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I'm working on a cmos image sensor that has a electronic rolling shutter, at this time I'm operating at 15fps and getting RAW data from it.

I was wondering how to combine a xenon flash with the rolling shutter operation, of what I understand regarding cmos image sensors is that they expose the sensor row by row hence all the pixels are not exposed at the same time.

I haven't found much reading material on this online and hence don't know when should I trigger the flash so that all the pixels in the sensor have even exposure.

Is xenon flash feasible at all with CMOS sensors with rolling shutters?

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I'm afraid this simply isn't going to work unless your flash duration is longer than 1/15th of a second (very unlikely unless you're discharging a truly giant capacitor) , or you can pulse the flash over the 1/15th of a second period.

The reason for this is simply that when your flash duration is shorter than the length of time it takes to sample the image then the flash output will have ceased by the time part of the image is sampled and thus it will appear dark.

The same thing occurs when using a mechanical shutter once you go above the sync speed (usually 1/250s). Beyond this point the shutter starts to close before it is fully open so there is no point in time where the entire sensor is exposed to the light flash so some of the image will be dark.

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  • \$\begingroup\$ Sorry, I'm a noob in photography, what is sync speed? Is it something related to the flash. \$\endgroup\$
    – Kevin Boyd
    Jan 13, 2011 at 12:22
  • \$\begingroup\$ Sync speed is a property of the shutter and it means the shortest exposure time for which the shutter is fully open. Any shorter than this and the shutter starts to close before it is fully open. Eventually you end up with a narrow slit moving across the image, which is very similar to your electronic rolling shutter. \$\endgroup\$
    – Matt Grum
    Jan 13, 2011 at 13:20
  • \$\begingroup\$ See: photo.stackexchange.com/questions/1615 \$\endgroup\$
    – mattdm
    Jan 13, 2011 at 13:21
  • \$\begingroup\$ @Matt Grum and mattdm: are there any online references that will enable me to understand the internal workings of a cmos image sensor? \$\endgroup\$
    – Kevin Boyd
    Jan 13, 2011 at 13:46
  • \$\begingroup\$ @kevin The internal workings of a CMOS sensor aren't really relevant to your problem. Essentially it's the reading-the-image-line-by-line approach (which is not unique to CMOS) which causes problems. If the rows of pixels are only refreshed every 1/15s (i.e. your exposure time is 1/15s) and your flash duration is very short then you could just get away with firing the flash when the first row is read... \$\endgroup\$
    – Matt Grum
    Jan 13, 2011 at 15:01
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The rolling shutter principle is similar to the focal-plane shutter mechanism. For high speed photography (= exposure times shorter than the sync-speed, about 1/250 sec for most SLRs), the sensor is not exposed in whole at the same time. Rather, the first curtain is opening and then the second curtain follows it to close the exposure in a delay equal to the selected shutter speed.

High speed sync flash is able to fire continuously for the required amount of time while the open slit is traveling across the sensor. This is done by firing the flash at a very high rate (tens of thousands of fires a second), so effectively it lights the scene as long as the sensor is exposed.

You may be able to use similar mechanism in your case.

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  • \$\begingroup\$ If I do use high speed sync flash, where do I fire. I mean at what position in the image sensor exposure should I trigger the flash and generally what would be the time period of the flash. \$\endgroup\$
    – Kevin Boyd
    Jan 13, 2011 at 11:57
  • \$\begingroup\$ The point of HSS is that it mimics continuos lighting. See photo.stackexchange.com/questions/1615 \$\endgroup\$
    – mattdm
    Jan 13, 2011 at 13:22
  • \$\begingroup\$ High a sync speed flash acts like a continuous light source (by turning on and off very quickly) so you need to fire it at some point before the frame you want to shoot, keep it on for the duration of the readout (at least 1/15s in your case) and then turn it off. There may have been a misunderstanding, so if you could describe in more detail your particular scenario we could probably help better! \$\endgroup\$
    – Matt Grum
    Jan 13, 2011 at 13:32
  • \$\begingroup\$ @Matt Grum: What details do you need? \$\endgroup\$
    – Kevin Boyd
    Jan 13, 2011 at 13:47
  • \$\begingroup\$ @Kevin what sort of camera are you using? What are you taking photos of? Why are you trying to get stills from a system with a rolling shutter? Are you even trying to take stills or are you shooting video? \$\endgroup\$
    – Matt Grum
    Jan 13, 2011 at 14:47

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