On completion of this chapter, you should be able to:
11. outline the difference between incandescent, gas-discharge, arc, and solid-state categories of lighting, giving at least one example of each.
12. describe the construction and principle of operation of a tungsten incandescent lamp.
13. describe the construction and principle of operation of a tungsten-halogen incandescent lamp – in particular, describe the halogen-regenerative cycle.
14. describe the general principle of operation of all gas-discharge lamps.
15. sketch the basic circuit diagram of a standard fluorescent lamp, and identify and describe the function of each of its major components.
16. describe the starting cycle for a standard fluorescent lamp.
17. describe methods of overcoming the potential hazards of a fluorescent lamp’s ‘stroboscopic effect’.
18. compare the advantages/disadvantages of the compact fluorescent lamp (CFL) and the tungsten (GLS) lamp it is intended to replace.
19. describe the construction and principle of operation of the
(a) high-pressure mercury-vapour lamp.
(b) low-pressure sodium-discharge (LPS), or SOX, lamp.
(c) high-pressure sodium-discharge (HPS or SON) lamp.
In this section, we will only study incandescent and gas-discharge lamps in any great detail. This is because, except for some archaic applications (e.g. older theatrical follow-spots, military searchlights, etc.), carbon-arc lamps are now essentially obsolescent, while solid-state lighting is still very much in its infancy and its future is far from clear.
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Incandescent lamps
Incandescent tungsten-filament lamps
Although the manufacture and sale of the familiar GLS (‘general lighting service’) incandescent tungsten-filament lamps has now been banned throughout Europe as well as in other parts of the world, there are still enough in service (and in reserve) for us to dwell on their construction and operation.
If asked, ‘Who invented the electric lamp?’ at a pub quiz, most people would probably answer ‘Thomas Edison’. However, this is not the correct answer, because, contrary to popular belief, Thomas Edison didn’t invent the electric lamp, although he did help perfect and market it! In fact, no one individual can be credited with ‘inventing’ the incandescent lamp; rather it is the result of the work of numerous individuals, each of whom contributed to its gradual improvement over a long period of time.
For example, in 1802, the English chemist Sir Humphry Davy (1778–1829) demonstrated that passing an electric current through a thin strip of metal would cause it to become white hot and emit visible light. This, the heating effect of an electric current, is the basic principle by which all incandescent lamps work.
Following Davy’s demonstration, many other scientists and inventors tried to develop a practical incandescent lamp. For example, in 1840, the Welsh judge and physicist, Sir William Grove, Q.C. (1811–1896), demonstrated the world’s first incandescent lamp but, unfortunately, the filament of his lamp had an unacceptably short lifespan because traces of oxygen in the air within the lamp’s glass envelope caused it to combust and eventually self-destruct.
But this problem was overcome, thanks to a German chemist working in London at that time, called Hermann Sprengel (1834–1906), who invented a vacuum pump that could reduce the amount of air in the glass envelope to around one-millionth of its original volume, thus preventing Grove’s lamp filament from burning out and, therefore, significantly extending its life.
Having overcome this problem, the research then changed to finding a more robust and long-lasting filament material that would operate at a higher temperature and, in 1860, the English chemist and physicist, Joseph Swan (1828–1914), invented an electric lamp using a treated-cotton filament. Unlike the gas lamps of the time, Swan’s electric lamps produced no flame or smoke, and they so impressed the impresario, Richard d’Oyly Carte, that he had them installed throughout his new Savoy Theatre in London, where he staged Gilbert and Sullivan’s popular operettas – making the Savoy Theatre the first public building in the world to be illuminated throughout by electricity.
Almost twenty years’ later, and without his permission, part of Swan’s research was used by the American inventor, Thomas Edison (1847–1931), to produce the first mass-produced incandescent lamp, using an improved filament made from tungsten. Tungsten proved to be an excellent choice as a lamp filament material because it has an extremely high melting point (approximately 3420°C). Swann was able to successfully sue Edison for patent infringement, thereby acquiring half of Edison’s company in compensation!
According to the Guiness Book of Records, as of 2013, the world’s oldest continuously burning incandescent lamp has been in use for 109 years, and holds pride of place in Fire Station Number 6, in the town of Livermore, northern California.
While the first practical incandescent lamps were made using a vacuum, it was later discovered that, by filling the glass envelope with an inert gas (usually an argon-nitrogen mixture), the resulting increased pressure within the glass envelope slowed down the evaporation of the filament by increasing the temperature at which this evaporation took place (equivalent to the way in which the boiling point of water
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increases with pressure), thus raising the operating temperature, and resulting brightness, of the lamps. Furthermore, most of the tungsten atoms evaporating from the filament collided with the surrounding gas atoms, and are deflected back to the filament. This combination of effects within a gas-filled lamp allows the filament to operate at a higher temperature compared with a vacuum lamp, resulting in a greater radiation of visible light. At the same time, the upper temperature of the filament is limited by heat transfer, through convection, by the gas.
Not all of the evaporated tungsten is returned to the filament; some tungsten is deposited on the inside of the glass envelope where it blackens the glass and reduces the luminous efficacy of the lamp.
A modern filament consists of around one metre of fine tungsten wire, wound to form either a coil, or a ‘coiled coil’ – i.e. a coil that is itself formed from a coil. Coiled, and especially coiled-coil, filaments are mechanically stronger, and each winding of the coil acts to heat the adjacent windings.
What is, perhaps, not widely realised is that modern incandescent lamps from reputable manufacturers typically incorporate a pair of fast-acting fuses in their filament leads, close to the cap (see Figure 18.22). These fuses are designed to operate much faster than the consumer-unit fuse or circuit breaker that are intended to protect the lighting circuit rather than individual lamps. This additional protection prevents the lamp from possibly exploding in the event of the filament failing and creating a high-temperature arc within the lamp.
Parts of a GLS incandescent lamp
Ordinary incandescent lamps are referred to as ‘GLS’, or ‘general lighting service’ lamps, and their construction is delicate and surprisingly complex, as illustrated in Figure 18.22.
Figure 18.22
To give a more pleasing appearance, and to diffuse the emitted light, a GLS lamp’s glass envelope is often ‘frosted’. It may also be internally coated to change the perceived colour of the light that it emits: enabling a shift towards the blue end of the spectrum, which is perceived as being a ‘cooler’ colour, or a shift towards the red end, which is perceived as a ‘warmer’ colour. Note that, in this sense, our perception of
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blue being ‘cooler’ and of red being ‘warmer’ is psychological, and has nothing to do with colour temperature!
