I power the incandescent lamp in an enlarger through a dimmer. Framing and focusing can be done with the lamp dimmed, and the exposure made at full power. Dimming an incandescent lamp does prolong life, but the lamp still produces nearly as much heat and is inefficient in utilizing power. Long ago, when a 16mm movie projectionist in a remote Navy installation, reducing voltage to the projector by 10% through a stepdown autoformer prolonged lamp life by a factor of several times. I once used 40 and 60 Watt lamps in series for greatly extended lamp life (years of continuous use). The 60 Watt lamp barely glowed. This is practical only where lamp life is much more important than efficiency. The system has been replaced with a LED lamp which produces much better light and consumes much less energy. The high cost of the LED is no more than a few months of the power consumed by the series lamps.