We switch to LED or other efficient solid state source light bulbs when a bulb needs replacing.
Incandescent, fluorescent, mercury, and metal halide bulbs use more energy than LEDs. This applies to overhead lights as well as to equipment lights (e.g. microscope bulbs). In all cases LEDs (or solid state lighting) are more energy efficient and last longer.
For information about LEDs click here.
For information about solid state lighting for microscopes click here.