Discharge Graphs of an Electronic Flash

Electronic flash units work by passing strong electrical current through ionized gas (xenon). Part of the electrical energy is thereby converted into light. The source of the electricity is a capacitor, which discharges (loses charge) very rapidly in the process. For more details on the workings of electronic flash, please read the Electronic Flash FAQ.

In February 1997, Jeremy Stein, an amateur photographer and physicist working in Albuquerque, New Mexico, measured the light output of Canon 430EZ and 380EX electronic flash units. The results are described in the following pages.

For the impatient, here is the most important result:

This is the discharge curve of the 380EX flash unit at full power. The rapid rise and a slower decay of the light output are well visible. The horizontal axis is time, and the whole graph represents a duration of 5 milliseconds (1/200 of a second).

Now the full story, mostly as written by Jeremy himself:

• • The experimental setup used for the tests;

  • Canon 430EX flash at different power levels;

  • Canon 380EX flash in high-speed sync mode;

  • Dependence of the output of the flash on the charge time;

All images are Copyright © Jeremy Stein, 1997. Unauthorized reproduction prohibited.