Part of the Computing fundamentals glossary:

Grayscale is a range of shades of gray without apparent color. The darkest possible shade is black, which is the total absence of transmitted or reflected light. The lightest possible shade is white, the total transmission or reflection of light at all visible wavelength s. Intermediate shades of gray are represented by equal brightness levels of the three primary colors (red, green and blue) for transmitted light, or equal amounts of the three primary pigments (cyan, magenta and yellow) for reflected light.

In the case of transmitted light (for example, the image on a computer display), the brightness levels of the red (R), green (G) and blue (B) components are each represented as a number from decimal 0 to 255, or binary 00000000 to 11111111. For every pixel in a red-green-blue ( RGB ) grayscale image, R = G = B. The lightness of the gray is directly proportional to the number representing the brightness levels of the primary colors. Black is represented by R = G = B = 0 or R = G = B = 00000000, and white is represented by R = G = B = 255 or R = G = B = 11111111. Because there are 8 bit s in the binary representation of the gray level, this imaging method is called 8-bit grayscale.

In the case of reflected light (for example, in a printed image), the levels of cyan (C), magenta (M), and yellow (Y) for each pixel are represented as a percentage from 0 to 100. For each pixel in a cyan-magenta-yellow (CMY) grayscale image, all three primary pigments are present in equal amounts. That is, C = M = Y. The lightness of the gray is inversely proportional to the number representing the amounts of each pigment. White is thus represented by C = M = Y = 0, and black is represented by C = M = Y = 100.

In some systems that use the RGB color model, there are 2 16 , or 65,636, possible levels for each primary color. When R = G = B in this system, the image is known as 16-bit grayscale because the decimal number 65,536 is equivalent to the 16-digit binary number 1111111111111111. As with 8-bit grayscale, the lightness of the gray is directly proportional to the number representing the brightness levels of the primary colors. As one might expect, a 16-bit digital grayscale image consumes far more memory or storage than the same image, with the same physical dimensions, rendered in 8-bit digital grayscale.

In analog practice, grayscale imaging is sometimes called "black and white," but technically this is a misnomer. In true black and white, also known as halftone, the only possible shades are pure black and pure white. The illusion of gray shading in a halftone image is obtained by rendering the image as a grid of black dots on a white background (or vice-versa), with the sizes of the individual dots determining the apparent lightness of the gray in their vicinity. The halftone technique is commonly used for printing photographs in newspapers.

In some cases, rather than using the RGB or CMY color models to define grayscale, three other parameters are defined. These are hue, saturation and brightness . In a grayscale image, the hue (apparent color shade) and saturation (apparent color intensity) of each pixel is equal to 0. The lightness (apparent brightness) is the only parameter of a pixel that can vary. Lightness can range from a minimum of 0 (black) to 100 (white).

This was last updated in May 2010
Posted by: Margaret Rouse

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