into average luminance values from details of the mean reflectance value of the room surfaces.
The equation relating luminance and illuminance is:
Figure 46.21 shows a typical office with relative illuminance values (from an overhead general lighting system) on the main room surfaces together with suggested reflectances. The human eye tends to be drawn to that part of the visual scene which is brightest. It follows that higher luminance values usually occur at a visual task area. The eye acknowledges detail within a visual task by discriminating between lighter and darker parts of the task.
The variation in brightness of a visual task is determined from calculation of the luminance contrast:
The vertical lines in this equation signify that all values of luminance contrast are to be considered positive.
The contrast of a visual task will be influenced by the reflectance properties of the task itself. See figure 46.21 .
Figure 46.21 Typical relative illuminance values together with suggested reflectance values
Optical Control of Lighting
If a bare lamp is used in a luminaire, the distribution of light is unlikely to be acceptable and the system will almost certainly be uneconomical. In such situations the bare lamp is likely to be a source of glare to the room occupants, and while some light may eventually reach the working plane, the effectiveness of the installation is likely to be seriously reduced because of the glare.
It will be evident that some form of light control is required, and the methods most frequently employed are detailed below.
If a lamp is installed within an opaque enclosure with only a single aperture for the light to escape, then the light distribution will be very limited, as shown in figure 46.22 .
Figure 46.22 Lighting output control by obstruction
Reflection
This method uses reflective surfaces, which may vary from a highly matt finish to a highly specular or mirror-like finish. This method of control is more efficient than obstruction, since stray light is collected and redirected to where it is required. The principle involved is shown in figure 46.23 .
If a lamp is installed within a translucent material, the apparent size of the light source is increased with a simultaneous reduction in its brightness. Practical diffusers unfortunately absorb some of the emitted light, which consequently reduces the overall efficiency of the luminaire. Figure 46.24 illustrates the principle of diffusion.
Figure 46.24 Light output control by diffusion
Refraction
This method uses the “prism” effect, where typically a prism material of glass or plastic “bends” the rays of light and in so doing redirects the light to where it is
required. This method is extremely suitable for general interior lighting. It has the advantage of combining good glare control with an acceptable efficiency. Figure 46.25 shows how refraction assists in optical control.
Figure 46.25 Light output control by refraction
In many cases a luminaire will use a combination of the methods of optical control described.
Luminance distribution
The light output distribution from a luminaire is significant in determining the visual conditions subsequently experienced. Each of the four methods of optical control described will produce differing light output distribution properties from the luminaire.
Veiling reflections often occur in areas where VDUs are installed. The usual symptoms experienced in such situations are reduced ability to read correctly from the text on a screen due to the appearance of unwanted high-luminance images on the screen itself, typically from overhead luminaires. A situation can develop where veiling reflections also appear on paper on a desk in an interior.
If the luminaires in an interior have a strong vertically downward component of light output, then any paper on a desk beneath such a luminaire will reflect the light source into the eyes of an observer who is reading from or working on the paper. If the paper has a gloss finish, the situation is aggravated.
The solution to the problem is to arrange for the luminaires used to have a light output distribution which is predominantly at an angle to the downward vertical, so that following the basic laws of physics (angle of incidence = angle of reflection) the
reflected glare will be minimized. Figure 46.26 shows a typical example of both the problem and the cure. The light output distribution from the luminaire used to overcome the problem is referred to as a batwing distribution.
Figure 46.26 Veiling reflections
Light distribution from luminaires can also lead to direct glare, and in an attempt to overcome this problem, local lighting units should be installed outside the 45-degree “forbidden angle”, as shown in figure 46.27 .
Figure 46.27 Diagrammatic representation of the forbidden angle
Optimal Lighting Conditions for Visual Comfort and Performance
It is appropriate when investigating lighting conditions for visual comfort and performance to consider those factors affecting the ability to see detail. These can be sub-divided into two categories—characteristics of the observer and characteristics of the task.
Characteristics of the observer.
These include:
· sensitivity of the individual’s visual system to size, contrast, exposure time
· transient adaptation characteristics
· susceptibility to glare
· age
· motivational and psychological characteristics.
Characteristics of the task.
These include:
· configuration of detail
· contrast of detail/background
· background luminance
· specularity of detail.
With reference to particular tasks, the following questions need to be answered:
· Are the task details easy to see?
· Is the task likely to be undertaken for lengthy periods?
· If errors result from the performance of the task, are the consequences considered to be serious?
In order to produce optimal workplace lighting conditions it is important to consider the requirements placed upon the lighting installation. Ideally task lighting should reveal colour, size, relief and surface qualities of a task while simultaneously avoiding the creation of potentially dangerous shadows, glare and “harsh” surroundings to the task itself.
Glare.
Glare occurs when there is excessive luminance in the field of view. The effects of glare on vision can be divided into two groups, termed disability glare and discomfort glare.
Consider the example of glare from the headlights of an oncoming vehicle during darkness. The eye cannot adapt simultaneously to the headlights of the vehicle and to the much lower brightness of the road. This is an example of disability glare, since the high luminance light sources produce a disabling effect due to the scattering of light in the optic media. Disability glare is proportional to the intensity of the offending source of light.
Discomfort glare, which is more likely to occur in interiors, can be reduced or even totally eliminated by reducing the contrast between the task and its surroundings. Matt, diffusely reflecting finishes on work surfaces are to be preferred to gloss or specularly reflecting finishes, and the position of any offending light source should be outside the normal field of vision. In general, successful visual performance occurs when the task itself is brighter than its immediate surrounds, but not excessively.
The magnitude of discomfort glare is given a numerical value and compared with reference values in order to predict whether the level of discomfort glare will be acceptable. The method of calculation of glare index values used in the UK and elsewhere is considered under “Measurement”.
