A thermal discoloration printer uses a row (or rows) of metal pins arrayed in a printer head. These pins are heated in a pattern according to the image desired. As a special, heat-sensitive paper is fed past the printer head, the hot pins discolor the paper where they make contact. The pins cool down very rapidly after each heating, and are reheated according to the new character or image desired as the head moves across the page.
The resolution of the thermal discoloration printer, as well as its printing speed, are usually much lower than other types of printers. Also, the special paper it uses is more expensive than normal printer paper and is susceptible to sunlight, heat, and certain chemicals which will cause the image to fade.
