Web Design in a Nutshell, 3rd Edition

Grayscale

PNGs can also support 16-bit grayscale images —that’s as many as 65,536 shades of gray (2¹⁶), enabling black-and-white photographs and illustrations to be stored with enormous subtlety of detail. This is useful for medical imaging and other types of imaging where detail must be maintained, but it is not much of an advantage for images intended for web delivery due to the inherent limitations of low-resolution images. Grayscale images are supported at 1-, 2-, 4-, and 8-bit depths as well.