By manufacturing the glass envelope in various shapes, incandescent lamps can be made to look more pleasing or to complement the design of lighting fittings – e.g. to simulate the look of a candle. A variety of examples are illustrated in Figure 18.23, together with their descriptions and codes.
Figure 18.23
GLS lamps are available with a variety of different types of base, or ‘cap’ (Figure 18.24). The most common for residential use are the ‘bayonet collar’ (B) and the ‘Edison screw’ (E) types, illustrated below. The bayonet collar is the standard cap for residential lighting in the United Kingdom, although the ‘Edison screw’ cap is required for some types of fixtures, such as the luminaires attached to ceiling fans. The Edison screw cap, however, is the standard cap for lamps manufactured in the United States and Canada.
Figure 18.24
There are, however, a great many other types of cap, of various sizes and configurations, depending on the physical size and application of the lamp and the national standards for the country in which they are used.
It is very important to ensure the correct ‘polarity’ (connection) when wiring Edison-screw lampholders. The Edison-type lampholder must be wired such that its centre terminal is connected to the switched line conductor, and the outer (female) screw must be connected to the neutral. Failure to do this might result in electrocution, as it is very easy to accidentally make contact with the outer screw terminal when unscrewing the lamp from its lamp holder.
The main reason that GLS lamps burn out is because the evaporation of tungsten from the filament is uneven, so some parts of the filament tend to become thinner than the rest. With less surface area for cooling, these spots become even hotter and evaporate more quickly – leading to fragility and/or early melting at those points.
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‘Long-life’ GLS lamps are simply incandescent lamps that are ‘under-rated’ – that is, they are intended to be operated at a lower voltage than they are designed for. For example, an incandescent lamp designed to operate at, say, 300 V, will last very much longer when operated at 230 V although, of course, its design power-rating will also have to be very much higher than the power it is expected to produce at 230 V. At the other end of the spectrum, ‘photoflood’ lamps, once popular with studio photographers, are incandescent lamps that are ‘over-rated’ – that is, they are intended to be operated at a higher voltage than they are designed for, thus producing an intensely bright light output at the expense of lifespan.
After many years of excellent service, the familiar and inexpensive incandescent GLS lamp is gradually being assigned to the pages of history as governments, worldwide, introduce bans on their manufacture and sale in the interests of energy conservation. The European Union, for example, has banned the sale of all GLS lamps since September, 2012. This ban, however, doesn’t extend to those incandescent lamps manufactured for special purposes, such as those used inside ovens or in refrigerators, for example.
Incandescent tungsten-halogen lamps
Incandescent tungsten-halogen lamps (also called ‘halogen’, ‘quartz-halogen’, or ‘quartz-iodine’ lamps) are bright, non-blackening, lamps originally introduced for use in film or slide projectors and as vehicle headlights, but which have since become widely used in domestic and commercial lighting installations as a replacement for the GLS lamp.
The term ‘halogen’ is applied to a group of elements that includes bromine and iodine. In tungsten-halogen lamps, the glass envelope contains a mixture of a halogen and inert gases at around five times the pressure of the gas in an ordinary tungsten GLS lamp.
One of the most common halogens used in the manufacture of tungsten-halogen lamps is iodine. At temperatures between 250°C and 750°C, for example, iodine vapour and tungsten chemically combine to form tungsten iodide; however, above 1250°C, tungsten iodide separates back into tungsten and iodine.
The temperature difference or gradient between the lamp’s tungsten element (around 2500°C) and its glass envelope (between 250–750°C) results in what is termed a ‘halogen regenerative cycle’ within the lamp. This four-step cycle is described in Figure 18.25, below.
Figure 18.25
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The result of this halogen-regenerative cycle is that evaporated tungsten cannot become deposited on the inside surface of the lamp which, therefore, never blackens, and the life of the filament itself is extended.
The luminous efficacy of tungsten-halogen lamps can be as high as 35 lm/W – roughly three times higher than a standard GSL lamp – which, thanks to the halogen regenerative cycle, they are able to maintain throughout their life span which, at around 2000 hours, is roughly twice that of a GSL lamp.
The lamp’s glass envelope must be capable of withstanding high operating temperatures and pressures. Consequently, the ‘glass’ is manufactured from various forms of quartz silica, which have very high melting points, and is thicker in order to withstand an internal pressure of about five times that of the atmosphere.
To maintain the high temperatures and pressures required for operation of the halogen cycle, tungsten-halogen lamp quartz-glass envelopes are significantly smaller and thicker, than those of non-halogen incandescent lamps of comparable power ratings. The lamps may be tubular (designated: ‘T’) or globe shaped (designated: ‘G’), as illustrated in Figure 18.26.
Figure 18.26
To support their halogen regenerative cycle, tubular lamps are best installed in the horizontal position, while globe-shaped lamps are best installed in the vertical position.
Replacements for the incandescent GLS lamp are now available, and consist of an outer glass envelope in a variety of shapes, including the classical ‘pear’ shape, inside of which is supported either a tubular- or globe-shaped halogen lamp.
A variation of the halogen lamp, described above, is the tungsten-halogen dichroic-reflector miniature spot lamp, as illustrated in Figure 18.27. Originally, this type of lamp was designed for use in slide projectors to provide a bright, yet relatively cool light, intended to avoid ‘popping’ (distorting) or melting the transparencies while they were being viewed.
These lamps have since become very popular in homes – particularly as reading lamps or for establishing ‘mood lighting’, and are typically available as 12-V bi-pin types and as 230-V bayonet-base types (illustrated below). Both types are available in power ratings of 20 W, 35 W, and 50 W.
Important! Even when cold, the quartz-glass envelope of a tungsten-halogen lamp should never be handled with bare fingers, because any natural oils
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deposited on the lamp may cause brittleness, which may accelerate the failure of the lamp.
Figure 18.27
The term, ‘dichroic’, describes the behaviour of the coating applied to the surface of the glass reflector, which reflects visible light while allowing invisible infrared light to pass out through its reflector – resulting in a beam of light that is ‘cooler’ than it would be if it projected both visible light and infrared.
Interestingly, the manufacture of ‘dichroic’ glass exhibiting this characteristic for decorative purposes dates back to the mid-fourth century, when it was used to produce Roman glass goblets that exhibited interesting and decorative reflection/transmission colour characteristics.
