How Electronic Flash Works
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....
86 PART 2 HOW DIGITAL CAMERAS CAPTURE IMAGES
AC is needed for the next step in the dance:
thestep-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.2
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.)3
Whether the electrical source is a studio’s AC
wall outlet or a single AA battery in the cam-
era or flash, it’s not powerful enough to gener-
ate 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).1