CIE Color System
The CIE system developed by the Commission Internationale de l’Eclairage of France is the universally accepted standard for color specification and measurement.
- The CIE system is based on the appearance of color to the theoretical standard observer.
- The CIE definition followed experiments with a sample of 17 human observers. The results have since been confirmed with larger samples.
- The CIE definition of the standard observer is based on three specific wavelengths of light in the RGB regions respectively (435.8, 546.1 and 700nm, see the figure below).
- The device independent tristimulus values (X,Y,Z) are derived from the relative amounts of these characteristic wavelengths present in a color.
- The tristimulus values can be determined with a colorimeter or a spectrophotometer.
- The chromaticity coordinates (Y,x,y) are derived from the tristimulus coordinates (x = X/(X + Y + Z), y = Y/(X + Y + Z))
- The x and y chromaticity coordinates can be plotted on the two dimensional chromaticity diagram. The observed colors define the horseshoe shaped region shown in the figure. This region is similar to the color wheel in that each point represents hue and saturation values. See figure below.
- The X, Y and Z values are usually measured on a 0-100 scale. (Thus the luminance, Y, of the chromaticity scale is measured 0-100 by construction.) Higher numbers are lighter, lower darker.
- The CIE system underlies all color measuring and encoding systems. However, x,y values may be difficult to interpret as they don’t relate directly to the perceptual attributes of color.
- In addition the chromaticity diagram is not uniform, i.e. equal distances do not represent equal shifts in color appearance.
The 1976 CIELAB system - The CIELAB coordinates L* (lightness), a* (red-green) and b* (blue-yellow) can be obtained by mathematical transformation of the X, Y and Z tristimulus values. The chromaticity diagram becomes a circle in the CIELAB system. The CIELUV system is similar but has a horseshoe shaped chromaticity diagram similar to that for the standard chromaticity diagram. CIELAB is the native model for Photoshop.
The YCC color Model - The YCC model is becoming increasingly important as it is the basis for the Kodak Photo CD system.
- The YCC system consists of a luminance component, Y and two chromatic components C1 and C2.
- It is designed to encode the full range of luminance values in an original scene.
- When YCC data are saved to a file 8 bits (0-255) are assigned to Y, but only 6 bits (0-63) are assigned to C1 and C2. This leads to smaller files.
PostScript Supported Color Models - Postscript supports RGB, CMYK, HSB and CIE-based color models as well as single-component grayscales. It also supports Separation, Indexed and Pattern colors.
- Separation color space is used to define the individual colors of an object. It supports separations corresponding to the process colors and spot colors.
- Index space is used to define an image in terms of fewer than the full 24-bit palette. It allows up to 4096 colors. This leads to smaller files.
- Pattern space stores small images which can be tiled and used as fill for graphic elements.
- PostScript includes the ability to convert information among different color spaces. Level 1 uses a simple conversion from one device to another, while levels 2 and 3 use CIE X, Y, Z