Many halogen lamps are internally fused for exactly the same reason as already explained for GLS lamps. However, due to their relatively small sizes, halogen lamps often incorporate what is termed a ‘Ballotini’ fuse. This is a trade name for a miniature fuse whose element is surrounded by tiny glass beads that melt when the fuse operates, quenching the arc and replacing the melted fuse element with a solidified insulating medium.
Gas-discharge lamps
Technically, gas-discharge lamps are also known as ‘arc lamps’, although they should not be confused with ‘carbon-arc’ lamps, which are quite different. For this reason, we will avoid using the term, ‘arc lamp’, and call them ‘gas-discharge’ lamps throughout this section.
Carbon-arc lamps were first demonstrated by Sir Humphry Davy, who actually called them ‘arch lamps’, because of the upward curve of the electric arc produced between the lamp’s electrodes. The term, ‘arch’, has since become corrupted to ‘arc’!
The operation of gas-discharge lamps rely on the phenomena of ‘fluorescence’ and ‘phosphorescence’.
XX Fluorescence describes the emission of visible light by a substance that has absorbed electromagnetic radiation. Of particular relevance to us is when the
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absorbed electromagnetic radiation is within the ‘invisible’, ultraviolet, region of the spectrum, and the emitted light is within the ‘visible’ region. XX Phosphorescence describes a process in which energy absorbed by a substance
is then released relatively slowly as visible light. A common example of a phosphorescent substance is the ‘luminous paint’ applied to the hands of a watch. But an example of more relevance to this section, is the coating used on the inside surface of a fluorescent tube.
Fluorescence and phosphorescence were the subjects of a great deal of research by a French physicist, Alexandre ‘Edmund’ Becquerel (1820–1891), who, as early as 1857, suggested manufacturing fluorescent tubes as a source of lighting.
Becquerel wasn’t the only person to investigate these phenomena. A pair of German physicists, Julius Plucker (1801–1868) and Heinrich Geissler (1814–1879), who was also a skilled glass blower, found that passing an electric current through a glass tube (now known as a ‘Geissler tube’) containing a tiny amount of gas, would produce light. Plucker and Geissler’s research led not only to the development of the gas-discharge lamp, but also to the development of the cathode-ray tube used in radar displays, computer monitors, and television sets before the advent of flat-screen displays.
A gas-discharge lamp, then, doesn’t produce light by means of an incandescent filament but, instead, by the excitation of a gas and/or metallic vapour contained within its glass envelope.
These lamps typically use an inert gas (such as argon, neon, krypton, or xenon) or a mixture of these gases, usually combined with additional metallic elements, such as mercury or sodium (in which case, we sometimes refer to these lamps as metal-vapour discharge lamps).
An inert gas is one that does not react, chemically, with other substances.
Gas-discharge lamps offer higher levels of luminous efficacy and a longer life, compared with incandescent lamps. However, they are much more complicated to manufacture, and they all require external circuits in order to start and to maintain the correct levels of current-flow during their operation.
In operation, when an electric field is applied to the gas, it becomes ionised – i.e. the gas atoms release electrons, which are accelerated by the field and collide with other gas atoms. This activity raises the energy-level of the gas atoms, causing their electrons to move to a higher orbit. Then, as each of these electrons returns to its original orbit, a photon is released – resulting in either visible or ultraviolet radiation.
In the case of metal-vapour discharge lamps, the heat of the gas discharge vaporises the metal and the discharge is then maintained almost exclusively by the metal vapour.
As already explained, gas- and metal-vapour discharge lamps do not emit visible light across the entire colour spectrum. Instead, they produce light of specific wavelengths and, therefore, specific colours according to their unique ‘bright-line emission spectra’. For example, neon emits a predominantly orange/red colour, sodium a bright yellow, and mercury a light blue/intense ultraviolet.
Fluorescent lamps
The research of Becquerel, Plucker and Geissler led an American engineer, Peter Hewitt (1861–1921), to develop, in 1901, a prototype of the modern fluorescent tube. Hewitt’s prototype led the General Electric Company in Britain to produce the first practical fluorescent tube in 1934, although it was not until
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four years’ later that General Electric in the United States became the first company to manufacture and market these lamps commercially. Since then, the fluorescent tube has become the most important source of lighting, accounting for around 80% of all artificial lighting!
A fluorescent lamp is a low-pressure gas-discharge lamp that contains a mixture of mercury vapour mixed with argon, krypton, neon, or xenon gas which, when excited, produces a light-blue visible light together with intense, but invisible, ultraviolet. This process we call ‘fluorescence’, hence the name ‘fluorescent’ lamp.
The interior surface of which is coated with a powder containing phosphors which, when excited by the emitted ultraviolet light, then emit visible light through a process we call ‘phosphorescence’.
Modern fluorescent tubes use what are known as ‘triphosphors’, which produce red, blue, and green light (corresponding to the red, blue, and green photo-sensors in the human eye) which, when blended or mixed together in appropriate proportions, results in a white light of any desired hue.
The operation of a fluorescent lamp depends on an electric current flowing through the gas within the glass tube. In their normal state, the gas atoms are electrically neutral, and there are not enough free electrons available to support conduction. So, when a fluorescent lamp is switched on, it is first necessary to introduce sufficient free electrons into the gas to enable it to conduct. The way in which this is done depends upon the type of lamp of which there are two main types: the ‘switch-start’ type or the ‘quick- (or instant-) start’ type.
Switch-start fluorescent lamp
The ‘switch-start’ type of fluorescent lamp has an external ‘bypass’ or ‘starting circuit’ as shown in Figure 18.29.
Located within each end of the fluorescent tube itself is a pair of electrodes – these are actually small heating filaments (Figure 18.28) which, as we shall see, function both as heaters and as electrodes. When the lamp is first switched on, the path of least resistance is through the ‘starting circuit’, comprising: a choke or ballast (an inductor), two combination electrode/filaments, and a starter switch (sometimes known as a ‘glow starter’).
Figure 18.28
The starter switch (Figure 18.33) consists of a pair of normally open bimetallic contacts, inside a sealed glass envelope containing neon gas. When the fluorescent lamp is first switched on, the resulting voltage-drop across the starter’s bimetallic contacts is sufficient to excite and ionise the neon gas, causing its temperature to increase, resulting in the bimetallic contacts bending towards each other and closing the bypass circuit (Figure 18.29).
