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Electronic Guidebook for High-Speed Flash Photography

revised 12-00

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Activity 3. Measuring flash duration

Equipment needed:

  • Vivitar 283 flash unit
  • 4 AA batteries or SB-4 AC adapter
  • 120-V AC extension cord
  • 100-kΩ variable resistor
  • High-frequency clock (clip clock)
  • Electronic stroboscope
Safety consideration:  The paper disc on the clock rotates at high-speed.  Keep body parts away from the disc in order to avoid paper cuts.



Background: The duration of the flash can’t be read directly from a dial on the unit, but there are methods of measuring it. The method that will be used in this activity is to set up a motor as a high-frequency clock. A black cardboard disc is centered on the axle of  a fan motor to serve as the clock face. A white, radial line painted on the disc serves as the hand, as shown in the figure to the right. If the motor rotates fast enough, the hand produces a perceptible blur under the illumination of the electronic flash. (Instructions for constructing such a clock from a clip fan are given here.)

  1. Clip the fan motor onto the edge of the table. Then turn the motor on to top speed. (Don't leave the motor running too long at one time, because it's not designed to rotate this fast indefinitely.)  Use the electronic stroboscope to measure the frequency of the motor. This is done by adjusting the stroboscope frequency until the hand of the clock disc appears stationary. To insure that the clock hand is making only a single rotation between flashes, double the stroboscope frequency. If two equally-bright images of the hand are seen 180° apart, the original frequency was the correct one.
  1. Record the clock frequency in flashes per minute.
  2. Calculate the number of flashes per second.
  3. Calculate the time in seconds for the clock to make 1 full rotation.
  4. Calculate how long it would take for the hand of the clock to move through an angle of 90°.
  5. Repeat part d for 1°.  This gives an approximate lower limit for the time intervals that the clock can measure.
  1. In order to use the clock to make accurate measurements of flash duration, it’s necessary to take photographs of the rotating disc. For this activity, however, you can simply view the disc and make relative judgments about the angle through which the hand moves. Connect an extension cord to the SB-4 adapter and plug it into the flash unit.   (The extension cord is needed, since you'll be moving around the room with the flash unit.) Set the flash unit on the purple automatic mode and point it at the rotating disc, about a meter away. Discharge the unit, watching the disc as you do so. Back up to greater and greater distances from the disc, noting the amount of blur each time. Then repeat with the flash unit set on the yellow automatic mode. Also try with the auto-thyristor set on M. Finally, make some tests with the 100-kΩ resistor replacing the auto-thyristor module. Describe the results of the tests.
  1. effect of increasing distance:
  2. effect of yellow compared to purple:
  3. effect of M setting:
  4. effect of increasing variable resistance:


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