How Camera Flashes Work

You know that it takes a lot of light to expose a vivid image onto film. For most indoor photography, where there is relatively little ambient light, you either need to expose the film for a longer period of time or momentarily increase the light level to get a clear picture. Increasing the exposure time doesn't work well for most subjects, because any quick motion, including the movement of the camera itself, makes for a blurry picture.

Electronic flashes are a simple, cheap solution to this inherent problem in photography. Their sole purpose is to emit a short burst of bright light when you release the shutter. This illuminates the room for the fraction of a second the film is exposed.

In this article, we'll find out exactly how these devices carry out this important task. As we'll see, a standard camera flash is a great demonstration of how basic electronic components can work together in a simple circuit.

Making a Flash

A basic camera flash system, like you would find in a point-and-shoot camera, has three major parts.

• A small battery, which serves as the power supply
• A gas discharge tube, which actually produces the flash
• A circuit (made up of a number of electrical components), which connects the power supply to the discharge tube

The two components on the ends of the system are very simple. When you hook up a battery's two terminals to a circuit, the battery forces electrons to flow through the circuit from one terminal to the other. The moving electrons, or current, provides energy to the various things connected to the circuit.

The discharge tube is a lot like a neon light or 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.

                                                                           

A typical camera flash tube, removed from its housing, looks like a miniature neon light.

The tube sits in front of the trigger plate.

The trigger plate is hidden by reflective material, which directs the flash light forward.

This is the metal trigger plate's job. If you briefly apply a high positive voltage (electromotive force) to this plate, it will exert a strong attraction on the negatively charged electrons in the atoms. If this attraction is strong enough, it will pull the electrons free from the atoms. The process of removing an atom's electrons is called ionization.

The free electrons have a negative charge, so once they are free, they will move toward the positively charged terminal and away from the negatively charged terminal. As the electrons move, they collide with other atoms, causing these atoms to lose electrons as well, further ionizing the gas. The speeding electrons collide with xenon atoms, which become energized and generate light.

To accomplish this, you need relatively high voltage (electrical "pressure"). It takes a couple hundred volts to move electrons between the two electrodes, and you need a few thousand volts to introduce enough free electrons to make the gas conductive.

A typical camera battery only offers 1.5 volts, so the flash circuit needs to boost the voltage substantially. In the next section, we'll find out how it does this.

The Boost

In the last section, we saw that a flash circuit needs to turn a battery's low voltage into a high voltage in order to light up a xenon tube. There are dozens of ways to arrange this sort of step-up circuit, but most configurations contain the same basic elements:

• Capacitors - Devices that store energy by collecting charge on plates 
• Inductors - Coiled lengths of wire that store up energy by generating magnetic fields 
• Diodes - Semiconductor devices that let current flow freely in only one direction 
• Transistors - Semiconductor devices that can act as electrically controlled switches or amplifiers 

The diagram below shows how all of these elements come together in a basic flash circuit.

Taken in its entirety, this diagram may seem a little overwhelming, but if we break it down into its component parts, it isn't that complicated.

Let's start with the heart of the circuit, the main transformer, the device that actually boosts the voltage. The transformer consists of two inductors in close proximity to each other (for example, one might be wound around the other, with both might be wound around an iron core).

You know that passing current through a coiled length of wire will generate a magnetic field. If you've read .You know that a fluctuating magnetic field, generated by fluctuating electric current, will cause a voltage change in a conductor. The basic idea of a transformer is to run current through one inductor (the primary coil) to magnetize another conductor (the secondary coil), causing a change in voltage in the second coil.

Master and Slave Professional photographers often set up flashes all around a subject to achieve better lighting effects. In this arrangement, onemaster flash may be triggered by the camera shutter, while other flashes are triggered by the master. Some slave flash designs use the master flash's light itself as a trigger. The slave flash has a small light sensor that triggers the flash circuit when it detects a sudden pulse of light.

If you vary the size of the two inductors -- the number of loops in each coil -- you can boost (or reduce) voltage from the primary to the secondary. In a step-up transformer like the one in the flash circuit, the secondary coil has many more loops than the primary coil. As a result, the magnetic field and (by extension) voltage are greater in the secondary coil than in the primary coil. The trade-off is that the secondary coil has weaker current

To boost voltage in this way, you need a fluctuating current, like the AC current (alternating current) in your house. But a battery puts out constant DC current (direct current), which does not fluctuate. The inductor's magnetic field only changes when DC current initially passes through it. In the next section, we'll find out how the flash circuit handles this problem.

Oscillator and Capacitor

In the last section, we saw that transformers need fluctuating current to work properly. The flash circuit provides this fluctuation by continually interrupting the DC current flow -- it passes rapid, short pulses of DC current to continually fluctuate the magnetic field.

The circuit does this with a simple oscillator. The oscillator's main elements are the primary and secondary coils of the transformer, another inductor (the feedback coil), and a transistor, which acts as an electrically controlled switch.

When you press the charging button it closes the charging switch so that a short burst of current flows from the battery through the feedback coil to the base of the transistor. Applying current to the base of the transistor allows current to flow from the transistor collector to the emitter -- it makes the transistor briefly conductive 

When the transistor is "switched on" in this way, a burst of current can flow from the battery to theprimary coil of the transformer. The burst in current causes a change in voltage in the secondary coil, which in turn causes a change in voltage in the feedback coil. This voltage in the feedback coil conducts current to the transistor base, making the transistor conductive again, and the process repeats. The circuit keeps interrupting itself in this way, gradually boosting voltage through the transformer. This oscillating action produces the high-pitch whine you hear when a flash is charging up.

The high-voltage current then passes through a diode, which acts as arectifier -- it only lets current flow one way, so it changes the fluctuating current from the transformer back into steady direct current.

The flash circuit stores this high-voltage charge in a large capacitor. Like a battery, the capacitor holds the charge until it's hooked up to a closed circuit.

The capacitor is connected to the two electrodes on the flash tube at all times, but unless the xenon gas is ionized, the tube can't conduct the current, so the capacitor can't discharge.

The capacitor circuit is also connected to a smaller gas discharge tube by way of a resistor. When the voltage in the capacitor is high enough, current can flow through the resistor to light up the small tube. This acts as an indicator light, telling you when the flash is ready to go.

The capacitor in a typical camera flash circuit can store a lot of juice. We charged this one up and then discharged it by connecting the two terminals. 

The flash trigger is wired to the shutter mechanism. When you take a picture, the trigger closes briefly, connecting the capacitor to a second transformer. This transformer boosts the 200-volt current from the capacitor up to between 1,000 and 4,000 volts, and passes the high-voltage current onto the metal plate next to the flash tube. The momentary high voltage on the metal plate provides the necessary energy to ionize the xenon gas, making the gas conductive. The flash lights up in synch with the shutter opening.

Different electronic flashes may have more complex circuitry than this, but most work in the same basic way. It's simply a matter of boosting battery voltage to trigger a small gas discharge lamp.

For much more information on camera flashes,
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