Legacy Document

Inside Macintosh: Advanced Color Imaging on the Mac OS /

Chapter 3 - Introduction to the ColorSync Manager / Introduction to Color and Color Management Systems

Color: A Brief Overview

Color is created through the interaction of a light, an object, and the eye. There must be a light to illuminate the object. White light contains many different colors of light. This can be seen by observing sunlight broken into its components when passed through a prism. The resulting rainbow represents the "visible spectrum" consisting of the colors visible to the eye. Each color of light has a particular wavelength. An object appears to have a certain color because its pigments absorb some wavelengths of the light that illuminates it while reflecting others back to the eye.

Color Perception

The eye contains three types of cone receptors. Each receptor is sensitive to about one-third of the visible spectrum. These are blue light, green light, and red light. The color the eyes see in an object depends on how much red, green, and blue light is reflected to the eye. Black is perceived when no light is reflected to the eye. When red, green, and blue lights are reflected to the eye in equal amounts, then white is perceived.

Even the conditions in which color is viewed greatly affect the perception of color. The light source and environment must be standardized for accurate viewing. When viewing colors, people in the graphic arts industry, for example, avoid fluorescent and tungsten lighting, use a particular illuminant, and proof against a neutral gray surface.

Color images frequently contain hundreds of distinctly different colors. To reproduce such images on a color peripheral device is impractical. However, a very broad range of colors can be visually matched by a mixture of three "primary" lights. This allows colors to be reproduced on a display by a mixture of red, green, and blue lights or on a printer by a mixture of cyan, magenta, and yellow inks or pigments. Cyan absorbs red, magenta absorbs green, and yellow absorbs blue. Black is printed to increase contrast and make up for the deficiency of the inks.

The use of only three colors to reproduce thousands of colors is possible because the eyes are basically responsive to these three broad sections of the spectrum. The three color values constitute the specification for the matching of properties of a color.

Hue, Saturation, and Brightness

Color is described as having three dimensions. These dimensions are hue, saturation, and brightness. Hue is the name of the color, which places the color in its correct position in the spectrum. For example, if a color is described as blue, it is distinguished from yellow, red, green, or other colors. Saturation refers to the degree of hue in a color, or a color's strength. A neutral gray is considered to have zero saturation. A saturated red would have a color similar to apple red. Brightness describes differences in the intensity of light reflected from or transmitted by a color image. The hue of an object may be blue, but the terms dark and light distinguish the brightness of one object from another.

Additive and Subtractive Color

The additive color theory refers to the process of mixing red, green, and blue lights, which are each approximately one-third of the visible spectrum. Additive color theory explains how red, green, and blue light can be added to make white light. Red and green projected together produce yellow, red and blue produce magenta, and blue and green produce cyan. With transmitted light, all the colors of the rainbow can be matched.

The subtractive color theory refers to the process of combining subtractive colorants such as inks or dyes. In this theory colorants of cyan, magenta, and yellow are used to subtract a portion of the white light that is illuminating an object. The color of an object is the result of the color lights that are not absorbed by the object. An apple appears that red because the surface of the apple absorbs the blue and green light.