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Q & A: Black on a TV screen

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Most recent answer: 10/22/2007
Q:
on a computer or tv screen i want to know how it makes black i know white is made all the collors being on so is black just no lights on?
- James
A:
Yup, you guessed it -- black is just the absence of light. But objects that do not generate their own light are often not black enough. The plastic around my computer screen is rather a noncommittal beige. So the trick is to get your screen to absorb the light that hits it when it is switched off or when a particular pixel isn't emitting light.

This is accomplished in different ways depending on what the screen technology is. A black-and-white TV screen simply has a coating of phosphor on the inside of the tube that's dark colored, but which glows when an electron beam hits it. Color CRT TVs have a mask and a layer of phosphors of different colors on the inside of the glass. The background color between the dots or lines of phosphor (look at the TV under a magnifying glass) is colored as black as possible.

Black and white LCD displays use crossed polarizers to achieve a black result. Light of one direction of polarization is absorbed by polarizing plastic film, while the perpendicular direction is transmitted. The LCD layer provides a controllable second polarizer, which absorbs light polarized in the other direction when switched on. If all of the light is absorbed, the result looks black.

For fun, put on a pair of polarizing sunglasses and look at a black-and-white LCD display. You might see interesting colors (due to the plastics used in the displays which can rotate the plane of polarization depending on the frequency of light).

I'm not much of an expert on color LCD displays or plasma TV displays, but they too are designed to be as black as possible when not emitting light.

Tom

(published on 10/22/2007)

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