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28 November 2003

A Homebrew Flash Lamp

by C. L. Stong
Excerpted from "Scientific American's The Amateur Scientist", first published August, 1974.

"Motion of the subject can be stopped in photographs only by short exposures, which require bright light. Each of the four colored slits of my instrument is 5/8 inch long and 1/8 inch wide-quite large compared with the dimensions cited in the technical literature. By using 'Sun Gun' lamps of the tungsten-halogen type with built-in reflectors of the kind designed for slide projectors, I could make photographs 45 millimeters square with exposures of a sixtieth of a second. This exposure is not fast enough, however, to stop motion as slow as hot air rising from a flame if the camera has a 400-millimeter lens.

"The solution of the problem of stopping fast action lies in an electronic flash lamp. The duration of most 'strobe' lamps owned by amateurs is about second, which is sufficiently brief for many events of interest. The flash unit is placed behind and close to the colored slits, as is the alternative incandescent lamp. Indeed, the schlieren apparatus can be made with a rotatable fixture for conveniently interchanging an incandescent lamp and an electronic flash unit. The continuous source serves for composing the picture and is replaced by the flash for making the exposure.

 

Harold E. Edgerton's flash lamp. Click image to enlarge.

"The simplest technique for taking photographs is to black out the studio, open the camera shutter and trigger the flash. For photographing bullets in flight a microphone can be employed to pick up the passing shock wave and trigger the flash. Timing difficulties are avoided by opening the shutter of the camera in advance. The color photographs reproduced in this article were made with a 35-mm. camera having a zoom lens of 170-mm. aperture and maximum focal length of 410 mm. I used ASA 160 high-speed Ektachrome film and a Microflash unit manufactured by EG&G, Inc. (35 Congress Street, Salem, Mass. 01970).

"This unit generates white light of remarkable intensity by discharging a capacitor in 3 x 10^-7 second through an air gap arranged so that the resulting arc adheres to the outer surface of a slender quartz tube. As first described by Edgerton in 1961, the flash unit consists of a power supply that charges a .05-microfarad capacitor of low inductance to a potential of about 18,000 volts. The flash is generated by discharging the capacitor through a spark gap consisting of two conductors in contact with the surface of the quartz [see illustration above, left]. This assembly is housed in a Pyrex test tube about an inch in diameter and six inches long. The quartz tube and the electrodes are supported by a rubber stopper that, along with the test tube, tends to suppress the noise of the spark.

Circuitry for the lamp. Click image to enlarge

"An appropriate electronic circuit for developing the spark consists essentially of a step-up transformer of 6,500 volts, a pair of solid-state diodes designed for service at 20,000 volts (such as the Varo Type 7715-20) and a pair of .1-microfarad capacitors of extraordinarily low internal inductance [see illustration, right]. Such capacitors are made by interleaving sheets of conducting foil with sheets of insulation and connecting the many alternate conducting sheets at the edges. The fabrication technique is costly, and the capacitors are also expensive.

"Edgerton found that the shortest flashes could be developed by discharging the capacitors in air. Gases such as krypton and xenon, which are found in conventional discharge lamps, continue to glow strongly for a substantial period of time after the electric current has ceased. Edgerton also learned by experiment that contact with the quartz tube tends to cool the arc and quench it quickly. He noted that the extended air spark over quartz has a higher resistance than an open gap. A critically damped spark must present to the circuit a resistance of about 13 ohms, whereas the resistance of an open spark is a small fraction of one ohm. The guided arc helps to damp out the oscillations in the circuit and reduce the flash duration. Moreover, the guided arc is more efficient for producing actinic (ultraviolet) radiation. The design generates a Hash of about five million candlepower within an interval of less than 300 nanoseconds [see illustration lower left].

 

Output of light by the lamp. Click image to enlarge.

"Triggering the spark to catch the action at the desired instant can pose a problem. In the case of a speeding bullet the problem has an interesting solution. Position a microphone to pick up the shock wave as the bullet speeds toward the target. After amplification the resulting pulse of current can be applied to a spark coil, such as the EG&G Type TR-50 trigger transformer. The output, at a potential of some 25,000 volts, is applied to a bare wire in the bore of the quartz tube. The resulting ionization initiates the discharge on the outer surface of the tube.

"The energy stored at 18,000 volts in the low-inductance capacitors is dangerous. The capacitors must never be touched until they have been short-circuited with a length of bare copper wire fastened to the end of an insulating handle at least a foot long. The triggering transformer, however, is no more hazardous than an automobile ignition coil."