CIELab Fundamentals for HRSSA
The CIE 1976 L*a*b* color space is the most widely used method for measuring and ordering object color. It is routinely employed throughout the world by those controlling the color of textiles, inks, paints, plastics, paper, printed materials, and other objects. It is sometimes referred to as the CIELAB color difference metric. The 1976 CIELAB color space is a mathematical transformation of the colorimetric system first published by the CIE in 1931.
Both the 1931 and 1976 color spaces share the same fundamental principles, that:
• Color is a sensation resulting from the combination of a light, an object, and an observer. A light source illuminates an object. An object modifies light, and reflects (or transmits) it to an observer.
• An observer senses the reflected light.
• Tristimulus values are coordinates of color sensation, computed from the CIE (light, object, and observer) data.
1976 CIE L*a*b* - Perceptual Uniformity
The 1976 CIELAB system organizes colors so that numeric differences between colors agree with visual perceptions. The CIELAB simplifies the communication of color difference information between parties.
L Star Example
A Star Example
B Star Example
Opponent Color Coordinates
The method of describing (and ordering) colors by an opponent type system has proven to be useful, and widely accepted. This approach follows the idea that somewhere between the eye and the brain, information from cone receptors in the eye gets coded to light-dark, red-green, and yellow - blue signals. The concept follows the "opponent" basis that colors cannot be red and green at the same time, or yellow and blue at the same time. However, colors can be considered as combinations of red and yellow, red and blue, green and yellow, and green and blue.
In the CIE L*a*b* uniform color space, the color coordinates are:
•L* - the lightness coordinate.
•a* - the red/green coordinate, with +a* indicating red, and -a* indicating green.
•b* - the yellow/blue coordinate, with +b* indicating yellow, and -b* indicating blue.
The L*, a*, and b* coordinate axis define the three dimensional CIE color space. Thus, if the L*, a*, and b* coordinates are known, then the color is not only described, but also located in space.