Digital cameras usually have both an electronic and a more traditional mechanical shutter. In this article we are going to see how an electronic shutter is realized in CMOS images sensors.

In CMOS image sensors the electric charge created in each pixel by incoming photons is read by an amplifier within the pixel itself. This analog information is then read out one row at a time. The time required for the reading operation of a whole frame is quite long. For instance, in a 3 megapixel sensor it takes about 82ms. This time corresponds to 1/12s. Compared to a typical shutter speed (1/15s - 1/1000s), such 1/12s is a very long time. So it is not feasible to reset all the rows (to clear out any accumulated signal), letting them integrate the incident photons over the set-up exposure time and then read them all out. This can not be done as the time required for the readout is not negligible compared to the exposure time. As an example, let us assume we selected an exposure time of 1/125s. After this amount of time, we start reading the frame. The first row is read after exactly 1/125s. While the first row is being read, all other rows keep transforming incoming radiations into electrons, thus effectively making the exposure longer. The last row will be read after 1/125+1/12s = 1/11s. So its real exposure time will not be the set-up 1/125s but unacceptably longer (1/11s).

An electronic shutter should work differently. The smart idea is resetting all the rows not together before starting integrating the incident light, but one at a time, in sequence. Each row is reset so that the time over which it will integrate the light will be exactly that selected by the user for the shutter speed. In the example above, each row is reset 1/125s before being readout. So here is what happens. The first row is reset and, after 1/125s, read out. It takes about 50 microseconds (1/20000s) to read out a row. So, 50 microseconds after the first row has been reset, the second row is reset, too. After further 50µs the third row is reset and so on. 1/125s after the first row has been reset, it is read out. 50µs after that, the second row will be read out, too; after still other 50µs it will be the turn of the third row and so on. In this manner, all rows are exposed exactly for 1/125s but will have an offset between each other in the time they are actually exposed. This "rolling" mechanism gives the name to this kind of shutter: "electronic rolling shutter". It can give exposure times from as low as 50µs up to an arbitrary amount of time (e.g. 1 minute).

The same rolling shutter can be used for shooting movies, which require 25-30 images per second. CMOS imagers usually also have a global shutter release mode to reset all the rows simultaneously to be combined with a mechanical shutter. However, electronic shutters are much more reliable than mechanical ones and don't require maintenance.

Andrea Ghilardelli runs an online photo retouching service. For more articles and to get your photographs transformed into something special, please visit his site: www.ilghila.com.

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