Lamp Guide
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Here are brief answers to some of the more common questions asked about lamps
Q. Is a GLS lamp life affected by switching rate?
A. Life is tested with one switching per 12 hours. One switching per hour reduces life about 1%.
Q. What is the effect of switching frequency on the electrical life of linear fluorescent lamps?
A. A guide to this effect is shown in Fig 6, p.24. With electronic circuits those including preheating will extend lamp life but those with ‘cold start will not.
Q. Is there high energy consumption at switch-on of fluorescent and discharge lamps?
A. The energy taken during starting is a small fraction of the energy taken during one minute’s operation. Current taken is usually higher than during operation. There can be a high transient current where capacitors are across the supply.
Q. Should tungsten halogen lamps be dimmed?
A. Life will not usually be affected, but the extra efficacy of the halogen lamp will be lost during dimming. If lamps are rarely operated at full light output, blackening can develop earlier than usual, and could be unacceptable. Blackening can be removed by operating the lamps at full power for a short period.
Q. Will high pressure discharge lamps go out if the supply voltage drops?
A. A transient drop may put out the lamp, which must then cool before restarting. A slow decline in supply voltage (say down to 85%) can usually be tolerated. There are twin arc tube lamp types and hot restrike ignitor systems that can minimise ‘lamp out’ time. Modern electronic circuits can compensate for variations in supply voltage and supply constant lamp operating conditions.
Q. Should ‘energy limiters’ (supply voltage reducers) be used?
A. Lamp life, starting, and operation may be impaired. Lamp replacement arrangements may be annulled. In some instances these devices can render equipment unsafe. LIF strongly recommends that energy should not be saved by reducing lighting levels, but by the selection of more efficient lamps and equipment, or by engineered regulation of output.
Q. Is there a high level of UV from unenclosed tungsten, fluorescent or discharge lamps?
A. Even at 1000 lux, UV levels are lower than outside on an average day. Observe any special instructions supplied with these lamps. Lamp standards have been amended to include maximum allowable UV output of relevant lamps and UV block lamp types have been developed. Special consideration should be given to display situations where delicate materials are exposed to high lighting levels e.g. retail clothing displays or museums and art galleries.
Q. Why replace existing T12 tubes with triphosphor T8 lamps rather than the less expensive halophosphate T8 versions?
A. Although both will provide approximately 10% energy saving, the triphosphor tube will give about 12% more light and greatly improved colour rendition. Combined with excellent lumen maintenance, the service life of the triphosphor tubes can be almost twice that of the halophosphate tubes, virtually halving annual lamp replacement and maintenance costs, which will more than offset the extra cost of the triphosphor tubes.
Q. Why do T5 tubes give their maximum light output at 35°C instead of the normal 25°C for other fluorescent tubes?
A. Because the ambient temperature in the new smaller enclosed T5 luminaires is closer to 35°C. If the maximum light output was at 25°C it would be necessary to design larger luminaires and not exploit the opportunities offered by the physical reduction of T5 tubes.
Q. What are amalgam CFLs and what are their advantages?
A. The lamps use an alloy of mercury instead of pure mercury. The amalgam enables a relatively constant mercury vapour pressure over a wide temperature range and thus maximum light output. This offers flexibility in luminaire design and acceptable environmental conditions. Amalgam CFLs emit 90% or more lumens in 5°C to 65°C ambient temperatures. Mercury CFLs only emit 90% or more lumens in 20°C to 45°C ambient temperatures.
Q. Can T12 tubes be operated on high frequency ballasts?
A. It is possible to operate some T12 tubes on HF ballasts designed for T8 lamps. However this is not recommended as the T12 lamps will only operate at a reduced light output.
Q. Why do metal halide lamps shift in colour in use and can it be prevented?
A. Low supply voltage or incorrect ballast tapping can under power the lamp and produce a noticeable colour shift. Also there is a slow diffusion by some of the metals through the quartz arc tube, changing the metallic mixture and hence the colour of the lamp. This problem has been overcome by making arc tube from polycrystalline alumina which prevent diffusion losses and ensures colours stability throughout life.
Q. Do energy saving light bulbs (CFLis) fit in ordinary light fittings?
A. Energy saving light bulbs (CFLis) fit in ordinary light fittings - and if you have a dimmer switch you can buy special dimming energy saving light bulbs bulbs.
Q. How much do energy saving light bulbs cost?
A. At the moment, energy saving light bulbs are a little more expensive than tungsten filament light bulbs, typically costing around £2 - but you'll easily make back the difference on your electricity bill in about a year. Energy saving light bulbs will become even cheaper as more are produced, because manufacturers will be able to make them more efficiently.
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