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Hi Shalin,
> I would like to understand how the scientific CCD cameras do sampling in
> time. All CCD pixels are the same size, so the sampling is uniform in space.
> But, do we get uniform sampling in time as well? When the camera specs say
> that the frame rate is 100 fps, are we really getting one image every 1/100
> s. Does the camera isolate itself from non-deterministic control by
> computer?
Depends on the camera and how it is operated. Specifically, most scientific grade cameras will have the option to use a software trigger and internal trigger. Using software trigger will cause the interval between images to be determined by the computer. Internal triggers are generated by a clock inside the camera, this clock should be very regular. The frame rate should be such that the camera is ready to fire again upon the next trigger. Unless your camera's manufacturer (please do check with them) made a mistake, the frame rate 'should' be as specified.
Things get more interesting with the new CMOS cameras that can operate in global shutter or rolling shutter mode, which can cause different parts of the image to be exposed at different times.
> We have a time-sequence of beating sea urchin sperm, and I tried to estimate
> the beating frequency by Fourier transforming the intensity variation along
> time at a given pixel. The results are quite off the mark from what I would
> expect. It seems there is jitter in temporal sampling.
If your camera has a trigger out TTL, then you could visualize the camera triggers on an oscilloscope. This would still not let you check if the CCD indeed starts and ends exposure when the camera clock fires. I'd be interested to hear about experimental setups to check the regularity with which images are exposed at high frame rates.
Best,
Nico
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