1 ISAT 413 ─ Module II: Lighting Topic 1: Vision and Color; Lighting Concepts and Fundamentals Vision and Color Human Eye (see http://www.tedmontgomery.com/the_eye/ ) CIE (Commission Internationale de l’Eclairage = International Commission on Illumination) Color (see http://www.cim.mcgill.ca/~image529/TA529/Image529_99/p rojects97/58_Al-Tikriti/image.html) Lighting Concepts and Fundamentals Lighting Levels Types of Lamps and Tubes Energy Saving (with an Example)
Transcript
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ISAT 413 ─ Module II: Lighting
Topic 1: Vision and Color; Lighting Concepts and Fundamentals
Vision and Color
Human Eye (see http://www.tedmontgomery.com/the_eye/ )
CIE (Commission Internationale de l’Eclairage = International Commission on Illumination) Color (see http://www.cim.mcgill.ca/~image529/TA529/Image529_99/projects97/58_Al-Tikriti/image.html)
Lighting Concepts and Fundamentals
Lighting Levels
Types of Lamps and Tubes
Energy Saving (with an Example)
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Human EyeImportant elements of human eye are (i) the cornea, (ii) the iris, (iii) the lens, (iv) the fovea, (v) the retina and (vi) the optic nerve.
The Retina & Cones and Rods.
• The retina is the lining inside the eyeball.
• It consists of myriad “photoreceptors” or photosensitive cells.
• There are two types of photosensitive cells:
– Cones
– Rods
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• There are three distinct types of Cones:
– Red (40)
– Green (20)
– Blue (1)
• For every 1 blue cone there are 20 green cones and 40 red cones.
• The ability to detect colour is called “photopic vision”.
• Cones are sensitive to colour, whereas rods are sensitive to light levels.
• There are 3 types of cone, and 1 type of rod.
• Cones send signals to the brain.
• There the electric signals are perceived as colour.
• We are sensitive only to the visible part of the electromagenetic spectrum.
Cones
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Rods• Rods detect movement.• Rods are very sensitive to light.• Rods are active when cones have stopped responding.• Rods are particularly important for low levels of light
intensity.• Rods enable ‘seeing in the dark’.• Rods determine the intensity of light levels rather than color.
Cones have about 7 million receptors are in sensitive to dim light. Rods have about 10 million receptors are sensitive to dim light and motion. Each has its necessary radiance to excite a threshold response as shown in the figure on the right.
Threshold Response of Rods and the Various Types of Cones
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The Fovea and the Iris
The iris is the colored part of the eye. The iris changes size. The iris dilates to let more light into the
eye. The iris contracts to keep light out of the
eye. Like a lens shutter. In sunshine we tend to screw up our
eyes: the iris contracts.
Rods and cones are not distributed evenly over the retina. The Fovea is the name for the central area of the retina around
the blind spot. The Fovea contains only rods. The rods are more tightly packed in the Fovea. This enables the eye to deal with spatial resolution. Good spatial resolution means recognizing shading and small
details.
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Human Eye (continued)
We do not “see” with our eyes but, rather, with our brains; our eyes merely assist with the process.
The eye allows us to see and interpret the shapes, colors, and dimensions of objects in the world by processing the light they reflect or emit. The eye is able to see in bright light or in dim light, but it cannot see objects when light is absent.
Light from an object (such as a tree) enters the eye first through the clear cornea and then through the pupil, the circular aperture (opening) in the iris.
Next, the light is converged by the crystalline lens to a nodal point immediately behind the lens; at that point, the image becomes inverted.
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Human Eye (continued)
The light progresses through the gelatinous vitreous humor and, ideally, back to a clear focus on the retina, the light impulses are changed into electrical signals and then sent along the optic nerve and back to the occipital lobe of the brain, which interprets these electrical signals as visual images.
If the incoming light from a far away object focuses before it gets back to the back of the eye, that eye’s refractive error is called “myopia” (nearsightedness).
If incoming light from something far away has not focused by the time it reaches the back of the eye, that eye’s refractive error is “hyperopia” (farsightedness).
In the case of “astigmatism,” one or more surfaces of the cornea or lens are not spherical but, rather, are cylindrical or toric.
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Color and Vision
Humans, apes, and Old World monkeys are unique in routinely possessing trichromatic color vision. In this form of color vision the retina of the eye has three types of cone pigment maximally sensitive to blue (437 nm), green (533 nm), and yellow-red (564 nm) wavelengths.
Color vision is trichromatic, implying sensitivity to three colors.
Three classes of cones exist, and each has a different pigment and therefore its own spectral response.
Each type of cone shows a different maximum sensitivity, but responds to all wavelengths.
The cones are commonly termed red, green, and blue receptors.
