Flash Unit of Digital Camera comes under the collective name of combined lighting equipments used for the need of illumination of objects to be captured. Flash Unit can be mounted on a Digital Camera as in build system, typically for the consumer grade cameras, where as DSLR cameras are equipped with a in built Flash Unit as well as a Hot Shoe Mounting System for attachment of External Flash unit.
Optical Properties, Principles of Light and Flash Unit
Flash unit is fired for a relatively short exposure time. For this, the time in which the triggered flash and illuminated object with reflected light, needs to synchronized with the shutter speed of the camera. Since the flash unit, by design briefly illuminates, it must be initiated at a time at which the shutter is fully open. The fastest shutter speed at which a focal plane the shutter exposes the sensor completely is called flash sync speed . By different flash methods, different effects and moods can be achieved in the photo.
An important criterion in the flash unit is the color of the light color which is expressed in Kelvin. Most current flash unit emits a natural white light ranging from 5500 K to 6500 K, quite similar to direct sunlight. These Flash Unit can therefore be easily combined with sunlight for example, for the fill in effect of dark areas under bright sunlight.
The angle of illumination of a flash unit is the maximum focal length which can be captured by the camera image as fully illuminated. Typically for this narrow angle angle of illumination of built in flash units, we need external flash unit.
Types of Flash Unit and their illumination system
Image : Action of a Xenon Flash Bulb in slow motion
It is beyond a single article’s capability to cover all the details of each type of Flash Unit and their illumination system. Flash Unit itself a vast topic, however, we are covering the basics only.
Open Flame Flash Unit : Usually not used these days.  Basically magnesium like light producing while ignited chemicals were used.
Electronic flash unit : Electronic flash unit again can be classified in to various types – by design of the electronic flash unit or by the method of illumination used (like xenon Flash, LED Flash etc.)
