Canon Inc.'s EOS flash system refers to the photographic flash mechanism used on Canon's film (35mm and APS) or digital EOS single-lens reflex cameras. The line was first introduced in 1987. It has gone through a number of revisions over the years, as new flash exposure metering systems have been introduced. The main light-metering technologies are known as A-TTL, E-TTL, and E-TTL II.