It takes a lot of light to expose an image on film. This presents a challenge when taking pictures indoors especially when there is little available light. Camera flashes momentarily boost the amount of light in a room to get a good exposure. Electronic flashes are a simple, cheap solution.

An electronic flash system has three main parts:

• A small battery, which serves as the power supply

• A gas discharge tube, which actually produces the flash

• An electrical circuit connecting the power source to the discharge tube

The discharge tube is a lot like a neon light or a fluorescent lamp. It consists of a tube filled with xenon gas with electrodes on either end and a metal trigger plate at the middle of the tube.

The trigger plate introduces free electrons into the xenon gas to make it conduct an electric current. A high positive voltage is applied to the plate. The positive charge pulls the negatively charged outer electrons away from the gas, causing it to become ionized. The speeding electrons collide with xenon atoms, energizing their remaining electrons. When the electrons give up that energy, they emit light.

To make this possible, the flash circuit needs to boost the 1.5 volts of the camera's flash battery substantially. A transformer boosts the voltage to 200 volts. The capacitor, stores the high-voltage charge.

When you take a picture, the flash trigger closes briefly, connecting the capacitor to a second transformer. The second transformer boosts the 200-volt current to between 1,000 and 4,000 volts and passes the current on to the metal trigger plate. The flash then fires, and for an instant the room is filled with light -- just long enough to take a picture.