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Pt.1: Units and Terms Electric Lighting In this new series on electric lighting, we will look at the different types of lights available and describe how they work. But let’s first examine the basic units and terms. By JULIAN EDGAR Looking around as you travel at night through city streets, you can’t help but wonder at all the different lights. Bright yellow street lights, white fluorescent tubes positioned behind glowing signs, small intensely bright lights used in shop dis­plays – they all use different technology to turn night into day. Like most tech4  Silicon Chip nology, we tend to take the presence of electric light for granted – until the power goes off or a blown lamp makes our car a one-eyed monster. But did you know that the output of a fluorescent tube decreases at lower temperatures, or that more infrared energy than visible light is emitted by the humble light bulb? That the pressure of the gas inside a light bulb changes as it gets hot­ter? That it’s not just your imagination that objects change colour under different lights? That excess lighting in offices places a large load on the airconditioning equipment? That not cleaning lights can effectively decrease their output by 25% after a few years? In this series we will answer questions like these and also examine all of the common types of electric lighting used. There’s certainly a lot more to it than initially meets the eye and that includes answering an apparently simple question – how do we describe the amount of light produced by a lamp? Luminous intensity Luminous intensity is measured in Candela (cd) in both the imperial and metric systems. The origins of the unit can be directly traced back to candles made of whale fat. In 1860, a unit of luminous intensity known as the “candle” was established. This used, as the base standard, a candle made from a specific quantity of sperm whale fat burning at a speci­fied rate. Later gas flames also used this unit, with a then-typical gas flame having a luminous intensity of 16 candles. Early incandescent lights had a luminous intensity of a similar magnitude! In 1909, the candle was redefined in terms of a group of carbon filament incandescent lamps having precise filament dimen­ sions and operating with a defined voltage. By 1937, the defini­tion included a blackbody radiator which at the temperature of solidification of platinum had a luminous intensity of 60 candles per square centimetre. In 1948, the unit was renamed the candela and in 1979 its definition was changed to involve the radiation of light of a single wavelength at a precise power. As an example of a real world use, luminous intensity is used to describe the amount of light emitted in selected direc­ tions from lamps and fittings. Fig.1 shows an example of the intensity distribution of a 150 watt PAR (“Portaflood”) bulb. Fig.1: the luminous intensity distribution of a PAR-type 150W bulb. Luminous intensity is measured in candelas. Here it can be seen that directly in line with the beam axis, the bulb has an intensity of 12,000 candelas, falling off to only 1,000 candelas at 20° to the tightly-focused beam. (Murdoch, B. Illumi­nation Engineering). Fig.2: the eye is most sensitive to light with a wavelength of 550 nanometres (yellow-green light). At wavelengths either side of this, the sensitivity falls rapidly. At 450nm (violet), the sensitivity of the eye has typically dropped by over 96%! This change in sensitivity must be taken into account when measuring luminous flux. (Murdoch, B. Illumination Engineering). Luminous flux Luminous flux is measured in lumens, which is abbreviated to lm. Just as there is an electrical power input measured in watts, there is a “light power” output measured in lumens. The reason that “light power” is not measured in watts is because the response of the eye to different colours needs to be taken into account. The part of the radiation spectrum that we can see lies between wavelengths of 380 nanometres (blue) and 780 nanometres (red). While an instrument designed to measure radiation power will read the same at all wavelengths (assuming equal power across the spectrum), the eye has varying sensitivity to differ­ ent wavelengths. A close light source producing one watt of radiation at 555nm (yellow-green light) gives a very strong sensation of light because the eye is very sensitive to this wavelength. However, at wavelengths either side of 555nm, the sensitivity of the eye rapidly decreases, as shown in Fig.2. This means that expressing the light power output in watts is not helpful – if the light power is at a wavelength that we can barely see, then even kilowatts of light power may be useless for practical illumination. Instead, to obtain a measure of the luminous flux of a light, the radiant flux (measured in watts) is weighted by the frequency response curve of the eye. This means that if the light emits a great deal of radiation at 555nm, its lumen rating will be high. Conversely, if the light radiates at a wavelength to which the eye has a low sensitivity, it will have a low lu- minous flux value even if the radiated power is quite high. The lumen is therefore a unit based on human response and cannot be defined as a purely physical quantity, as can the watt. Interestingly, individual response curves often differ from the typical curve shown in Fig.2. That means that my 5 lumens may not be quite the same as your 5 lumens! Luminous flux measurements are widely used in lighting. A typical application is in expressing luminous efficacy, a meas­urement of how much light output there is for a given electrical power input. It is expressed in lumens/watt, abbreviated to lm/W. A typical incandescent light bulb November 1997  5 Above: the reduction in illuminance that occurs at increasing distances from directly beneath a lamp can be seen in this photo. This pattern of illum­inance can be plotted on an isolux diagram such as the one shown in Fig.4. compared with the traditional white painted backing plates. Illuminance has a luminous efficacy of 8-17 lm/W, while a low pressure sodium discharge lamp (the yellow ones used for highway lighting) has a vastly better effic­acy of 100-200 lm/W. If you were paying the electrical bill (and ultimately you are), which one would you use to light a highway? Another use of luminous flux is to Location express the actual light output of a luminaire (light fittings are known as luminaires in lighting parlance.) The total light output of the luminaire divided by the light output of the lamp gives the Light Output Ratio (LOR). The LOR of a fluorescent luminaire can be increased by up to 40% by the use of high quality reflectors, Maintained Illuminance (Lux) Instrument assembly 1500 Garment manufacture - sewing 750 School classroom 500 Cinema auditorium 50 Kitchen work areas 500 Hospital ward at night Operating theatre (local lighting) 1 100,000 Toilets 100 Supermarket 750 Fig.3: the CIE recommended illuminance levels for various activi­ties. (Philips Lighting Manual). 