If computer screens render different colours by filtering white background light through red, green and blue pixels, and black is the absence of colour, how do computer screens reproduce the colour black?

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The most intuitive answer would seem that black pixels get “switched off”, but I know that’s not true because I can tell when a monitor is switched on and off even if the screen is just black. There’s a sort of “black glow” to them.

In: Engineering

So this applies to LCD, and other screen panel types where the pixels dont produce their own light.

On an lcd the backlight is always on in some form, and the way they make “blacks” can be done in a few diffetent ways and a combination of ways.

LCDs generally have a layer behind the pixels that is a polarizing layer, that can be turned on and off to block the backlight from going through that zone of pixels. What you reference when you know your monitor is on but supposed to be black is called “backlight bleeding”

Fancier LCDs use a technique called Local dimming where the backlight is a panel of millions of white LEDs broken into a grod of zones,i beleive current high end lcds have 192+zones of dimming. This is where the tv will actually shut off areas of the backlight to produce a much deeper black, though this effect can create “halos” most noticible on white text on a black background.

I think part of what you are experiencing is called “color constancy”. Your brain is telling you that the black parts of the image are actually blacker than they are, because they are part of an image that your brain needs to make sense of in changing light conditions. There’s a related thing called “lightness constancy”. Your monitor can’t physically make things blacker than what you see when the monitor is turned off.

Not all screens work this way.

OLED displays don’t have a backlight; they use red/green/blue pixels that are individually self-illuminating. So if that pixel needs to be black, it just doesn’t light up at all, and it really does look black.

But cheaper displays do work with a backlight, and the red/green/blue pixels control how much of the backlight gets through. To display black, a pixel will reduce the light coming through it by the maximum amount possible. Since this isn’t perfect, you actually get a slightly glowing dark gray color instead of black – but you perceive it as black because you’re comparing it to the other colors on the screen. Fancier displays have what’s called “local dimming” where sections of the backlight can be made dimmer or turned off, to make the black areas closer to black.