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Turning down the brightness will also provide significant power savings. Generally when you're using the computer, you're not staring at the desktop wallpaper, so the biggest gains are available when the system is not currently in use. If you want to reduce power consumption, configure your system to sleep the display after 10 or so minutes, as this uses the least amount of power and will allow LCD displays to turn the backlight fully off. OLED technology works without a backlight, and a black background would reduce power consumption.If it doesn't block light by default, and requires power to block light, then a white background uses less power. If it blocks light by default, and requires power to unblock the light, then a black background is better. A white or black background will use less power depending on the resting state of the LCD. LCDs with CCFL and LED backlights will have little to any power change based on if the screen is white or black because the backlight is always on (an LED backlight will use less power than a CCFL backlight, however).CRT technology works without a backlight, but uses a high-energy electron beam to make phosphors glow, which means that a black background will significantly reduce power consumption.(In order from most power-consuming to least power-consuming) There are some technologies that aim to reduce power when large sections of the screen are black (you might have heard of "localized dimming" or "dynamic contrast"), but these features are largely found in TVs where movies might actually have large swaths of black on screen. If you need to make just a few black pixels, you can't just turn off the backlight in that area, so the backlight is always on and we just block light for those few pixels)
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(This might make you ask "Why?", but remember that the backlight does not have as fine-grained control as the pixels themselves.
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When an LCD is black, it is doing full light output, and then also blocking all of that light to create the appearence of black. Most LCD devices use a backlight which is always on.
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This depends on the display technology being used.
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