High Speed Photography Strobe Light

Posted on Feb 4, 2014

In order to take a high speed photograph one must either have a very fast shutter or light source. We followed the latter course and first examined an commercial flash unit to determine its light profile. This was accomplished by exposing a phototransistor connected in an emitter follower configuration with a relatively low impedance emitter resistor to the flash and viewing the resistor voltage

High Speed Photography Strobe Light
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with an oscilloscope. Since we did not have a fast digitizer or camera available the observed trace was roughly hand sketched. The figure to the left shows the observed data. The phototransistor circuit appears in the right of the figure and the light curve of the commercial flash unit is shown at the lower left. It can be seen that the pulse length is around 175 µs which is far too long. For example, a 130 m s-1 object (295 mph) will travel 22 mm in that time. We set about to build our own light with a significantly shorter pulse length. Fortunately, a U-shaped xenon flash lamp and companion pulse transformer were available from a local Radio Shack (and still are although the pulse transformer is now a special order item) and there were a few high voltage capacitors in the junk box. A high voltage (several hundred volt) power supply was also laying about which was built for another project at least twenty years ago. A very nice reflector was obtained from a hand-held lantern, drilled out to pass the flash lamp, and attached to a small plastic box with epoxy. The electrical components were mounted free-standing inside the box, a male DB-9 connector was used to connect to the high voltage source, a "short-to-fire" connector was implemented with a female 1/8" phone jack, and a momentary contact switch was utilized as a "push-to-test" button. The following figure shows a collection of views of the home-made strobe flash unit....

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