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Why color LCDs need backlights

 

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Kevin Reid
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Why color LCDs need backlights

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I used to wonder why color LCD displays always have backlights (and when the light is off they're very dark). I haven't actually researched the matter, but I recently thought of a highly plausible explanation:

A LCD per se is just a device which either blocks or transmits light under electrical control. A simple monochrome LCD (as in a digital watch) just has a reflective back panel; so the opaque segments are dark and the transparent ones pass the ambient light in both directions. If there's a backlight, then the light from it goes through the LCD in one direction.

A color LCD simply adds red, green, and blue filters aligned with the LCD elements, filtering the white backlight.

So, an ambient-light color LCD will be throwing away at least 2/3 of the light hitting it, because for any given red element, the filter is throwing away all the green/blue light, and similarly for the other colors. Therefore a reflective LCD displaying pure "white" can only achieve dark gray (compared to ordinary white objects which reflect nearly all of the light hitting them). Furthermore, the ambient light is not going to be coming in perfectly perpendicular, but from all directions — so it won't necessarily arrive through the same color filter as it departs through, leading to further losses.

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