If you've read How Cameras 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.
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 (see How Batteries Work for more information).
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.
The basic idea is to conduct electrical current -- to move free electrons -- through the gas in the tube, from one electrode to the other. As the free electrons move, they energize xenon atoms, causing the atoms to emit visible light photons (see How Light Works for details on how atoms generate photons).
You can't do this with the gas in its normal state, because it has very few free electrons -- that is, nearly all the electrons are bonded to atoms, so there are almost no charged particles in the gas. To make the gas conductive, you have to introduce free electrons into the mix.
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 (see How Fluorescent Lamps Work for more information).
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.
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. All of these components are explained in other HowStuffWorks articles:
Capacitors - Devices that store energy by collecting charge on plates (see How Capacitors Work)
Inductors - Coiled lengths of wire that store up energy by generating magnetic fields (see How Inductors Work)
Transistors - Semiconductor devices that can act as electrically controlled switches or amplifiers (see How Amplifiers Work)
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).
If you've read How Electromagnets Work, you know that passing current through a coiled length of wire will generate a magnetic field. If you've read How Inductors Work, 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.
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 than the primary coil. (Check out this site for more information.)
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.
Master and Slave
Professional photographers often set up flashes all around a subject to achieve better lighting effects. In this arrangement, one master 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.
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 (see How Amplifiers Work for details).
When the transistor is "switched on" in this way, a burst of current can flow from the battery to the primary 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 a rectifier -- 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 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, including flashes that "read" the subject in front of them, check out the links on the next page.