Jasey Michelle's 365/366 Photo a Day Blog

The earliest flashes consisted of a quantity of magnesium flash 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. Later bulbs substituted zirconium for the magnesium which produced a brighter flash.

Flashbulbs took longer to achieve full brightness and burned for a longer duration than electronic flashes, and slower shutter speeds (typically from 1/10 to 1/50 of a second) were used on cameras to ensure proper synchronization. One of the most widely used flash bulbs up through the 1960s was the number 25. This is the large (approximately 1 inch in diameter) flash bulb often shown used by newspapermen in period movies, usually attached to a press camera or a twin-lens reflex camera.

Today's flash units are often electronic xenon flash lamps. 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.)

The intensity (or illuminance or irradiance) of light or other linear waves radiating from a point source is inversely proportional to the square of the distance from the source. So an object twice as far away, receives only 1/4 the light.