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With the starter’s bimetallic contacts closed (Figure 18.30), current is able to flow through the lamp’s two filaments and around the starting circuit:
Figure 18.29
Figure 18.30
This current causes the lamp’s filaments to heat up and emit electrons through a process termed ‘thermionic emission’ (equivalent, if you like, to steam leaving the surface of boiling water), and providing just sufficient charge carriers to establish a current path through the tube (Figure 18.31), thereby exciting the gas.
Figure 18.31
The low-resistance current path now offered by the ionised gas short-circuit’s or bypasses the starter switch, causing the bimetallic contacts to reopen. This stops current flow through the filaments, which then cease to act as heaters and, instead, behave as electrodes between which the lamp’s discharge current flows.
When the gas ionises, its resistance flow falls significantly (in fact, it exhibits a negative temperature coefficient of resistance), so some means must be used to limit the corresponding increase in current. This is the job of the ‘choke’ or ‘ballast’ which is in series with the tube.
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‘Choke’ and ‘ballast’ are both terms used to describe an iron-cored inductor when it is used to limit current. It is the inductive reactance (symbol: XL) of this choke that acts to limit the value of current flowing through the lamp. Because a choke has very little resistance in comparison with its inductive reactance, very little energy is lost in the choke.
Additional components
The need for a choke to limit the current through the lamp means that the fluorescent lamp would have an unacceptably low (lagging) power factor, unless something is done to improve it. This is achieved by placing a capacitor (C1) in parallel with the choke – raising the power factor above 0.85.
Another capacitor (C2) is connected across the starter’s bimetallic contacts in order to prevent any arcing from damaging those contacts and to suppress any radio interference.
These additional components are illustrated in Figure 18.32.
Figure 18.32
What’s inside a fluorescent lamp starter switch?
Figure 18.33
The starter (sometimes called a ‘glow starter’) is a set of normally open bimetallic contacts, enclosed within a glass envelope containing neon gas. When a voltage is applied across the contacts, the neon gas ionises, and the resulting increase in its temperature causes the bimetallic contacts to close. The function of the small capacitor, which is connected in parallel with the contacts, is to minimise any arcing
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between those contacts as they reopen and to suppress any radio interference that would otherwise result.
A faulty starter will likely result in either of two extremes: either the lamp won’t start at all (although, in some cases, its ends may glow), or it will start but then begin flashing (usually accompanied by a ‘pinging’ sound from the starter) as the starter continuously attempts, but fails, to start the lamp.
Figure 18.34
When the supply is first switched on, the supply-voltage is applied across the length of the tube, creating a potential gradient between each electrode and the earthed strip. At the same time, the autotransformer’s secondary windings (S and S‘) supply current to each of the electrode filaments, causing their temperatures to rise (Figure 18.35, left).
Quick-start fluorescent lamp
The ‘quick-start’, or ‘instant-start’, type of fluorescent lamp removes the need for a starter switch (which, as explained above, is often the source of problems with fluorescent lamps), and increases its speed of starting.
The schematic diagram, shown in Figure 18.34, shows an example of a quick-start circuit. It consists of an autotransformer whose primary winding (P) is common to both the supply voltage and the electrodes located at each end of the tube. Its secondary windings (S and S‘) are connected to the electrode filaments. A narrow, earthed, metallic strip is attached along the length of the tube.
Figure 18.35
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The combination of the heating-effect of the electrode filaments (left-hand diagram, above) and the potential gradient between each electrode and the earth strip (right-hand diagram, above) is sufficient to ionise the mercury vapour in the vicinity of the electrodes. This ionisation then rapidly spreads along the complete length of the tube (Figure 18.35, right).
Figure 18.36
Immediately the lamp is fully illuminated, the large current drawn by the lamp increases the voltage-drop across the choke, reducing the voltage across the autotransformer primary and the lamp itself to around half the supply-voltage – thus limiting the amount of current flowing through the tube (Figure 18.36).
Additional components
Figure 18.37
The combination of autotransformer and choke causes the basic circuit to have an unacceptably low (lagging) power factor. This is improved by connecting a capacitor (C2) across the supply. In order to suppress any radio interference, another capacitor (C1) is connected across the lamp electrodes. These additional components are shown in Figure 18.37.
There are variations of this circuit – e.g. in some cases, the autotransformer itself also functions as the choke. In practice, the choke and autotransformer are not separate, but incorporated into one sealed package.
Not all quick-start fluorescent fittings require a metallised strip running along the length of the tube itself. Instead, the close proximity between a standard fluorescent
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tube and the earthed metal reflector will achieve exactly the same result – such fittings may have a label, stating: ‘Mount tube within 15 mm of earthed metal reflector’.
Nature of the light emitted by fluorescent tubes
The greenish-white colour emitted by some fluorescent lamps is generally considered to be harsh and unpleasant, with individuals appearing to have a unhealthy skin tone under their lighting.
Fluorescent lamps, therefore, can be manufactured to a more appropriate colour temperature by altering the mixture of phosphors that coat the inside of the tube.
A phosphor emits light within a narrow range of wavelengths, unlike an incandescent filament, which emits the full spectrum (although not all colours equally) of visible light. A mix of phosphors, however, can give a good approximation of that from an incandescent light.
As already explained, modern fluorescent lamps use ‘triphosphors’, which produce red, blue, and green light that corresponds to the red, blue, and green photo-sensors in the human eye and which, when blended together in appropriate proportions, result in a white light of various hues.
Table 18.4
Type of fluorescent lamp Colour temperature
‘warm-white’ 2700 K
‘neutral-white’ 3000 K – 3500 K
‘cool-white’ 4100 K
‘daylight’ 5000 K – 6500 K
Again, it’s important to understand that the terms, ‘cool’ and ‘warm’, in this context, refer to our psychological perception of those colours, which is the reverse of their actual colour temperatures.
Unfortunately, the efficiency of phosphors falls over time and, by around 25 000 h of operation, a fluorescent lamp may be only half as bright as a new one – although this may only be obvious if compared alongside the new lamp!
Compact fluorescent lamps (CFLs)
As already explained, the European Union has banned the manufacture and sale of incandescent GLS lamps, and is gradually removing other, what it describes as, ‘wasteful’ lamps from the European market. Similar legislation has been introduced in both the United States and Canada, as well as in various other countries worldwide such as Australia and New Zealand.
