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Rolling and global shutter (CMOS sensors)

For machine vision use, CMOS sensors that feature a global shutter enable sharp images of fast moving objects as all pixels are exposed at the same time in the same way as progressive scan CCD sensors. However not all CMOS sensors use Global shutters, some use a rolling shutter.

In a "rolling shutter" sensor, the start and end of exposure on each row or column or individual pixel happens sequentially, so not all the pixels are exposed at the same time. It can take up to 1/frame rate for all of the pixels on the sensor to become active. The effect will be noticed if the object is moving. See image below.

Rolling shutter sensors allow for a pixel to be designed with fewer transistors, reducing cost, and in some circumstances, increasing the well capacity and hence the quality. There are some modes of operation that can be implemented in rolling shutter cameras, such as a global start.

This is where all the pixels are turned on at the start of exposure, but are turned off in sequence. This can lead to a longer exposure at the bottom of the image, resulting in some blurring. However with a carefully designed system that uses a short exposure strobe light at the start of the sensor's exposure, it can yield good results in dark areas.

Taking this workaround further, some cameras that use rolling shutter sensors provide a strobe output signal that indicates when all pixels are being exposed and the point at which the first pixels are being turned off. If a strobe is light is activated during this time and the camera is in a dark environment, again a good image of a fast moving part can be obtained. The drawback to this technique is that the strobe might be delayed by up to half a frame after the trigger is received by the camera. Therefore best practice is to use a global shutter sensor wherever possible.