Wednesday, March 18, 2009

Colour and Colour Mixing

A coloured surface appears the coloured because light is reflected from it in that colour's wavelengths only. All other wavelengths predominantly are absorbed by it. A blue object reflects blue light but absorbs most of the red, orange, yellow, green and violet. Black and white is slightly different. They are not, strictly speaking, colours. White surfaces reflect all or nearly all wavelengths and black surfaces absorb them totally. Response curves for the rods (colour sensitive photoreceptors of the human eye). The peak response values occur at 580 nm for red, 540 NM for green, and 440 NM for blue. There is no excitation of the blue cones above 550 NM .The overall response of all three cones together is at 560 NM (yellow-green).

Additive Colour Mixing
The absence of light is darkness, add light to it
Superposition (lamp overlap)
Rapid alternation (biased LED) "persistence of vision"
Small elements (TV pixels, halftones)

The basic rules of additive colour mixing.
red + green = yellow;green + blue = cyan
blue + red = magenta ; red + green + blue = white

Subtractive colour mixing
Subtractive colours are produced when white light falls on a Coloured surface and is partially reflected. The reflected light reaching the human eye produces the sensation of colour. Subtractive colour is based on the three colours Cyan, Magenta and Yellow. Varying the mixture of these primary colours produces other colours. When these three colours are mixed together at 100% they produce black. The absence of CMY pigments would result in white.

Metamerism :
When two object with different spectral graphs present identical coloured appearance in specific lighting and observation conditions, and different appearance when lighting or observation conditions are changed ,then metamerism is said to be in existence. Metameric index (D65 ~ A) = { (LD-LA)2 + (a*D65-a*A)2 +( b*D65 - b*A)2 }1/2

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