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Additive and Subtractive Colours
By Carly Pope
The human eye is able to distinguish up to 10,000,000 colours. All of these colours derive from two main types of light mixture, either additive or subtractive.
Additive colours
Additive colours occur when beams of light are combined and parts of the spectrum are mixed. It is used mainly to produce coloured images on television screens, for quick stage scenic changes in the theatre and in colour printing.
The three additive primaries are red, blue and green, and when additively mixed, in varying amounts, all other colours can be produced. When combined together, all three primaries make white light.
Adding two primary colours together makes the secondary colours, to produce:
Red and blue = magenta
Red and green = yellow
Blue and green = cyan
The process of additive colour mixing can be demonstrated using three slide projectors, each fitted with filters. One projector throws red light onto a white screen and then blue light and finally green light is added. Additive mixing occurs where the beams overlap. If the red and green beams overlap, yellow is produced. If more red light is added or the intensity of the green light is decreased, the mixture will become orange.
An additive effect can also be achieved by the use of subtractive colours. For example, if minute red and green dots are placed extremely close together, i.e. on a television screen, then the human eye, to give the impression of yellow mixes the reflected rays of red and green light.
Subtractive colours
The subtractive process produces colours that are naturally occurring. These react to the light in three different ways: -
?They can absorb some of the light rays from white light and reflect the remainder. The reflected light is the colour we see.
?They can subtract all the light rays; we perceive this as black.
?They can reflect all the light rays; we perceive this as white.
Subtraction entails the removal or absorption of spectral components and occurs when colorants (in the forms of pigments and dyes etc.) are mixed.
The subtractive primary colours are red, yellow and blue, and when mixed together, in specific proportions, all other colours can be produced. When all three primaries are combined, the colour black is produced.
The secondary colours are made by combining the primaries together, whilst reflecting and subtracting the light. This produces: -
Red and yellow = orange
Red and blue = purple
Blue and yellow = green
The subtractive process can be demonstrated when several coloured filters are inserted into a single beam of white light, e.g. a projector is fitted with a red filter, and it transmits red light and absorbs all other colours. If it is fitted with a green filter, then red is absorbed, as are all other colours and only green is transmitted. If however, the projector was fitted with a red and a green filter (yellow and blue), all colours would be absorbed and no light transmitted. This would result in the colour of black.
Similarly, a yellow pigment will absorb blue, whilst reflecting yellow, green and red light (they combine to produce yellow), whilst blue will absorb mainly red light and red will absorb mainly green light.
The additive primaries of red, green and blue, apply to coloured light, whilst the subtractive primary colours, favoured by artists, of red, yellow and blue, apply to pigments.
About the Author
Carly Pope is the webmaster of The Rosette Company http://www.therosettecompany.co.uk and Rooster Rosettes http://www.roosterrosettes.co.uk
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