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Steve Boyd
03-04-1996, 12:40 AM
Dear colleagues:

Due to the small size of the markers I would like to track to obtain 3D motion
analysis data, I have chosen to use fluorescent orange markers rather than the
traditional reflective tape (the tape is too difficult wrap on the small
markers, ~1.5 mm diameter, and reflective paint is hard to find). However,
using the fluorescent markers, I have encountered a problem that I hope someone
else may have experienced. The images of my fluorescent markers are fading from
visible to non-visable during tracking.

The problem is that I collect my video data at a certain fixed frequency (60,
120, 180, or 240 Hz). The fluorescent UV light source used to illuminate my
orange markers gets its A.C. power source from a wall socket which has a
non-constant frequency of close to 60 Hz. The small and fluctuating difference
between my video data collection frequency, and the UV light power source
frequency causes 'beats' in the intensity of my marker video image.

It should also be noted that that video cameras normally operate using red LEDs
that are synchronized with the camera collection frequency. Therefore, I have
access to a 'synch' signal and a 12 volt D.C. power supply from the camera to
operate the fluorescent light source. Unfortunately, I don't think 12 V D.C.
fluorescent lamps are available.

Some possible solutions I have come up with are as follows, each with their own
problem:

1. Use a constant UV light source (however, low intensity of the UV source
becomes a problem).

2. Synchronize the UV light with the output signal from the camera (I don't
know of an A.C. generator where the frequency can be controlled by an external
signal).

3. Control the frequency of the wall power source to be exactly 60 Hz (I've
been advised this is not easy to do).

If anyone out there has any comments or other possible solutions I would very
much appreciate the input. I'll post the results.

Sincerely,
Steve Boyd

M.Sc. Student at:
Human Performance Laboratory,
University of Calgary,
Alberta, Canada.

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boyd@kin.ucalgary.ca