High-Speed Photos of Low-Speed Impacts

Posted on Feb 7, 2014

The goal is to freeze different stages of some fairly quick process (at least when compared to the time resulotion of the human eye). Since I don`t have fancy toys like a high speed film camera, lack of good equipment has to be compensated with a smart setup applying just a an ordinary consumer-type digital camera and some self-made electronics. U

High-Speed Photos of Low-Speed Impacts
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nfortunately, the camera`s minimum exposure of about 1ms is too long for the purpose and furthermore the delay from pressing the button to actually opening the shutter is around a second - quite long. So, the basic idea is to make the complete room dark, open the shutter of the camera, drop the ball, ignite the photo flash after some time and close the shutter again. This way the real exposure time is given by the duration of the photo flash which was measured by me (using a DSO) to be less than 100us (better than 1/10, 000 second) when set up accrodingly. So, first we need to hold the ball and make sure it falls down each time at the same point, so using one`s own hands is not an option. Instead, since steel balls are magnetic, we built a dropping device made of an induction coil. (In the image on the right, white paper is wrapped around the coil. ) When switching off the current through the coil, the ball drops down. To prevent sparks and high voltage peaks, we were actually using a RC-damped MOSFET-based switch to slowly turn on/off the the current through the coil. Hence, using the switch time as start signal is not precise enough and a self-made light barrier was used. The light barrier consists of a orange LED and a photo diode. In front of the LED, a lens was introduced to focus the light onto the LED. Light interruption by the ball is the start signal. After the start signal, some time passes during which the ball...

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