There seems no doubt that CFL lamps are highly unpopular with consumers but, despite this, the EU argues that, by changing to compact fluorescent lamps (CFLs), ‘families and companies can “reduce their negative contribution to climate change” and save money at the same time, because the initial higher cost of CFLs is quickly recouped as they consume around just one-quarter of the energy used by incandescent GSL lamps, while lasting up to ten times longer’.
Despite these words, anecdotal evidence suggests that the EU’s figure regarding the lifespan of CFL lamps seems significantly optimistic, with many of the ‘unbranded’ CFLs sourced from factories in the Far East barely exceeding or, indeed, matching that of incandescent lamps! This seems to have been confirmed by research data published during 2014 by the consumer magazine, Which?
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A compact fluorescent lamp, then, is a type of fluorescent lamp that has been designed as an energy-efficient, albeit more expensive, alternative to the standard incandescent GLS lamp. They are manufactured in a variety of shapes (either to maximise the light output, or to make them look more attractive) and fitted with a bayonet collar or an Edison-screw cap (Figure 18.38). Although CFLs can fit some of the light fixtures originally designed to accept incandescent lamps, in many cases, the size of the base (which contains the starting circuit, called an ‘electronic ballast’) makes it impossible to fit in many existing light fixtures which must, therefore be replaced – thus adding to the overall cost of adopting CFLs!
As already mentioned, the starting circuit for CFLs is termed an ‘electronic ballast’. A detailed description of operation of this type of ballast is beyond the scope of this book, but it uses electronic components to change the 50/60-Hz supply input to a high-frequency output, of 20 kHz or more, to supply the lamp itself. This essentially refreshes the lamp’s phosphors so quickly that it practically eliminates any flicker or resulting stroboscopic effects (described later in this section).
Figure 18.38
CFLs are relatively fragile and potentially hazardous if broken, so, to avoid breakage, a CFL should always be handled by its base when it is inserted into or removed from a lamp holder, and never by its glass envelope.
Types of CFL
There are two types of CFL: the integrated ballast type and the non-integrated ballast type.
Integrated CFLs combine a miniature fluorescent tube, electronic ballast, and Edison or bayonet base into the one unit, as shown in Figure 18.39, and easily interchanged with standard tungsten-filament GLS lamps. These are the types sold for domestic use.
Non-integrated CFLs (Figure 18.40) do not incorporate an electronic ballast which, instead, is built into a special fixture into which the CFL tube is then inserted. These are not, therefore, interchangeable with tungsten-filament lamps, and their fixtures are relatively expensive and tend to be used only in commercial buildings.
Drawbacks of using CFLs
The two most commonly encountered problems when attempting to change to CFLs are the size of the lamp, and the warm-up period with the very low initial light levels that some lamps exhibit.
Figure 18.39
Figure 18.40
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Because of the size of its base (which accommodates the electronic ballast), a CFL is much bulkier than its tungsten-filament equivalent. As explained, the enclosures of many existing lighting fixtures (‘luminaires’) may not be large enough to accommodate a CFL and may need to be replaced.
Although it varies from brand to brand, many current CFLs have a warm-up period of up to five minutes, during which they may produce, in some cases, less than 20% of their full brightness. This problem occurs each time they are switched on! This may not be a problem in a commercial environment, where they tend to remain on throughout the working day, but this is appears to be one of its major drawbacks to a widespread acceptance for domestic use. However, with advances in CFL technology this problem seems to be diminishing, and there already exist dedicated CFLs that warm up almost as fast as incandescent lamps.
Frequent switching has an adverse affect on the life of most CFLs and, so, they should not be installed where frequent switching (defined, according to the literature, as ‘more than three times per day’!) is likely – such as in situations where they might be controlled by motion detectors.
Where CFLs cannot be used
There are some applications for which CFLs cannot be used. For example, the special-duty tungsten-filament lamps used in ovens and refrigerators are designed to function at extreme operating temperatures. Compact fluorescent lamps contain plastic components and electronic devices that simply cannot withstand the heat of an oven, and fluorescent lamps do not work well at low temperatures.
Importantly, the vast majority of compact fluorescent lamps for the domestic market are not designed to work with dimmers. This is not merely a recommendation; CFLs must not under any circumstances be connected to circuits controlled by dimmers because they present a potential fire hazard.
CFLs that cannot be dimmed should be labelled with the pictogram: white on a blue background, as illustrated in Figure 18.41.
Dimmable CFLs are available, but they must only ever be used with a dimmer specifically designed to work with these particular lamps, and these do not present any potential hazard.
Regular dimmer switches are designed to work with incandescent lamps and work by rapidly turning the lamp on and off at different points during each half-cycle of the a.c. waveform. Because CFL ballasts use completely different technology, when they are supplied from a dimmer (even when that dimmer is set to ‘maximum brightness’), they can draw up to five times as much current than they are designed to carry and, in some cases, this has led to CFLs overheating and igniting.
CFLs (and most fluorescent lamps) cannot be used for emergency lighting systems. This is because emergency lighting systems usually switch to battery power in an emergency. Fluorescent fixtures with magnetic ballasts won’t work at all if d.c. is used, and electronic ballasts may malfunction if d.c. power is used.
Finally, CFLs should not be used in areas of high humidity, such as bathrooms, as humidity may damage the components in their electronic ballast.
Disposal
One of the biggest problems with CFLs (and fluorescent tubes in general) is their content, as they can contain up to around 5 mg of mercury. Mercury is highly toxic,
Figure 18.41
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and absorbing or inhaling it may cause brain damage in adults, children, unborn children, and pets. Ironically, while banning incandescent lamps in favour of CFLs that contain mercury, the European Union has also banned the sale of mercury for use in thermometers, barometers, etc!
Rather disturbingly, the US Environmental Protection Agency, for example, publishes a four-page leaflet describing their recommended CFL clean-up procedure. It advises that cleaning up a broken CFL lamp should be done with care: keeping children and pets away from the area, opening all windows for at least five minutes, and not using a vacuum cleaner (which will disperse mercury vapour into the air!). Instead, the cleanup should be done using cardboard and sticky tape, and the broken parts then sealed in a glass jar. Finally, it’s important to thoroughly wash our hands after disposing of the materials.
Furthermore, the glass jar and other clean-up materials must not be disposed of with our household waste, but at waste recycling centres which local councils are obliged to set up for this purpose.
