A stroboscope is a tool which uses a bright flashing light to allow a user to view objects which are rapidly moving in a periodic manner. When the frequency of the flashing light is matched to the speed of the object, the object can appear to be stationary.
Stroboscopes are commonly used to examine moving machinery in an industrial setting. The stroboscope allows the equipment to be inspected or monitored while it is operating.
If the strobe frequency is high enough, the observer won’t perceive the flashing of the light, similar to how one does not notice the fact that a movie projection actually consists of individual frames.
This article details construction of a control circuit that can drive an array of LEDs for use as a stroboscope. The LED array can be made by straightforward modification of inexpensive LED flashlights, or a custom array can be built.
One note of caution. Some people who have epilepsy are susceptible to seizures caused by rapidly flashing lights. Do not use the device shown here, or any commercial strobe light around anyone who is known to have such a susceptibility!
Here is a YouTube video demonstration of just a few ways that this stroboscope unit can be used to view moving objects.
The stroboscope works by producing very brief yet very bright pulses of light. If the frequency of the light pulses is correct, the rotating object will be illuminated at the same position during each flash of light, giving the appearance that is stationary. This phenomenon is called the stroboscopic effect.
The stroboscopic effect is really a phenomenon of aliasing, which is the result of under sampling. If you are familiar with digital signal processing, you may be aware that a signal that is sampled can produce “aliases” depending on the sample rate used. A signal that is sampled at less than twice its frequency can produce a result called an alias, which has a lower frequency than the original signal.
If a signal is sampled at a rate which is exactly the same frequency as that of the input signal, then the sample will take place at the same point in its cycle, and so the same value will always be read. Because the sampling results in the same value each time, the resulting sampled signal representation appears as DC value instead of the alternating waveform of the actual signal. The original signal has therefore been downshifted in frequency to a DC signal. The same concept applies to the stroboscope. The actual rotational speed will be downshifted to zero in the eyes of the observer, giving the false perception that the object is standing still.
The object will also appear stationary when the strobe frequency is some integer fraction of the rotational frequency, such as one half, one third, or one forth, etc. This is because these cases will also result in the object being illuminated when it is in the same position each time.
If the strobe frequency is slightly lower or higher than the rotational rate (or an integer fraction of the rotational rate) of the object, it will appear to rotate slowly forward or backward. In these cases the rotational speed has been downshifted to a speed slightly greater than zero in the eye of the observer.
The appearance of a rotating object illuminated by a strobe can show more complex behavior if the rotating object has multiple identical sectors, like the spokes of a wheel or the blades of a fan or propeller. In these cases, the object can appear stationary when the period of the strobe frequency is an integer multiple of the rotational period divided by the number of sectors. If the individual sectors are similar enough in appearance that they are identical to an observer, then the object will appear stationary for any strobe frequency where any of the identical sectors is in a given position. As the strobe frequency is swept, the observer may notice several points at which the rotation appears to first slow , then stop, and then begin rotating in the opposite direction.