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Camera Shutter Time Measurement -- August 14, 2011

Camera Shutter Time Measurement
A while back I received an email from someone who needed a simple way to measure the speed of a photographic camera.  I thought about this and suggested the method below.  It relies on an oscilloscope and a cheap photodiode to measure the speed.

The illustration below depicts how this speed measurement is set up.  A photodiode with a 1K resistor wired in parallel is positioned in the photographic camera’s film plane.  A bright incandescent lamp is placed in front of the camera lens.  When the camera shutter opens and closes, a voltage pulse is produced by the photodiode, which can easily be observed on the oscilloscope screen.  The width of the pulse is the camera shutter speed.  By adjusting the scope’s sweep speed, pulses ranging from milliseconds to seconds can be measured.

I would suggest a BPW34 photodiode made by Osram.  The 1K load resistor should be about right, so the DC component from ambient light will be small, relative to the light hitting the photodiode when the shutter is open.  The light hitting the photodiode generates a current proportional to the light intensity.  The 1K load resistor converts the current into a voltage.  A bright flashlight is all that is needed as a light source.  Make sure that the flashlight is an incandescent type, since this type of light will be more easily detected by the photodiode.

With a careful adjustment of the scope’s trigger setting, you can capture both fast and slow shutter speeds.  I would recommend a “normal” scope trigger setting with a positive slope. In this mode, the scope screen will be blank and the trace will not start until the photodiode sees light from the flashlight through the lens and shutter. The end of the pulse will be seen as a negative slope.

 

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