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Grayscale is the collection or the range of monochromic (gray) shades, ranging from pure white on the lightest end to pure black on the opposite end. Grayscale only contains luminance (brightness) information and no color information; that is why maximum luminance is white and zero luminance is black; everything in between is a shade of gray. That is why grayscale images contain only shades of gray and no color.
Grayscale is also known as achromatic.
A digital image usually contains both color information and luminance or grayscale. If you remove the color information, you are left with grayscale, resulting in a black and white image. Grayscale is an important aspect of images, and it is the only portion that is not removed; otherwise, a pure black image would result no matter what color information there is.
A digital image is composed of groups of three pixels with colors of red, green and blue (RGB), also called channels in digital imaging. Each channel also contains a luminance value to determine how light or dark the color is. To get a grayscale image, the color information from each channel is removed, leaving only the luminance values, and that is why the image becomes a pattern of light and dark areas devoid of color, essentially a black and white image. Most digital imaging software applications, even the most basic ones, are able to convert an image to grayscale. This is also very important when printing, since it only consumes black ink, as opposed to printing in color, which consumes all three print colors (cyan, magenta and yellow) as well as black.