Thankfully, perhaps, the CFL lamp is already approaching the end of its short history, as it seems highly probable that sales of these very unpopular lamps will shortly be outstripped by that of LED (light-emitting diode) lamps. It seems likely that the increasingly popular LED lighting will eventually consign CFLs to the dustbin of lighting history.
Stroboscopic effect of fluorescent tubes
Imagine that we are looking at a rotating machine, such as a lathe’s chuck, that is rotating at, say, 100 rev/s – or at exact multiples of this speed. Now imagine that the machine is illuminated by means of a series of short flashes occurring at 100 flashes per second. Each flash will, therefore, illuminate the machine at exactly the same position in its rotational cycle. The result is that, if the fluorescent lamp is the main source of illumination, then the machine will appear to be stationary although it is actually spinning!
If the flashes occurred at, say, a little more than 100 flashes per second, then they would illuminate the machine a little earlier in its rotational cycle, and it will appear to be moving slowly backwards. If the flashes occurred at a little less than 100 flashes per second, then the machine would appear to be moving slowly forward.
This phenomenon is called the ‘stroboscopic effect’ and, if it can ‘fool’ someone into thinking that a machine is stationary or rotating slowly when it is, in fact, rotating at full speed, then this could prove to be very dangerous indeed!
The stroboscopic effect, described above, only applies to conventional fluorescent lamps, as the electronic ballast used in CFLs drives the lamp at around 25 kHz, unlike conventional fluorescent lamps that operate at supply frequency.
A fluorescent lamp is a source of ‘flashing light’, because it switches off each time its a.c. supply voltage falls below its ‘striking voltage’ (i.e. the voltage that causes the gas to ionise) – flashing at twice the frequency of the mains supply (i.e. one-hundred times per second, in the case of a 50-Hz supply) – as shown in Figure 18.42. Of course, we can’t actually see this effect, because of our ‘persistence of vision’ (which is why we see a moving picture, in the cinema or on television, when what we are really being presented with are 24 individual pictures, or ‘frames’ per second for films, or 25 frames per second for television).
In practice, however, the phosphors used in the coatings of modern fluorescent lamps exhibit an ‘after glow’ or ‘memory’ – i.e. they glow for a short period after the ultraviolet discharge has stopped (this effect is more pronounced with ‘warmer’
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coloured fluorescents). Because of this, a stroboscopic effect with modern fluorescents is unlikely.
Nevertheless, however unlikely, there is a potential danger that this stroboscopic effect may occur whenever machines operate under the illumination of fluorescent lighting, which could result in injury to the operator. Accordingly, steps must be taken to remove this risk.
For single-phase supplies
There are a number of ways of preventing this stroboscopic effect.
One method is to use at least two adjacent fluorescent lamps, and arrange for their pulses of light to overlap each other – essentially providing a continuous (in practice, a ripple) light output.
In order to do this, it is necessary to shift the voltage applied to one of the two lamps by plus or minus one-quarter of a cycle. This can be achieved by placing a large capacitor is series with the supply to one of the lamps – as shown in Figure 18.43:
Figure 18.42
Figure 18.43
The capacitor provides sufficient phase-shift to cause the light pulses from one lamp to overlap the other’s, and provide a more-or-less continuous light output, as shown in Figure 18.44.
For three-phase supplies
For buildings with a three-phase supply, an alternative method to avoid the stroboscopic effect is to connect adjacent fluorescent lamps to alternate phases – as illustrated in Figure 18.45.
Figure 18.44
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With the supply voltage to each fluorescent lamp displaced by 120°, the resulting flashes of light will completely overlap each other – providing continuous illumination as shown in Figure 18.46.
Figure 18.45
Figure 18.46
Flicker effect of fluorescent lamps
‘Flicker’ is a different problem from the stroboscopic effect, described above, and many people claim to be affected by it. People suffering from migraines, for example, have complained about fluorescent lights, and it is thought that it is the flicker from
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these lamps that causes the problem (defective fluorescent lamps may even trigger an epileptic attack). This effect can be particularly severe in the case of a windowless room, such as a conference hall, that is illuminated only by fluorescent lamps – worse, if one of the fluorescents is faulty, causing it to continually flash!
As already explained, the 100-Hz (twice mains-frequency) flickering of a fluorescent cannot be detected by the human eye, thanks to what is called persistence of vision. Persistence of vision describes how the human eye perceives a series of still images presented at around 16 images per second or higher, as motion – it’s thanks to persistence of vision that we are able to see movies and television!
Some people, however, appear to be affected by flicker at 50 Hz. This can happen during the final few hours of tube life, when the emission coating on one of the electrodes runs out before that on the other electrode. This results in a slight rectifying effect that produces uneven light-output during each half-cycle – causing it to operate bright/dim over each full cycle.
High-pressure mercury-vapour discharge lamp
A high-pressure mercury-vapour lamp is a discharge lamp that uses mercury vapour to produce light. With a rated life of 7500 h, this type of lamp is widely used for street lighting, illuminating shopping malls and gymnasiums, and for area floodlighting.
As shown in Figure 18.47, this lamp consists of two glass envelopes. The inner, tube-shaped, quartz glass envelope, contains a mixture of argon and liquid mercury in which the gas discharge takes place – producing a bluish light accompanied by an intense ultraviolet radiation. The outer glass envelope, which may be clear or coated with a phosphorescent layer, provides thermal insulation and protects the user from ultraviolet radiation.
Figure 18.47
Like a fluorescent lamp, mercury-vapour lamps require a method of starting. This method consists of a third electrode (‘starting electrode’) mounted near one of the main electrodes, and connected via a resistor to the other main electrode. When the lamp is first switched on, the voltage is sufficient to ionise argon gas surrounding these two electrodes. As the lamp warms up, the liquid mercury is vaporised – allowing an arc to then strike across the two main electrodes. It takes a little over five minutes for the lamp to then achieve its final brilliance.
These lamps have very high values of efficacy – ranging from 1800 lm/W (for a 50-W lamp) to 54 000 lm/W (for a 1000-W lamp).
In order to limit the resulting current flow through the discharge, it is necessary to use an external ballast (inductor), whose inductive reactance opposes the flow of alternating current – as shown in Figure 18.48.
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All mercury-vapour lamps must incorporate some means of preventing the radiation of the high levels of ultraviolet-light that they produce. This is the function of the outer glass envelope, which is manufactured from a Pyrex-like toughened glass whose property blocks ultraviolet light. Where necessary, special care must be taken to protect this outer glass envelope from damage (e.g. through the use of wire guards), as there have been cases in sports centres where damaged lamps have been thought responsible for sunburn and eye-inflammation!
