How Does a Stroboscope Work
Stroboscope
A stroboscope uses a flash lamp driven by an electronic oscillator. The flash lamp is usually a xenon bulb, though LEDs are sometimes used. The oscillator triggers the lamp at a steady rate, settable from a few times per second to thousands of times per second. The flash lamp has a reflector to increase its brightness and to make the flash more directional.
There are two basic types of strobes: those used for entertainment and those for science. The party models are speed-limited, since it's been found that flashes at certain rates can induce epileptic seizures in some people. They may have other features, like multiple colored lights that flash in sequence. The scientific models don't have a speed limit; they must be able to capture high-speed phenomena.
Scientific and professional strobes may also have external trigger inputs. For example, if technicians are examining a machine that runs at a certain rate, they can easily sync the stroboscope to it and slow down its motion for study. The external trigger overrides the internal oscillator.
Aliasing
The actual stroboscopic effect is a kind of cognitive trick called temporal aliasing. You don't need an electronic strobe to experience this. It was discovered back in Victorian times when a spinning wheel was observed through a second slotted wheel. The slot was a form of shutter that allowed a glimpse of the first wheel at a point in time. The apparent motion of the wheel depended on the relative speeds of both wheels. It might appear spinning normally, slowed down, stopped or moving backward. The same thing, called the "wagon-wheel effect," happens in movies. The motion of car wheels and other objects aliases against the 24-frame-per-second movie camera shutter, altering the apparent motion.
You should keep in mind that the motion is perceived; that is, your brain "connects the dots" between glimpses and creates the apparent change in movement.
With an electronic strobe, you watch in a darkened setting with the only light coming from the strobe. You see a moving object in the glimpses permitted by the strobe's flash. The motion aliasing between the object and the strobe rate makes the object appear slowed down or stopped.
Freeze-Frame
Besides altering the apparent motion of things, strobes can be used in a one-shot mode to freeze motion entirely. This is useful in capturing fast phenomena like a bullet passing through a soda can or a balloon popping. The technician synchronizes the flash with an electronic detector, such as a photogate or microphone. A pulse from the detector is processed by a variable delay, which then triggers the flash. The delay lets the technician determine precisely when the flash will go off, fine-tuning it by millionths of a second, to capture the exact instant he wants.
Tags: apparent motion, flash lamp, altering apparent, altering apparent motion, down stopped