Only one type of rod exists, responsible for monochromatic night vision.
The differing excitations of these pigments are thought to produce over 2.3 million discriminable colors in the human perceptual system. Although human color vision is not nearly as good as some vertebrate animals (e.g. birds are tetrachromatic and can perceive ultraviolet light), our color perception is markedly better than other mammals, which are dichromatic and therefore cannot perceive long wavelengths of yellow, orange, or red light.
Figure above shows the Comparison of two scenes as seen by a trichromatic observer (a) and a dichromatic observer (b). Note: the dichromatic observer cannot discriminate between the red tulips and the green foliage (from Viénot et al. 1995)
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Color and Light Color is not a property of objects, but rather a human response to the different wavelengths of radiant energy incident on the retina.
The color appearance of an object changes when the spectral characteristics of the light change.
Object color, then, is due to selective absorption – that is, some wavelengths are absorbed, others are reflected, scattered, or transmitted.
Light of primary colors are added to produce the secondary colors of light (i.e. red and green combine to produce yellow). A computer monitor illustrates additive color mixing.
A pigment absorbs or subtracts a primary light color and reflects the other two. A yellow filter, for instance, transmits green and red. An ink-jet printer illustrates subtractive color mixing.
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CIE Color SystemThe system is based upon the principle that any color of light can be exactly matched by a combination of three primary light sources. The spectral colors are located along the edge of the diagram, and a straight line joins the red and violet ends.
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CIE Color System (continued)
The CIE XYZ model, as it is known, defines three primaries denoted X, Y, and Z that can be combined to match any color humans can see. This relates to the tristimulus theory of color perception, that states that the human retina has 3 kinds of cones with peak sensitivities of 580 nm (“red”), 545 nm (“green”), and 440 nm (“blue”). These primaries are combined in an additive manner. The primaries are then normalized in order to determine the chromaticity coordinates, x, y, and z.
Any physically realizable color is represented by a point on or within the closed curve, and points outside the curve are impossible colors, but can still be combined to produce real colors.
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CIE Color System (continued)
780
380
dxSX
Given the spectral distribution of a light source, S(), the spectral reflectance of an illuminated object, (), and the tristimulus distribution functions, ,z,,y,x
one can calculate the tristimulus values, X, Y, and Z, by integration of the product of the three functions over all visible wavelengths.
This calculation would be repeated twice using the appropriate tristimulus functions to determine Y and Z. The chromaticity coordinates, x, y, and z, are then determined by normalization of the tristimulus values, as shown for x below.
ZYX
Xx
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Lighting Levels
The basic SI unit of luminous intensity is the candela, cd, it is defined such that the luminance of the total radiator at the center of the temperature of the solidification of platinum (i.e., 1769oC), is 60 cd/cm2.
Light from a point source illuminates a spherical surface surrounding it and so the light emitted is defined in terms of solid angle (the unit of solid angle is the area of part of a spherical surface divided by the square of the radius of the sphere and is called the steradian, st).
Lumen, lm, as the luminous flux emitted within unit solid angle by a point source with a luminance of 1 candela.
The lux, lx, is then defined as an luminance of 1 lumen per square meter. (1 footcandle = 10.76 lux)
Lighting Concepts and Fundamentals
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Steradian
The steradian (symbolized sr) is the Standard International (SI) unit of solid angular measure. There are 4 pi, or approximately 12.5664, steradians in a complete sphere.
A steradian is defined as conical in shape, as shown in the illustration. Point P represents the center of the sphere. The solid (conical) angle q, representing one steradian, is such that the area A of the subtended portion of the sphere is equal to r2, where r is the radius of the sphere.
Based on the foregoing example, the geometry of which is independent of scale, it can be said that a solid angle of 1 sr encompasses about 1/12.5664, or 7.9577 percent, of the space surrounding a point.
Illumination follows a square law. For example, for any given reading, if the light meter is held twice as far away from the light, the meter will read only one-fourth as much; if the light meter is held half as far away from the light, the meter will read four times as much.
Typical Range (Lx) Situation
100,000 Bright sunny day
10,000 Cloudy day
1000 to 2000 Watch repairman's bench
100 to 1000 Typical office
200 to 1000 Night sports field
1 to 10 Residential street lighting
0.25 Cloudy moonlight
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Efficacy Efficacies of Today's Lighting Technologies
Lamp TypeEfficacy (lumen/Watt)
Candle 0.15
Edison's First Incandescent
1.4
Today's Incandescent 10-20
Halogen 20-35
Fluorescent 35-90
Mercury Vapor 40-60
Metal Halide 60-70
An electric light source producing a given amount of light from a given electrical power input has an efficacy defined as the luminous flux in lumens divided by the power input in watts, lm/W.