This lamp produces a bright-line spectrum, which results in a bluish light. To colour-correct the light emitted by these lamps, the inside surface of the outer glass envelope is commonly coated with fluorescent phosphors that emit red light when excited – thus extending the colour-spectrum of its light output. Without this colour-correction, people’s skin would appear unnaturally pale and ‘corpse-like’!
The production and sale of mercury lamps is scheduled to be banned in Europe from 2115.
Low-pressure sodium-discharge (LPS/SOX) lamp
A low-pressure sodium-discharge (LPS) lamp – also known as a sodium dioxide (SOX) lamp – is a discharge lamp that uses sodium vapour to produce its distinctive bright-yellow light. With a rated life up to 18 000 h, this lamp’s main application is for street and motorway lighting where correct colour rendition is not important.
The bright yellow output of these lamps has a wavelength that is close to the eye’s maximum sensitivity, giving them a good efficacy – ranging from 61 lm/W 160 lm/W, which does not decline with age.
The lamp consists of a u-shaped tube, manufactured from a sodium-resistant glass, and enclosed in an outer glass envelope, as shown in Figure 18.49. The lamp illustrated below is designed to fit a bayonet lampholder; an alternative type uses a bi-pin arrangement at opposite ends of the outer envelope (similar to that of a fluorescent tube).
Figure 18.48
Figure 18.49
The inner tube contains solid sodium together with a small amount of neon/argon gas mixture. This tube is contained in an evacuated outer glass envelope that is coated with an infrared-reflecting layer that allows the visible light wavelengths to pass
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through, while reflecting infrared (heat) back. The function of the vacuum is to retain the heat of the discharge in rather the same way as a vacuum flask.
In order to start the lamp, a voltage of around twice the mains voltage is applied across its electrodes. This causes the neon/argon gas to ionise, which then heats the sodium, causing it to vaporise and discharge. This process takes 5–10 min, during which time its colour gradually changes from a dull red, through orange, to a bright yellow.
When the lamp is switched off, the sodium solidifies and, to ensure that the sodium is then distributed throughout the whole tube, it is important that this lamp is mounted horizontally.
The external starting circuit for a low-pressure sodium-discharge lamp (Figure 18.50) requires a ‘leaky’ step-up autotransformer that will supply roughly twice the supply-voltage across the lamp’s electrodes. The term ‘leaky’ describes the fact that this autotransformer is manufactured with an air gap in its core, to deliberately increase its magnetic leakage, thus resulting in a poor voltage regulation (see the chapter: Transformer 2 -Theory). The effect of this is to make the autotransformer behave as though it has choke in series with its primary and secondary sides, which acts to limit the current flowing through the lamp.
Figure 18.50
High-pressure sodium-discharge (HPS/SON) lamp
High-pressure sodium-discharge (HPS or SON) lamps (Figure 18.51) are smaller than the low-pressure types and, in addition to sodium, contain additional metallic elements such as mercury. Like the low-pressure lamp, this lamp has an inner ceramic-glass envelope that contains the discharge, and an evacuated outer glass envelope designed to maintain the temperature of the discharge. The gas used to start the lamp is xenon, rather than the neon/argon mix. The high pressure generated in this lamp considerably widens the colour spectrum of the emitted light, producing a pleasant ‘golden-white’ light that allows correct colour rendition of illuminated objects (the ‘sun-like’ nature of the light has resulted in the manufacturers’ alternative designation: SON).
Figure 18.51
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A variation of the HPS lamp is the ‘white HPS’, which runs at a higher pressure and emulates the colour of light produced by a tungsten-filament lamp.
HPS/SON lamps are used outdoors, to illuminate factories, warehouses, car parks, and airports, or indoors to illuminate hotel lobbies and supermarkets. The efficacy of these lamps is between 100 and 150 lm/W, and their lifespan is around 4000 h.
Xenon, at a low pressure, is used as the starter gas. When the lamp is switched on, an electronic pulse-igniter (supplied by its ballast) applies around 2 kV to the xenon, causing it to discharge. This heats the sodium/mercury for a little over 5 min until it forms a vapour, and the lamp reaches its full brightness as its pressure increases.
The external starting circuit (Figure 18.52) for a high-pressure sodium-discharge lamp requires an ‘igniter’ circuit that will supply the 2-kV starting pulse, in addition to a ballast to limit the current flow through the gas. The operation of the igniter is beyond the scope of this chapter.
Figure 18.52
Switch ratings for gas-discharge lighting circuits
Due to the reactive load presented by the choke (ballast) and/or transformer used in the control circuits of some types of gas-discharge lamps, the UK’s wiring regulations require that the current-rating of any switch controlling such lamps must be twice that of the lamp’s steady load current.
The ‘steady-load current’ is derived from the total ‘steady-load’ power of an individual lamp, which is determined by multiplying its ‘nameplate’ power by a factor of 1.8 (for power factors greater than 0.85). This allows for any losses in the choke or transformer used in its control circuit.
Worked example
A gas-discharge lamp, rated at 250 W, operating at a power-factor of 0.9, is connected to a 230-V supply. Determine the rating of the switch that will control the lamp.
Solutiontotal steady-state power = (nameplate power rating loading factor
250 1.8
450 W
×= ×=
PE
load current =450230
1.96 A= =
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So, the required switch must be able to break twice this value:
�× 4A (Answermin. current - rating of switch = 2 1.96 )
High-voltage luminous-discharge display lighting
The final type of gas-discharge lighting that we will examine in this chapter is high-voltage luminous-discharge display lighting – better known (although technically incorrect) to most people as a ‘neon sign’.
This type of display lighting evolved from early experimental glass tubes, such as the Geissler tube already mentioned in this chapter, and was first demonstrated to the public at the Paris Expo of 1910 by France’s answer to Thomas Edison, inventor and businessman Georges Claude (1870–1960).
Claude’s original intention was to use these discharge lamps simply for general lighting purposes but, when a business associate recognised their commercial potential as a means of providing eye-catching and colourful advertising signage, Claude was quick to patent this application and set up an internationally successful manufacturing company, ‘Claude Neon’, producing neon signs. Unfortunately for Claude, his personal success came to an abrupt end when, in 1945, he was arrested, tried and convicted of being an active collaborator with the Nazi occupying forces in France, and imprisoned.
