I've got a Dell S23340M - an LCD monitor with an LED backlight, which has built-in options to adjust the brightness & contrast. The monitor features a nifty "Energy Usage" bar that changes as you increase/decrease the brightness. When I increase the brightness, the energy bar goes up quickly but when I increase the contrast, it stays the same (no matter how high I make the contrast).
At 100% contrast, my monitor is practically a giant flashlight but the energy bar still stays the same. I'm wondering if changing contrast actually does anything to energy usage even though the monitor screen seems to be visibly brighter? (If not, why not?)