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 edition of HowStuffWorks,
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 (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.
A typical camera flash tube, removed from its
housing, looks like a miniature neon
light.
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The tube sits in front of the trigger
plate.
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The trigger plate is hidden by reflective
material, which directs the flash light
forward.
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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.
Another camera flash tube design: In this
curved tube, the trigger plate is attached directly to
the glass on the
tube.
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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.
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. 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)
- Diodes - Semiconductor devices that let current
flow freely in only one direction (see How
Semiconductors 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.
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Master and
SlaveProfessional
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.
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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.
Flash capacitor from a regular
point-and-shoot
camera
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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.
Check
out this short video to see what happened. (Kids,
don't try this at
home!)
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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.
Lots More Information!
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