Contrast -- the range between light and dark objects in the image -- needs to be enhanced to emphasize color and brightness variations and minimize the dulling effect of atmospheric haze. This is done by digitally "stretching" the data so they span the full range of possible brightness (many satellite images are composed of 8-bit data, a possible range of 2**8 or 256 brightness values). ETM images have eight different spectral bands that sample discrete portions of the electromagnetic spectrum ranging from visible wavelengths of light through thermal infrared “heat.”