Display of any digital image requires that pixel values (numbers in a computer that range from 0 to some maximum) be allocated a brightness value on a computer monitor. This is achieved by use of a look up table (LUT); a simple LUT will be a linear "translation" of pixel values to monitor brightness as depicted by the figure below.
In the figure above, the level (L) corresponds to the pixel value that corresponds to the mid-gray brightness level on the monitor. Increasing the level will make the image darker, whereas decreasing the level value will make the image brighter. In the figure below, the window width (W) determines the range of pixel values that will be incorporated into the display width. Increasing W will reduce display contrast (see figure) whereas decreasing the W increases the brightness interval between two consecutive pixel values.
Numbers between 0 and 255 (for example) can be assigned to any arbitrary color value as shown in the example above. Individuals with normal color sensitivity can more readily distinguish subtle differences in color compared to similar differences in grayscale. Use of specific color LUTs can convert a grayscale image into a variety of color maps that can depict quantitative and qualitative changes in the images that are otherwise difficult to discern.
Often, color images contain three separate color intensities (red, green, blue or RGB) that are each stored in one byte with 256 intensity levels, with a total of 3 byte (24 bits) per pixel. Combination of RGB intensities provide a large number of unique colors (224 16 million) that can be used to advantage for certain applications.