It is a long way to go from the source of light to the perception of colour!


A range of illuminants;
() a tungsten filament lamp;
(---) D65 north daylight, new CIE standard;
() C north daylight, original standard

The first step on the way to colour perception is the emission of light from a light source. This emission is different from wave-length to wave-length. The emission spectrum usually is designated S().

When an object is illuminated by light, it absorbs a certain proportion of the radiation and remits another one. The reflectance spectrum is usually called (), or R().

Colourants absorb their complementary colour
complementary colours
 [Full screen 54 KB]
(Source: Fonds der Chemischen Industrie, Germany; imageseries "Farbstoffe und Pigmente")

The product S() · () = () is called the colour stimulus function (perhaps, literal translation from German).

When the light specified by () hits the eye with sufficient intensity, it irritates light-sensitive cells, called cones. There are three types of cones, which are sensitive to red, green and blue light, respectively. Together they create the perception of a certain colour.

In addition to the cones, there is another kind of light-sensitive cells, called rods. They are more sensitive than the cones, but cannot detect colour.


Similar to the sensitivity of the cones, but not exactly the same, the CIE standard observer response curves for "red", "green" and "blue" have been defined: (), (), and ().

Using these, the CIE tristimulus values for red, green and blue can be given: X, Y and Z. The relative shares of red, green, and blue, called the chromaticity coordinates, are calculated thus:= X / (X+Y+Z), etc.

Summarizing:

Source of light × Reflectance of object = Colour stimulus
S() × () = ()
Colour stimulus × Standard observer = Tristimulus value
(after integration and normalisation)
() × () =              X
Tristimulus value ÷ Sum of tristimulus values = chromaticity coordinate
X ÷ (X + Y + Z) =

So it is possible to specify a colour with only two chromaticity coordinates, as the third one is the complement to give 1.

Y also designates the luminosity of an object.

Note: The expression "colour stimulus" may be not correct English, because it is a literal translation from German.


Colour chart
Colour chart
 [Full screen 44 KB]
(Source: Fonds der Chemischen Industrie, Germany; imageseries "Farbstoffe und Pigmente")

Colour Synthesis

There are two methods of colour synthesis: additive and subtractive.


Additive and subtractive colour synthesis


In four-colour prints, both principles occur; this is called autotypical colour synthesis.

 

When a perfect black body is heated, it will eventually begin to glow dark-red, than orange, yellow, and, finally, emit bluish white light.

As there is a relationship between colour and temperature, the expression "colour temperature" was coined. The colour temperature of a body must not be confused with its actual temperature.

In Germany it is standardized, which hue (i.e. which shares of red and green) a certain lamp must have to be legally "green", etc.

Why is that strange shape of "white" ?

Colour TV uses additive colour synthesis. A colour monitor basically is an array of tiny red, green, and blue lamps.

As the colours with x = 1, y = 1 and z = 1 do not exist, they cannot be emitted. Real colours must be situated within the so-called "sole of a shoe". Hence, colour monitors cannot synthesize all possible colours.


Production of colour television screens
TV screen
 [Full screen 91 KB]
(Source: Fonds der Chemischen Industrie, Germany; imageseries "Reprographie: Kommunikation durch Chemie")





           Copyright © 1997     gomail Feedback     Updated on: 20. January 1998