Discharge Curves of Electronic Flash at Different Power Settings

In February 1997, Jeremy Stein, an amateur photographer and physicist working in Albuquerque, New Mexico, measured the light output of a Canon 430EZ electronic flash unit. He writes about the procedure:

First I set up a fixture so that the conditions would be the same for every shot, at least in terms of the light pickup and its relationship to the flash head. Then I ran a series of shots at the same scope settings over the manual range of power levels, from 1/1, 1/ 2, 1/ 4, 1/8, 1/ 16, and 1/32. I waited at least 30 seconds from the time the red light came on before firing each shot, so that the power level (or at least the charge level) would be the same. Then I reshot the lower power levels at faster sweep speeds so that it would be possible to get better data on them.

The risetime of the light from the flash unit is essentially the same for any power level. If not clipped off by the end-of-flash thyristor, the light output rises in about 100 microseconds, 10% to 90%. At the lower power levels, this time is apparently shorter because the light output is clipped off before it reaches full output.

A problem with making a measurement of this type is deciding what constitutes the end of photographically useful light output. At full and half power the light output level is strictly a function of the capacitor discharge; i.e., an exponential decay from the peak level. I chose to determine the time between the levels at half peak intensity and also those for a quarter of the peak intensity. These correspond to a range of one stop down from the peak and two stops down from the peak. It might be meaningful to check the timing for lower levels also, but only for the full and half power flashes, since the flash is clipped off and the time is essentially the same for any level less than about one stop down.

These data are in the form: power level, risetime (10% to 90%, microseconds), duration for one stop down, duration for two stops down.

Power
Level
rise
usec
-1 stop
usec
-2 stop
usec
Full 110 1000 1810
1/2 100 1000 1820
1/4 100 940 1010
1/8 115 365 405
1/16 105 155 195
1/32 90 88 120


Discharge graph at full power
 
 
 
This is the discharge graph at full power. One unit on the horizontal axis corresponds to 250 microseconds; the whole graph represents a time period of 2.5 milliseconds (0.0025 seconds). The rapid rise and exponential decay of the light output are clearly visible.
Discharge graph at 1/2 power
 
 
 
This graph is made at 1/2 power setting. The cut-off thyristor cuts the pulse at slightly after 1 millisecond after the flash has started.
Discharge graph at 1/4 power
 
 
 
1/4 power. The working of the cutoff thyristor is clearly visible. At less than 0.5 milliseconds, the current is sharply cut off.
Discharge graph at 1/8 power
 
 
 
1/8 power. The output is clipped almost immediately after full power has been reached.
Discharge graph at 1/16 power
 
 
1/16 power. Now the current is cut off well before it reaches its peak value. Cutoff time about 160 microseconds.
Discharge graph at 1/32 power
 
 
 
1/32 power. Cutoff occurs long before peak light output could have been attained.

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


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