6  Silicon Chip Illuminance is expressed in lux, abbreviated to lx and is a measurement of how many lumens there are per square metre. There are recommended values of maintained illuminance for various activities, with Fig.3 showing some International Commission on Illumination (CIE) suggestions. Because of the drop in illu­minance as lamps age and luminaires get dirty, “maintained” in this context refers to the actual illuminance obtained with regular maintenance. On a flat outside surface where there are few reflections, it is quite easy to plot lines of equal illuminance. These lines are called isolux contours and a typical isolux diagram is shown in Fig.4. Basically, it is a diagram of the “pool of light” found beneath outside street lights – the one so beloved of writers of detective fiction! Such a diagram is useful when designing the lighting system of a car park, for example. The pattern of illu­minance shown by the diagram can be clearly seen in the photo­graph of the McDonald’s car park (above), Mailbag : ctd from p.3 Fig.4: an isolux diagram shows lines of equal illuminance, as would be found beneath a single light illuminating a car park, for example. (Pritchard, D. Lighting). detection point) has already entered the “over charge” mode. This, he points out, is not desirable for long battery life. He also mentions that it is desirable to utilise some form of alternate charge and discharge, especially if one is charging at the fast charge rate; ie 1C. I must mention at this stage that I have had one of Horst Reuter’s fast chargers and have found that it has done wonders for cells and batteries which had become marginal for a variety of reasons and it is fast reaching the point where it has just about paid for itself. I would be very interested to hear your views on the points which I have raised. I do look forward to reading the many interesting articles which appear in SILICON CHIP each month. M. Fraer, New Zealand. Comment: the licensed technology used by Smart FastChargers does appear to be effective. What more can we say? TENS electrodes not easy to obtain The colour distribution of a light source can be directly exam­ined with a spectrometer, which uses a prism to split the light into its different colours. which is illuminated mainly by a single light source. Colour temperature An object at any temperature will emit radiation. At low temperatures, the wavelengths of the radiation are mostly in the infrared region and so cannot be seen. However, if the tempera­ture of the object is increased, that object (eg, a piece of steel) will start to glow (ie, it begins emitting radiation that can be seen). The temperature of the object can be measured in degrees Kelvin (K), which is its temperature in degrees Celsius plus 273.15. The radiation properties of a hypothetical so-called black body radiator mean that it will be red at 1000°K, I am writing to let you know of an experience that I have just had with your TENS kit, that you might want to pass on to your readers. I had a friend who wanted one made so I decided to purchase a kitset. The kitsets themselves are very hard to find. I had to ring around several Dick Smith Electronics stores before locating one. The kit itself is great. It is the electrodes that are the real problem. Your article states that the electrodes are available from most chemists. Unfortunately, that may not be entirely accurate. I tried over eight chemists in Sydney, none of whom had stocked them for at least six months. Only one chemist was able to provide details of where to get them. They can be bought from Masters Medical, 8 Palmer St, Parramatta, NSW 2150. Phone 02 9890 1711. They are about $15-$20 for a pair. J. Cowan, No address supplied. November 1997  7 colour temper­ atures, the perceived colour of different light sources varies relatively little. Daylight has a colour temperature of about 5500°K, while an incandescent light bulb is around 2800°K. Fluores­cent tubes are available with colour temperatures ranging from 2900-6500°K. Unlike the eye, however, camera film is very much affected by differing colour temperatures. Photos taken under 1500°K light­ ing will have a red cast, under 3000°K a yellowish cast and under 12,000°K a blue cast. Colour rendering Photos taken under different lighting clearly show the effect of varying colour temperature. This photo has a strong yellow cast and was taken under incandescent tungsten halogen lighting with a colour temperature of about 3000°K. Fig.5: (1) low pressure sodium lamp; (2) incandescent lamp; (3) high pressure mercury vapour lamp. The appearance of colours when illuminated by a lamp depends on the distribution of the wave­lengths of light emitted by the lamp. Under a sodium lamp, every­thing is yellow! (Pritchard, D. Lighting). yellow near 3000°K, white near 5000°K, blueish white near 10,000°K and pale blue near 30,000°K. This means that the colour of a light source can be specified in terms of its colour temperature. This is the temperature 8  Silicon Chip to which a blackbody radiator would have to be heated to match the colour of the light source. Electric lights have widely varying colour temperatures but because your eyes are very tolerant of differing Colour rendering refers to the appearance of an object when it is illuminated by the light source under consideration. Light sources of similar colour temperature can have completely differ­ e nt wavelength compositions and so can provide great differences in colour rendering. Fig.5 shows the spectra (mix of wavelengths) of various lamps. The low pressure sodium lamp (1) produces light at just a single wavelength and so the lamp reveals only that colour. Line 2 shows the spectrum of a incandescent lamp, which has an output that covers all wavelengths fairly evenly – although there is an emphasis on red. A high pressure mercury vapour lamp (3) has a mixture of some ‘lines’ (high outputs at specific wavelengths) mixed with a continuous background spectrum and a band of energy at the red end. Of these light sources, the incandescent lamp gives the best colour rendering, followed by the high pressure mercury lamp and then the low pressure sodium lamp. Colour rendering is measured on a colour rendering index (expressed as Ra) scale of 1-100, where 100 provides the best colour rendering. The Ra scale for a lamp is based on the illumi­ nated appearance of 14 different colour chips. These colours include saturated red, yellow, green and blue; and colours ap­proximating the (white) human skin and green foliage. The scale is based on the average colour shift that occurs when changing from the test to the reference illuminant. The colour rendering of incandescent lights is very good at 99Ra, while fluorescent lights vary from 85-90Ra. That’s all for this month. Next month, we will look at incandescent SC lamps.