With a Rolling Shutter, only a few rows of pixels are exposed at one time. The camera builds a frame by reading out the most exposed row of pixels (and ceasing exposure of that row), starting exposure of the next unexposed row down in the ROI, then repeating the process on the next most exposed row and continuing until the frame is complete. After the bottom row of the ROI starts its exposure, the process “rolls” to the top row of the ROI to begin exposure of the next frame pixels.
The exposure down each frame, and from frame-to-frame, remains consistent due to this continuous read-out.
The row read-out rate is constant, so the longer the exposure setting, the greater the number of rows being exposed, or integrated, at a given time. Rows are added to the exposed area one at a time. The more time that a row spends being integrated, the greater the electrical charge built up in the row’s pixels and the brighter the output pixels will be. As each fully exposed row is read out, another row is added to the set of rows being integrated.
Example of Rolling Shutter
A very short exposure may be obtained by having only three rows of integration. This means that as each row is being read out, the three rows ahead of it are being exposed. As each row is read out, another row is added to the group of rows being integrated.
When to Use
Rolling Shutter provides evenly exposed image data with the greatest possible speed. Because of its speed, a camera in rolling shutter mode is programmed to free-run, that is it sends frames across the bus as fast as it can.
When Not to Use
Each row of pixels has a slightly different exposure start and end times from the adjacent rows, so Rolling Shutter can produce a distorted effect when imaging fast moving subjects, even with very short exposure times. The distortion is due to the comparatively lengthy process of readout compared to exposure.
As an example, to readout the entire PL-A780 frame requires approximately 250 milliseconds. While a short exposure may stop a moving object, the same object can move appreciably in the quarter second that it takes to readout the frame resulting in distortion in the direction of motion.
For Best Results
Rolling shutter should be used with constant illumination and with a static subject. If strobe illumination is required or the object is moving, the Fast Reset Shutter mode should be used.