Within the normal range of supply voltage variations the light output will change by about 3.5% for a 1% change of voltage. The effect of voltage on life is much more pronounced; 5% over-voltage will roughly halve the lamp life, whereas 5% under-voltage will approximately double it. If it is found that lamps are consistently failing prematurely then the first thing to investigate is the agreement between the voltage marked on the lamp, and the actual voltage at the supply terminals. In certain cases, it might seem attractive to operate 240V lamps on a 230V circuit to secure the increased lamp life, until it is realised that in doing so the light output will fall considerably more than the corresponding reduction in electricity consumption.
It is generally unwise to operate a lamp at anything other than its design voltage, but there are some exceptions. For instance under-volting can be justified for aircraft obstruction lights at the top of a tower where the cost of replacement is very high. By contrast, over-volting is regularly practised in studio lighting, where the extra light output permits fewer projectors on simpler mounting structures, and this is a worthwhile exchange for renewing lamps before each filming session rather than after, say, a year of service.
The nomograph in Figure I17 shows the relation between supply voltage variation and the performance of gas-filled incandescent lamps. It applies only approximately to vacuum lamps and to tungsten-halogen types.
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Figure I17 - Voltage Variation Nomograph for Gas-Filled GLS Lamps