Grayscale is a range of monochromatic shades from black to white. Therefore, a grayscale image contains only shades of gray and no color.
While digital images can be saved as grayscale (or black and white) images, even color images contain grayscale information. This is because each pixel has a luminance value, regardless of its color. Luminance can also be described as brightness or intensity, which can be measured on a scale from black (zero intensity) to white (full intensity). Most image file formats support a minimum of 8-bit grayscale, which provides 2^8 or 256 levels of luminance per pixel. Some formats support 16-bit grayscale, which provides 2^16 or 65,536 levels of luminance.
Many image editing programs allow you to convert a color image to black and white, or grayscale. This process removes all color information, leaving only the luminance of each pixel. Since digital images are displayed using a combination of red, green, and blue (RGB) colors, each pixel has three separate luminance values. Therefore, these three values must be combined into a single value when removing color from an image. There are several ways to do this. One option is to average all luminance values for each pixel. Another method involves keeping only the luminance values from the red, green, or blue channel. Some programs provide other custom grayscale conversion algorithms that allow you to generate a black and white image with the appearance you prefer.
While grayscale is an important aspect of digital images, it also applies to printed documents. When you select "Print," the print dialog box that appears may include a grayscale option. If you choose this option, the color information will be removed from the document before it is printed. As long as your printer has an individual black ink cartridge, when you print in grayscale, it should only use the black ink and none of the color cartridges. Therefore, the "Print in Grayscale" feature is useful if you just need to print a document for reference or don't need a color version.
Updated: April 1, 2011