Measurement
Lighting surveys
One survey technique often used relies upon a grid of measuring points over the whole area under consideration. The basis of this technique is to divide the whole of the interior into a number of equal areas, each ideally square. The illuminance at the centre of each of the areas is measured at desk-top height (typically 0.85 metres above floor level), and an average value of illuminance is calculated. The accuracy of the value of average illuminance is influenced by the number of measuring points used.
A relationship exists which enables the minimum number of measuring points to be calculated from the value of room index applicable to the interior under consideration.
Here, length and width refer to the room dimensions, and mounting height is the vertical distance between the centre of the light source and the working plane.
The relationship referred to is given as:
where “x” is the value of the room index taken to the next highest whole number, except that for all values of RI equal to or greater than 3, x is taken as 4. This equation gives the minimum number of measuring points, but conditions often require more than this minimum number of points to be used.
When considering the lighting of a task area and its immediate surround, variance in illuminance or uniformity of illuminance must be considered.
Over any task area and its immediate surround, uniformity should be not less than 0.8.
In many workplaces it is unnecessary to illuminate all areas to the same level. Localized or local lighting may provide some degree of energy saving, but whichever system is used the variance in illuminance across an interior must not be excessive.
The diversity of illuminance is expressed as:
At any point in the major area of the interior, the diversity of illuminance should not exceed 5:1.
Instruments used for measuring illuminance and luminance typically have spectral responses which vary from the response of the human visual system. The responses are corrected, often by the use of filters. When filters are incorporated, the instruments are referred to as colour corrected.
Illuminance meters have a further correction applied which compensates for the direction of incident light falling upon the detector cell. Instruments which are capable of accurately measuring illuminance from varying directions of incident light are said to be cosine corrected.
Measurement of glare index
The system used frequently in the UK, with variations elsewhere, is essentially a two-stage process. The first stage establishes an uncorrected glare index value (UGI). Figure 46.28 provides an example.
Figure 46.28 Elevation and plan views of typical interior used in example
The height H is the vertical distance between the centre of the light source and the eye level of a seated observer, which is normally taken as 1.2 metres above floor level. The major dimensions of the room are then converted into multiples of H. Thus, since H = 3.0 metres, then length = 4H and width = 3H. Four separate calculations of UGI have to be made in order to determine the worst case scenario in accordance with the layouts shown in figure 46.29 .
Figure 46.29 Possible combinations of luminaire orientation and viewing direction within the interior considered in the example
Tables are produced by lighting equipment manufacturers which specify, for given values of fabric reflectance within a room, values of uncorrected glare index for each combination of values of X and Y.
The second stage of the process is to apply correction factors to the UGI values depending upon values of lamp output flux and deviation in value of height (H).
The final glare index value is then compared with the Limiting Glare Index value for specific interiors, given in references such as the CIBSE Code for Interior Lighting (1994).
REFERENCES
Chartered Institution of Building Services Engineers (CIBSE). 1993. Lighting Guide. London: CIBSE.
—. 1994. Code for Interior Lighting. London: CIBSE.
Commission Internationale de l’Eclairage (CIE). 1992. Maintenance of Indoor Electric Lighting Systems. CIE Technical Report No. 97. Austria: CIE.
International Electrotechnical Commission (IEC). 1993. International Lamp Coding System. IEC document no. 123-93. London: IEC.
Lighting Industry Federation. 1994. Lighting Industry Federation Lamp Guide. London: Lighting Industry Federation.
OTHER RELEVANT READINGS
Association française de normalisation. 1975. Couleurs d’ambiance pour les lieux de travail. Norme française enregistrée NF X 08-004. CIS document No. 76-1288. Paris: Tour Europe.
Bestratén, M, R Chavarria, A Hernandez, P Luna, C Nogareda, S Nogareda, M Oncins, and MG Solé. 1994. Ergonomía. Centro Nacional de Condiciones de Trabajo. Barcelona: Instituto Nacional de Seguridad e Higiene en el Trabajo.
Cayless, MA and AM Marsden. 1983. Lamps and Lighting. London: E Arnold.
Commission for the European Communities (CEC). 1989. Framework Directive. EC Directive No. 89/391/EEC. Brussels: CEC.
De Boer, JB and D Fischer. 1981. Interior Lighting. Antwerp: Philips Technical Library.
Department of Productivity. 1979. Artificial Light at Work. Occupational Safety and Health Working Environment, No. 6. Canberra: Australian Government Publishing Service.
—. 1980. Colour at work. Occupational Safety and Health Working Environment, No. 8. Canberra: Australian Government Publishing Service.
Gardiner, K and JM Harrington. 1995. Occupational Hygiene. Oxford: Blackwell Science.
Grandjean, E. 1988. Fitting the Task to the Man. London: Taylor & Francis.
Greene, TC and PA Bell. 1980. Additional Considerations Concerning the Effect of ‘Warm’ and ‘Cool’ Wall Colours On Energy Conservation. London: Ergonomics.
Illuminating Engineers Society of North America. 1979. American National Standard Institute. Practice of Industrial Lighting. ANSI/IES RP-7-1979. New York: Illuminating Engineers Society of North America.
—. 1981. Lighting Handbook. New York: Illuminating Engineers Society of North America.
International Labour Organization (ILO). N.d. Artificial Lighting in Factory and Office. CIS Information Sheet No. 11. Geneva: ILO.
Mandelo, P. 1994. Fundamentos De Ergonomia. Barcelona: Universidad Politécnica de Barcelona.
Moon, P. 1961. Scientific Basis of Illuminating Engineering. London: Dover Publications.
Walsh, JWT. N.d. Textbook of Illuminating Engineering. London: Pitman.
