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Repeatedly subjecting the incandescent element in one of these lights to this rapid heating/cooling causes thermal stress to the element, often causimg it to become physically brittle.įluorescent lights can also be damaged by repeated on/off switching of the "switch," still due to a startup-effect, but differently than incandescents.

The filaments in incandescent lights are rapidly heated to "white hot" during the first several ac cycles of power applied to them (through ohmic heating) remain at a relatively constant (hot) temperature, reardless of the 120hz "pulsing" of ac mains power through them (due to the damping of their thermal mass), until power to the light is "turned off " then rapidly cool off again.

That statement originated (to my knowledge) in the damage that can be caused to incandescent lights (those that produce light as a result of heating some element in the light).

They also contradict me regarding longer off times. Lamptech have an article on the subject with some graphs and response times. This was due to the rotary dimmer being turned up over, say, half a second and gradually increasing the current while the element warmed up. My domestic experience was that lamps on dimmer switches lasted 'forever' relative to the other lamps in the house. The longer the 'off' periods the harder it is for the lamp. (The thermal inertia of the element is high enough that it doesn't cool down between AC mains half-cycles.)ĭuring rapid switching the filament would not cool completely before the next switch-on so it would not be stressed so much. This explains why bulbs generally popped on switch-on rather than at a random moment when burning steadily. The result is that there is a large inrush current initially but as the element heats the resistance increases and the current decreases to its nominal value.

Incandescent filament lamps resistance changes dramatically - up to ten times - as they warm up.