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For LCD screens, you can make a screensaver, that shows white color instead of black. LCD works in that way, that power is needed to make black color. White is the natural state of the screen, so it only uses power for backlight. And if you turn of the screen light, then it saves power completely. Try white color with a voltmeter.
With plasma and CRT screens black is the natural state of them, so showing black should save power. But those screens are eating more of it naturally.
What about LED-backlit LCD ? Aren't they smart enough to selectively turn off some of the white LEDs when displaying black ?
Ah, can't wait until we have OLED and transflective screens everywhere
I believe that most LED backlights don't actually have a lot of LEDs behind the screen (known as "full array") that can be turned on and off, but rather have LED emitters at the edges of the screen that uniformly illuminate the area behind the LCD panel.
There is a technology known as "local dimming" that works like you describe, but it's only in high end TVs at this point. I don't know if anyone is making computer monitors with local dimming.
I think you are quite wrong there.
It is a fact that the LCD screen pixel structure is only about 5% efficient at letting the light through when fully on, that means 95% of whatever the light source is either CC tube or LED is blocked and is therefore turned to black body heat. If the screen is black it would just be 99.9% instead. To save power means controlling the light source power.
There was/is a company called Unipixel that had a LCD alternative that claimed 60% efficient light cell based on a neat MEMs opto capacitive structure. They even licensed it to Samsung in 2009, not a squeek since though. When driven by side lit LEDs, its power use would be far below todays LED LCD panels which are mostly already good enough.