Colour: Physics and Light

Visible light consists of a spectral distribution of electromagnetic energy having wavelengths in the range of 380-760 nm. Waves of light are measured in wavelengths, the length determining its color. The perceived colour of visible light is to some extent a subjective experience.

             

Spectral Response:

The retina has both rods and cones. The cones are responsible for colour perception. There are three types of cones, referred to either as B, G, and R, and their peak sensitivities are located at approximately 430nm, 560nm, and 610nm for the "average" observer. The photopigments in rods and cones are stimulated by absorbed light, yielding a change in the cell membrane potential. The different types of cells have different spectral sensitivies:

             

Metamers are colours that are perceptually identical when viewed even though the spectra is different for the two colours. The perception of colour is only related to the stimulation of three different types of cones. Thus,if two different spectra stimulate the three cones the same way, the colours produced will not be distinguishable for the eye.

Experiments have been carried out to determine the amount of R, G, B values needed for a given wavelength. On one side of the display a colour of a set wavelength is shown. Then on the other side the subject must mix three other pure wavelenths (such as R=700nm, G=546nm, and B=436nm) until the two sides of the display look identical.

             

As you can see in the diagram above red has a negative value for part of the graph. The CIE ("Commission Internationale d'Eclairage") defined three new hypothetical light sources x, y, and z to replace (red, green, and blue) which each yield positive matching curves.

             

To convert a given spectrum to the corresponding X, Y, and Z quanties, you integrate the product of the spectral power and each of the three matching curves over all wavelengths. The weights X, Y, and Z form a three-dimensional CIE XYZ space.

CIE in 2DColour Space:

To convert the 3D colour space to 2D colour space, project the 3D colour space onto the plane X + Y + X = 1, yielding a CIE chromaticity diagram (shown below). The projection is given in the following equations:

             

2D CIE Chromaticity Diagram

             

Colour Gamuts:

Using the chromaticity diagram above the different "gamuts" of various output devices: monitors, and printers can be compared.


             

RGB Colour Cube:

The RGB colour cube is used to represent the additive colour model. R, G, and B represent the colours produced by red, green, and blue phosphours.

             

The RGB colour cube fits into the CIE XYZ model, as shown below.