8-bit image:
This is a digital image composed of as many as 256 possible colors or shades of gray.
acquire: ...
8-Bit Image - A digital image that utilizes 8 bits of data for each pixel. The image may contain up to 256 colors.
24-Bit Image - A digital image that utilizes 24 bits of data for each pixel. The image may contain up to 16.5 million colors.
An 8-bit image where the input/output ("levels") curve is not a straight line must interpolate where the slope is steeper while real information can be used if more bits are available.
Standard 8-bit images use values from 0 through 255 to cover the range from black to white. 16-bit images up the ante by allowing you go from 0 all the way to 65,535. In both though, black is still black, and white is still white.
Recall that each channel in an 8-bit image can only have discrete color intensities from 0 to 255 (see "Understanding Bit Depth"). A stretched histogram is forced to spread these discrete levels over a broader range than exists in the original image.
There can be up to 256 shades of gray in an 8-bit image. Each pixel has a brightness value ranging from 0 (black) to 255 (white). The shades (gray levels) of the converted pixels represent the luminosity of the original pixels.
Gray level
A shade of gray assigned to a pixel. The shades are usually positive integer values taken from the gray-scale. In a 8-bit image a gray level can have a value from 0 to 255.
Digital images are often described by the number of bits used to represent each pixel. I.e. a 1-bit image is monochrome; an 8-bit image supports 256 colors or grayscales; while 24 or 32-bit supports an even greater range of color.
 See also: Image, Color, Light, Digital, Bit
  
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