Discharge Curves of Electronic Flash in High-Speed Synchronization Mode

In February 1997, Jeremy Stein, an amateur photographer and physicist working in Albuquerque, New Mexico, measured the light output of a Canon 380EX electronic flash unit. This page shows the results for the high-speed synchro mode.

The flash unit was attached to an EOS Elan II camera, which has the highest X-sync speed of 1/125 seconds. Jeremy writes:

This file consists of data taken from the same camera and flash combination, starting out with new batteries, and taking a series of shots with the flash set on H at successively faster shutter speeds. The sequence goes from 1/125 to 1/4000 of a second. The pictures show several things about this flash system. There is marked change in the behavior of the flash when the shutter speed goes above 1/125 second; at this and slower speeds the flash behaves as in the data I took on the 430EZ. That is, the light output rises in about 100 microseconds, and then decays exponentially unless the system decides that the exposure has been completed, in which case the flash output is chopped off early. At shutter speeds faster than 1/125, i.e. those for which the focal plane shutter is never completely open, the flash modulates its output light at about 40 kHz in order to maintain a nearly constant light level for longer than the time it takes for the shutter slit to travel completely across the film plane. You can see that this time is about 13 milliseconds for 1/180 and goes down to about 9.5 milliseconds for 1/4000 second. During this discharge time, the light level changes by about 1 full stop or maybe a little less.

The electronics in the flash unit most likely have a transistor which completely shuts off current flow to the flashtube for part of each 40 kHz cycle, and relies on the relatively slow rate of deionization of the flash tube gas (low-pressure Xenon, almost certainly) to keep it well enough ionized to maintain a rather constant load impedance. In addition, although the current is pulsed on and off, the light emission from the still-ionized gas continues in reasonably even fashion during the time the current is shut off. The overall decay time constant seen in the H mode flashes is much longer than that seen in the 1/125 second and longer exposures. The peak intensity in the two modes is very different. In the normal flash mode, peak intensity in this series is about 2.8 volts, while in the H mode, the peak intensity is from 350 to 450 millivolts. This accounts for the very low guide numbers for the H-mode flashes.

[Ed. note: "volts" means volts as measured from the output of the light-to-electrical signal converter. We expect to have a linear correspondence between the "volts" and light levels, but the numerical relationship between the two is not known to us.]

This is the discharge graph at 1/125 seconds. It is similar to those measured earlier for the 430EX flash.

A small change in shutter speed from 1/125 to 1/180 drastically changes the picture. The flash unit switches to high-speed synchronization mode, the operation of which is nicely described by Jeremy above.

The images for shutter speeds between 1/180 and 1/4000 seconds are similar, with the duration of the pulse slowly diminishing.

Further changes in shutter speed (this one is taken with the camera at 1/4000 sec) cause only a small reduction of the duration of the pulse.

This image shows the start of the flash in the high-speed synchro mode, with a much more detailed resolution than the previous images. As soon as a certain level of output is acieved, the electronics start a rapid on-off cycle at approximately 40000 cycles per second.

This is the end of the high-speed synchro flash, with a rapid cut-off, apparently by an independent timing circuit, since the last wave is not complete.

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