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Rolling shutter

The term "shutter" originates from the old days of photo cameras. After the opening of the shutter the image, created by the lens, encountered the film. At the end of the exposure time, the shutter was closed and the film was forwarded. Even today, in the era of digital cameras, many photo cameras are based on this method. Only the film is replaced by a CMOS sensor or CCD sensor.

For video and industrial cameras, and thus cameras that output image sequences, it is better to use an electronic shutter. They are very fast and do not wear out. Fortunately, most CMOS sensors and CCD sensors have such an electronic shutter. In the case of CCD sensors, building a shutter is very easy - it is part of their nature.

A rolling shutter is typical for CMOS sensors

In contrast to this, CMOS sensors have trouble dealing with an electronic shutter. Most CMOS sensors are only able to expose the image line by line, starting at the top. Therefore, it is called a "rolling shutter".

Thus, the top line of an image is taken first, while the bottom line is taken last. If, in the meantime, the object and/or the industrial cameras has moved, the image is "skewed". The flickr blog Rolling, Rolling, Rolling Shutter shows some impressive examples for this effect.

The higher the frame rate, the faster the lines are processed and the weaker the skewing effect.

In the case of consumer cameras, a rolling shutter is rarely a problem. In the context of industrial USB cameras, FireWire cameras or GigE cameras, however, a rolling shutter can get in the way. Since the machine vision market is becoming more and more interesting for the manufacturers of CMOS sensors, they are starting to provide sensors with a global shutter.