Light source based on ignition of combustible metal wire in a gas filled transparentenvelope. Popular sizes are usually blue-coated to give light approximating to daylight.Flash bulbscome in various sizes and types. All work by burning metal foil in an oxygen atmospherewithin the glass bulb. Because the light is caused by combustion inside the glassenvelope, light intensity increases from zero as combustion begins. It reaches apeak value and then falls off as combustion ends. The flash unit is fired or triggeredby the shutter mechanism in the camera. For some flashbulb types in some cameras,the shutter mechanism fires the flash and then waits for a specified time delay beforeit actually opens the shutter. This delay is to allow the flash bulb to get up tofull brightness. See more on FP (focal plane bulb) section.