Despite being widely used, the term ‘neon sign’ is somewhat misleading because, although such tubes may indeed contain neon, they may alternatively contain other types of inert gas or gas mixture – depending upon the desired colour emission.
High-voltage luminous-discharge signs, then, are manufactured from long glass tubes, in a range of external diameters, typically from 8–25 mm (the narrower the tube, the brighter the light), and shaped to form letters, symbols, or patterns, using a blowtorch and other heating methods, by skilled tradesmen called glass benders or tube benders. Individual letters or symbols can be separated by blacking out short sections of tube using an opaque paint.
Once shaped, the tubes are evacuated, and filled with an inert gas at a few hundredths of atmospheric pressure. While neon and argon are the most commonly used gases, others, such as krypton, helium, and xenon (including mixtures of these gases) may also be used to achieve the desired colour discharge.
The colours emitted depend upon the type of gas, or the combination of gases, used. Neon, for example, produces a characteristic red/orange colour, but around 150 colours and shades are possible using different combinations of gas, or by using mercury vapour combined with appropriate phosphorescent coatings inside the tube rather like a fluorescent tube.
Unlike fluorescent lamps, it is unnecessary to heat the electrodes and, for this reason, this type of lighting is sometimes referred to as ‘cold cathode’ lighting. Instead, the discharge is started by applying a high ignition voltage across opposite ends of the tube. The term, ‘ignition voltage’, describes the level of voltage necessary to ignite the tube and allow current to flow; this drops by about 50% to its ‘operating’ or ‘running voltage’ once a constant current is flowing.
The required value of ignition voltage is determined by the length and diameter of the glass tube, and the type of gas filling. The longer the tube, the higher the required ignition voltage, and the thicker the tube the lower the required ignition voltage. For example:
XX An 8-mm diameter, neon-filled, tube will require an ignition voltage of 1316 V per metre length, whereas a 15-mm diameter tube using the same gas will require just 970 V per metre length.XX An 8-mm diameter, argon-filled, tube will require an ignition voltage of 738 V per
Figure 18.53
Figure 18.54
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metre length, whereas a 15-mm diameter tube using the same gas will require 460 V.
The necessary voltages (typically up to 10 kV) are obtained using a step-up transformer designed for this purpose, whose secondary terminals are applied to opposite ends of the tube.
When determining the voltage rating of a step-up transformer necessary to supply an appropriate ignition voltage, it’s also necessary to add a further 250 V for each pair of electrodes, to allow for their emission losses.
As such voltages are classified as ‘high voltages’ and are potentially very hazardous, the relevant UK regulations specify that neither of the transformer’s secondary terminals must not exceed 5 kV measured with respect to earth. So for transformers having a secondary voltage of, say, 10 kV, this is achieved using an earthed centre-tapped secondary winding – as shown in Figure 18.55:
Again because of the levels of voltage involved, the transformer, together with all auxiliary equipment, must be contained within a secure earthed metal enclosure, labelled: ‘Danger – High Voltage’. Additionally, a means of isolating the system from the point of supply must be provided together with a method of securing the isolator in its ‘open’ position to prevent it from being accidentally closed during maintenance.
The future of lighting…?
Let’s finish this chapter with a brief glimpse into the future of artificial lighting…
In fact, the future of electric lighting is far from clear but what is obvious is that it is dependent upon the emergence of new energy-efficient technologies, or upon the further development of existing energy-efficient technologies, such as light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs).
LED lighting
LED lighting has emerged as the most-likely replacement for CFL lighting in the foreseeable future. Until quite recently, LED lighting has been prohibitively expensive for residential lighting but it is now evident that costs are falling rapidly as consumers start to embrace this new form of lighting. For example, the Dutch electrical giant, Philips, reported a 38% increase in sales of LED lamps during the first quarter of 2013 compared with the previous year, despite the then high cost. At the time of writing this book (2014) an LED-replacement for the familiar 60-W GLS incandescent lamp cost has fallen below £10. While this is still significantly more expensive than a corresponding incandescent lamp, the LED lamp operates at around one-tenth of its wattage, making it significantly cheaper to run. Furthermore, LED lamps eliminate practically all of the problems associated with CFLs – in particular, their fragility and toxic contents.
Visible-spectrum light-emitting diodes (LEDs) have been around since the early 1960s, when a red LED was first developed by the General Electric Company in the United States. We are already very familiar with the LEDs that are widely used as coloured indicator lamps on our camcorders, audio equipment, etc. But in recent years, there has been a rapid development in ‘bright white’ LEDs, originally designed in the early 1970s for use with optical-fibre data transmission but, now, widely used in the manufacture of inexpensive torches (flashlights) and lanterns, and even as car sidelight displays – not individually, but in ‘clusters’ to achieve the necessary light output.
High-intensity white-light LEDs use a yellow phosphorescent material to convert the monochromatic light emitted from either blue or ultraviolet LEDs to produce
Figure 18.55
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an intensely white light, using a similar process to the way in which white light is produced from fluorescent lamps.
The glass envelope of the Philips-manufactured lamp to the right in Figure 18.56, for example, is rather bizarre-looking because of its mustard-yellow colour when switched off, this being the natural colour of its phosphorescent interior coating. However, when switched on, the light emitted by its interior blue LED array causes the phosphorescent coating to emit a bright white light.
A minor drawback with LED lamps of the type illustrated in Figure 18.56 is that they are substantially heavier than a corresponding incandescent GLS lamp and could, therefore, put excessive strain on a lamp holder intended to support a GLS lamp.
OLED lamps
Organic Light-Emitting Diodes (OLEDs) are based on a property called ‘electroluminescence’, which occurs whenever an electric current passes through certain types of organic substance usually deposited on the surface of transparent polymer which form lighting panels. An ‘organic’ substance is defined as a gaseous, liquid, or chemical compound whose molecules contain carbon.
In its original sense, the term, ‘organic’, was used to describe substances found in living organisms. This is no longer the case, as many modern ‘organic’ substances are not found in living organisms.
Currently, the most common application for OLEDs is to provide flat-panel displays for television and computer monitors, mobile phones, etc., while eliminating the need for any backlighting. However, major research is now being conducted into their use as energy-efficient lighting panels.
Despite extensive research into OLED lighting panel technology by all the major lighting companies, we are probably several years away from their mass production.
