Naturally, all lamps are designed to produce their rated luminous output at their rated voltage.
An overvoltage on an incandescent lamp produces a brighter and whiter light because the filament temperature is increased.
Its operating life is, however, drastically reduced.
A 50% increase over its rated voltage will reduce the lamp life by 50%.
Conversely ,a supply voltage reduction will increase the operating life of a GLS lamp but it produces a duller, reddish light.
Lamps rated at 240V are often used in ship’s lightning system operating at 220V.
This under-running should more than double the lamp life.
Similar effects on light output and operating life apply to-discharge lamps but if the supply-voltage is drastically reduced (below 50%) the arc discharge ceases and will not re-strike until the voltage is raised to nearly its normal value.
A fluorescent tube will begin to flicker noticeably as the voltage is reduced below its rated-value.
The normal sinusoidal a.c. voltage wave form causes discharge lamps to extinguish at the end of every half cycle (every 10ms at 50Hz or every 6.7ms at 60Hz). Although this rapid light fluctuation is not detectable by the human eye, it can cause a stroboscopic effect where by rotating shafts in the vicinity of discharge lamps may appear stationary or rotating slowly could be a dangerous illusion to operators.