Color Specification

Printing or display processes can reproduce a wide range of different colors. In order to judge how well colors are reproduced it is necessary to be able to specify colors in terms of color models, color sample systems and color measurement.

Color Models - A color model is a systematic method of quantitatively classifying individual colors. Color models are invaluable tools for visualizing, quantifying and controlling colors. Shifts between two colors can be quantified precisely, since the color model provides a coordinate system respect to which distances can be measured.

Electronic images - Computer monitors and TV screens (CRTs) utilize sets of red, green and blue (RGB) phosphors to display a wide range of colors. Phosphors emit light when struck by an electron. The incident electron excites an electron in the phosphor molecule to a higher energy state, which then decays back to the ground state with emission of a photon.

Thus, the RGB model is used for display color. The RGB model is well tuned to the human perception system because of the red green and blue cones discussed previously. However, the actual colors perceived are characteristic of the particular display device so that care must be taken when transferring a given set of RGB values between different display devices. Even images displayed on identical monitors may look different because of different setup and calibration.

Printed Images - The process colors - cyan, magenta, yellow and black (CMYK) define a different color model. Just as different phosphors effect the perceived color of different monitors, different pigments effect the perceived colors of a given set of CMYK values. In addition, the paper and the printing mechanism effect the perceived colors. Thus, both RGB and CMYK are device dependent color models.

RGB and CMYK have different color gamuts, or ranges of reproducible colors. Monitors can display colors that cannot be printed, especially is the saturated red, green and blue regions. Also, some printed colors cannot be displayed. Other models are based on a device independent color space, where colors are specified in terms of an objective color measurement system closely related to human color perception.

Perceptual Color Models - When the human eye perceives a color, three things are observed. Its hue, i.e. the wavelengths are present. Its colorfulness, i.e. is the hue clearly visible or is it contaminated by other colors. Its brightness, i.e. how much light is reflected or transmitted. These attributes of human color perception form the basis of a number of similar color models such as HSL (Hue, Saturation and Lightness), HSB (Hue, Saturation and Brightness), HSV (Hue Saturation and Value) and LCH (Luminance, Chroma and Hue).

All of the perceptual color models are organized about a brightness axis and two axes that contain hue and colorfulness. The latter two axes form a two-dimensional chromatic plane or color wheel, where color information is defined independent of luminance.