How electronic light works | Camera flash operation

Whether those flashing lights are from the small flash built into point-and-shoot cameras, a professional flash attachment, a bank of studio lights, or even the frenetic flashing above a disco floor, they all work the same way. A gas is excited electrically, until it bursts forth with a high intensity blast of energy in the form of light. Of course, it's actually a little more complex than that. Here, using a high-end flash gun as an example, is the intricate electronic flash dance that begins when the photographer presses the shutter button—a dance that turns mere electrical excitement into enlightenment. Preparations for the dance begin early.

Whether the electrical source is a studio's AC wall outlet or a single AA battery in the camera or flash, it's not powerful enough to generate the blinding flash of light the camera needs. The typical 1.5-volt direct current (DC) of the batteries in a flash gun passes through the wire coils of an oscillator. The coils cause a transistor switch to open and close rapidly, reversing the current's flow so that it becomes alternating current (AC).

AC is needed for the next step in the dance: the step-up transformer. Here the current spirals through another coil, one wrapped about an iron core. As the alternating current flip-flops its direction, it creates magnetic fields in the core that expand and collapse with each reversal of the current. This is the source of the whining sound you hear as a flash attachment ramps up.

The fluctuating fields create an electrical current in a second iron core wrapped with many more coils of wire than the first core. The voltage of the new current is in direct ratio to the numbers of coils around the two iron cores. Lest anyone think this creates electrical energy out of nothing, note that the increasing voltage is accompanied by a decrease in amperage, or the amount of current, so that it all evens out.

The current, now boosted to thousands of volts, is stored in two capacitors, electrical components that hold large amounts of electricity for later use, much like a dam harnesses the energy in a flowing river. One of the capacitors stores only a small amount of the electricity. The other, called an electrolytic capacitor, is designed to store and quickly release the powerful charge destined to create the flash of light.

If a flash isn't built into a camera, flash attachments send and receive the signals necessary to coordinate the camera and flash in as many as three ways. The most common is to fit the flash in a hot shoe, mounted atop the camera, in line with the lens. The flash may also use a sync cable connecting it to a camera's PC terminal. The 'PC' has nothing to do with computers. The connector is named for German shutter makers Prontor and Compur. Finally, and not as common in everyday shooting, the camera and flash communicate with one another through radio signals, or it may be built into the camera. With any of the methods, when the flash is turned on, the first thing it must do is send an electrical notice to the camera that it's there. Some flash units also send the camera the Kelvin temperature of the light they produce to help the camera automatically set white balance. That knowledge may change the way the camera's processor would otherwise expose a photo based on only ambient lighting. Dedicated flashes are programmed to work with specific cameras, and swap information that extends the ranges of what the flash and camera can do when they combine their features.

When you press the camera's shutter button, the camera normally waits until the shutter is completely open. Then it sends a signal to the flash gun. The circuitry inside the flash sends another signal to the smaller of the two capacitors, telling it to unleash its electrical charge.

That relatively small charge, called the trigger charge, goes to a metal plate called the trigger plate located outside a tube of glass or fused quartz that will in an instant produce the flash of light.

The charge on the trigger plate contributes electrons to the xenon gas inside the tube, a process called ionization. At that moment, the larger capacitor discharges its electricity, which races through the xenon. The current excites the gas's electrons, causing them to gain energy. Immediately, the electrons shed that energy in the form of photons, or light. 

Techniques of using cameras flashes