In certain machinery applications the stroboscopic effect must be avoided; this occurs to varying degrees with certain types of lighting when the mains frequency of 60 Hz is close to the rotating speed of the machinery, i.e., 3600 rev/min.
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Types of Lamps and Tubes
Incandescent
The most common type of lamp, or light bulb, is the incandescent type in which a filament of wire is heated electrically to a high temperature such that the radiation emitted is in the visible waveband.
The wire used is made of tungsten and such lamps are therefore usually known as tungsten lamps; the basic tungsten lamp is frequently referred to as GLS, General Light Standard.
Incandescent lamps are more efficient and have a longer life when filled with a halogen gas; various proportions of halogens are used but the type is generally known as tungsten halogen.
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Fluorescent
In the fluorescent tube light is produced by the excitation of gas (originally neon) or a metallic vapor, by an electric discharge; a coating of phosphorus, say, on the inside surface of the tube produces a white light.
This type of light has a much higher efficacy than the incandescent lamp and has a longer life. The connection is usually made by a two-pin fitting on each end of the tube.
Fluorescent tubes require control gear to start the discharge process and to provide the correct discharge rate. Compact lamps may have integral control gear using an electronic ballast.
High frequency (28 kHz instead of 60 Hz) fluorescent tubes have 10% higher efficacy and almost none stroboscopic effect.
Various of improvements to the design have been made in the recent years to increase the efficacy:
1. Krypton gas is replacing argon gas in the tube;
2. Tube diameter has been reduced;
3. Tri-phosphorus coatings provide more stability and better color rendering;
4. Tubes have been made in a compact form in a size very little larger than the tungsten GLS lamp and with the same type of fitting.
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Other Discharge Types of Lamps
Lamps using the principle of discharge through a gas or metallic vapor but not necessarily using the fluorescent concept are also in common use.
1. The high pressure mercury fluorescent lamp produces light partly by discharge in the vapor and partly from ultraviolet radiation striking a fluorescent coating on the outer surface of the bulb (superseded by SON).
2. The metallic halide lamp produces light by radiation from a mixture of a metallic vapor (usually mercury) and the dissociated products of halides. For floodlighting, studio.
3. Low pressure sodium (SOX) lamps have a high efficacy but given an almost monochromatic yellow light. For street lighting.
4. High pressure sodium (SON) lamps are almost as efficient as the SOX lamps, have a much better color quality. For industrial use.
All types of discharge lamp are NOT suitable for instant lighting, 6 to 12-min warming up, and 5 to 8-min on re-start are common.
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Efficacy and Life Expectancy for Various Lamps
Type Luminous flux (lm)
Power (W)
Efficacy (lm/W)
Life Expectancy (h)
Tungsten GLS 1 200 100 12 1 000
Tungsten GLS 20 000 1 000 20 2 000
Tungsten Halogen 50 000 2 000 25 4 000
Compact fluorescent 1 200 28 43 8 000
Fluorescent tube 4 500 70 64 10 000
High frequency tube 5 000 62 80 10 000
Low pressure sodium 30 000 210 143 12 000
Metal halide 160 000 2 000 80 10 000
High pressure sodium 25 000 280 90 12 000
(Note: The efficacy quoted is the luminous flux divided by the total electrical input; the efficacy is sometimes defined as the luminous flux divided by the rated electrical power of the lamp)
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Energy Saving
• The first and perhaps most obvious way is to check that each lamp is switched on only when required.
• Certain types of lamp can be dimmed to lower levels of illuminance.
• The choice of lamp and luminaire (the apparatus which fixes the lamp to the structure, protects it, and at the same time controls the distribution of light to the areas to be illuminated, is known as the luminaire) for the purpose required.
• The cost of replacement must be considered as well as the running cost and hence the probable life of the lamp is an important factor.
• Lighting costs can be reduced by proper maintenance (clean at predetermined intervals).
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Energy Saving (Example II-1.1)
A factory building of 15000 m2 floor area is lit by 500 twin fluorescent luminaires each tube having a rating of 68 W and electrical input of 80 W with an efficacy of 60 lm/W and a life expectancy of 8000 h. It is proposed to replace the tubes with 100 high pressure sodium lamps of 560 W electric input, efficacy 100 lm/W, and a life expectancy of 10000 h. The building is occupied for 12 hours per day for five days per week for a 50-week year. Taking the cost of each sodium lamp as $20, the cost of each fluorescent tube as $3, the labor cost for replacement as $5 per luminaire, the capital cost of installing the new system as $6000, and the price of electricity as $0.045/kWh, calculate:
(i) The percentage increase in luminance;
(ii)The break-even time for the new system neglecting depreciation and inflation.
