What is Color?

Why do we need colorspaces?

Imagine you want a delivery of T-shirts in a particular shade of turquoise, and you are supposed to give your supplier a precise description of the color to be produced over the telephone. - Do you think that the T-shirts would be the right color? Highly unlikely.

We need aids in which every color can be described and classified unambiguously. Attempts were made over many years to find a color space that was both easy to interpret and had equal intervals in all color areas. Various systems evolved from this. The following are the most important systems in colorimetry today.

At first, the CIE X, Y and Z tristimulus values were used to describe color. X related to the observer's red stimulus, Y to the green stimulus and Z to the blue stimulus. Y also relates to the perception of lightness. It is useful to display the hue and chroma of a color without considering its lightness. To do this, the tristimulus values were converted into CIE x, y and Y values and the color displayed on a CIE x,y chromaticity diagram.

The CIELAB color space is an improvement over the CIE X, Y and Z color space. Three color values L* (lightness), a* (red-green axis), b* (yellow-blue axis) or L* (lightness), C* (chroma/saturation) and h (hue/shade) are calculated from the CIE tristimulus values.

The lightness (L*) ranges between 0 = black and 100 = white.

Positive +a* values represent red hues and negative -a* values green hues.

Positive +b* values represent yellow hues and negative -a* values blue hues.

The chroma (C*) is 0 for a purely achromatic color and increases as the color becomes more brilliant.

Hue (h) is a circle from 0° yellow through 90° red, 180° blue, 270° green and back to yellow. For example, a light, brilliant orange could have the values L*= 70; C* = 56.6; h=45° or L*= 70; +a*= 40; +b*= 40.