Using rolling shutter with flash is more complex

Industrial cameras nowadays are mostly global shutter cameras. Rolling shutter cameras provide some benefits, while using flash exposure is a bit more complex

More Details about Rolling Shutter and Flash

Rolling shutter is a sensor property

Rolling shutter refers to the way in which data is read from the CMOS sensor. This is determined by the design of the sensor. This article covers the standard rolling shutter mode. If supported by the sensor, flash operation of a rolling shutter sensor in global reset release mode is preferable.

Despite the speed of scanning one line after the other („rolling“) is very high, it is important to note that the instant of imaging a single line will be different to the point of time of the next line imaging. As this works out without any effect in the final image with still sceneries, with moving objects you get geometric distortions (see example of rotating propeller), showing fast moving structures in an predictable, in the first moment yet surprising way.

As it takes some time to read out a whole sensor (and the whole sensor has always to be read out!) you need to make sure that light conditions are stable while reading the sensor. This restriction applies especially to using PWM driven lights or flash lighting with rolling shutter. Unstable light conditions will result in a horizontal line structured pattern noise. Rolling shutter cameras in general are more sensitive in their light response than global shutter ones.

Using strobe flash with rolling shutter

Scanning sensor lines takes time. Be aware of the time slots where sensor is only partly open. There are 2 general options for lighting:

  • Make sure your flash is ON and stable the whole period of time while scanning/exposing (continous light) . Minimum strobe ON time is scanning time. In this case, while flashing you will get geometric distortions as mentioned above. Exposure will be determined by camera exposure time and light intensity
  • If your strobe time is less than the exposure time of the camera, you need to start strobe ON earliest as soon all sensor lines are sensitve to light. This means you have to start strobe ON with a delay of scanning time. Use dark environment. Exposure will be determined by flash time/intensity.

Be aware of the timings

As shown here, after triggering only part of the sensor is sensitive to light (scanning time). As soon as scanning time has finished, all pixels are sensitive to light, the sensor is fully open. While being fully open this is the time where flashing should happen. In the final scanning time, less and less pixels are sensitive to light until the sensor is fully closed.

Rolling shutter pixel lines light sensitivity versus time

What happens if you flash in scanning time?

If your flash is ON with not all pixels are in light-gathering mode, some lines of the sensor do not see the flash light thus are not or less exposed to light, resulting parts of the image being darker.