Studio Flash Explained:
Understanding Xenon Flash

Understanding Xenon Flash
Essentially all photo flash systems produce light by storing electrical energy in capacitors, then discharging the capacitor quickly into the flashtube. This process ionizes the Xenon gas in the tube, causing an “arc.” This is essentially the same process as a lightning strike.

Xenon gas is used for this purpose because its characteristic light spectrum closely resembles that of sunlight and is relatively free of color spikes found in other gas discharge lamps. This results in excellent color rendition.

Technically speaking, light thus emitted has a color temperature of between 5000K and 6000K, depending on the voltage and current supplied to the flashtube. This is in the same color range as unfiltered sunlight falling on Earth.

Light Intensity Vs. Amount of Light
Think of a camera sensor as a container of water. It takes a certain amount of light to expose the image, like it takes a certain amount of water to fill the bucket. We can fill the bucket by letting a trickle (low intensity) of water flow for a long time, or we can fill it almost instantly from a fire hydrant (high intensity).

In a flash unit, the intensity of light is extremely high - hundreds of thousands to millions times brighter than light from incandescent bulbs. It is emitted in a short pulse and therefore exposes the picture in a brief instant, freezing action.

Compare an ordinary 300 Watt bulb that produces, say 6000 Lumens (intensity) of light with a 150 Ws flash unit that produces 6000 Lumenseconds (amount) in a pulse lasting 1/1000 second. If a one second exposure is made under the ordinary lamp, the amount of light involved is 6000 Lumenseconds. If the subject is moving, the picture is blurred due to the long exposure time. With the flash, the same exposure is made in 1/1000 second and there is little blur. In order to take the picture at a 1/1000 second exposure time using continuous light (hot lights) you would have to use a 300,000 Watt lamp producing 6,000,000 Lumens.

Thus, it should be remembered that the exposure from continuous light sources is dependant on camera exposure time, while camera exposure time with flash has no effect on the exposure as long as the shutter is open during the brief instant the flash fires. If you are in a dark room, it makes no difference if you use a 1 second or 1/125 second camera exposure time.

But if your room is not dark, a flash picture will also contain some amount of light from the ambient room light that should be prevented from adding to the flash exposure. Most flash pictures are shot at camera exposure times from 1/60 to 1/250 second in order to exclude the ambient light.

> more info on Efficiency, Wattseconds and Units of Measure here

> more info on Syncing to the Camera and Sync Speeds here