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Help: Synchronization problem of CMOS camera



Jan 1, 1970
I am using a CMOS camera to detect some sample which is illuminated
with pulsed laser. The pulse duration is 10nS, and the duration of
light pulse to be detected is 10-20nS. The pulse repeat once every
second, and the laser provide a TTL synchronization signal which rises
10nS before the light pulse, lasts for 10uS and fall down to zero.

The exposure time of the CMOS camera is set to be 100mS (it is the max
exposure time for the camera). I use the TTL signal to synchronize the
camera--after each rising edge plus a delay, I request the camera to
sample a single frame.

I had thought that, if the delay is 900-1000mS, the next light pulse
will fall into the exposure time of the camera, and thus i can use the
camera to detect the light pulse. However, the experiment showed
that, only when the delay is 100-300mS can the light pulse be
detected. It seems that the exposure period is before my sampling
request, instead of being after the request.

What troubles me more is that, only part of the pixels detected the
light and the part rolls up or down. For example, my camera is with
320*240 pixels. In the first detected frame, only pixels at row 1-40
detected the light; for the second frame, only pixels at row 31-70
detected the light, and row 51-90 for the third time... During the
whole period, the delay between the light pulse and my sampling
request is fixed.

I can not explain why and can not figure it out. Does anybody know the
reason of this phenomenon?

Thanks a million!

Bob May

Jan 1, 1970
A camera's imaging pixels are always looking at the target. This means that
you will indeed capture the image before the readout of the image.
I don't know the camera (and probably won't be familiar with it anyway) so I
don't know what the sequence of the taking of an image is. For video
cameras, the image is taken, then moved to a storage section of the chip and
finally read out from the chip. The exposure time is dependent upon the
time between the start of the imaging cycle and the readout of the image to
the storage section.
That you are seeing only part of the image (generally the lower part if you
are late with the light) lit up then you need to adjust the timing so that
the imaging section of the camera sees the whole light flash before the
exposure is completed.
This is sort of one of those things where you more adjust to the reality
than to try to figure out why such a reality exists contrary to your


Jan 1, 1970
I got the answer.
Unlike CCD, most CMOS camera adopt electronic rolling shutter. The
camera pixels are exposured raw by raw (or block by block).