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Did you ever wonder why...
A room illuminated by a television appears blue to an outside viewer regardless of the colors that are being displayed on the screen?
Colour television pictures are produced by three different colours: red, green and blue. These combine to create a whitish light with a colour temperature close to that of daylight (5500 to 6500 kelvin).
Colour temperature is a way of describing the relative amounts of the different colours that make up the light from a particular source. It can be thought of roughly as the temperature an object would need to be heated to give off light with that mixture of wavelengths. At low temperatures, low-energy colours such as red and yellow predominate; at higher temperatures the proportions of green or blue increase.
For example, standard tungsten light bulbs have a colour temperature of 3200 K and are very yellow. Your eyes adjust to this light, making it appear whiter and more like daylight. At twilight or at night, anything with a higher colour temperature than tungsten lights, such as a television, appears distinctly blue.
The same effect is seen if you look out of a room lit by tungsten light. The twilight appears particularly blue, and yet if you were to go outside and look at the same scene, the eye would no longer compare it to the lit room and the colours would appear more neutral.
The converse is also true. Viewed from outside, the windows of houses lit by tungsten light sources look distinctly yellow. Fluorescent sources generally have a spectrum much closer to that of daylight and don't produce the same effect.
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