The earliest flashes consisted of a wad of magnesium powder that was ignited by hand. Later, magnesium filaments were contained in flash bulbs, and electrically ignited by a contact in the camera shutter; such a bulb could only be used once, and was too hot to handle immediately after use, but the confinement of what would otherwise have amounted to a small explosion was an important advance.
For the Kodak Instamatic camera, flash cubes of 4 bulbs were introduced, that allowed taking 4 images in a row as the cube automatically rotated 90 degrees to a fresh bulb upon firing. The later Magicube was noteworthy in that each bulb was set off by a plastic pin striking a pyrotechnic element in the flash, so that a battery was not required.
Today's flash units are often electronic. An electronic flash contains a tube filled with xenon gas, where electricity of high voltage is discharged to generate an electrical arc that emits a short flash of light. (A typical duration of the light impulse is 1/1000 second.) As of 2003, the majority of cameras targeted for consumer use have an electronic flash unit built in.
A separate flash unit may usually be mounted to a camera via a standardised accessory mount bracket.
See also: Flash synchronization