Rolling shutter is a method of image capture in which a still picture (in a still camera) or each frame of a video (in a video camera) is captured, not by taking a snapshot of the entire scene at single instant in time, but rather by scanning across the scene rapidly, either vertically or horizontally. In other words, not all parts of the image of the scene are recorded at exactly the same instant. (Though, during playback, the entire image of the scene is displayed at once, as if it represents a single instant in time.) This produces predictable distortions of fast-moving objects or rapid flashes of light. This is in contrast with "global shutter" in which the entire frame is captured at the same instant.The "Rolling Shutter" can be either mechanical or electronic. The advantage of this method is that the image sensor can continue to gather photons during the acquisition process, thus effectively increasing sensitivity. It is found on many digital still and video cameras using CMOS sensors. The effect is most noticeable when imaging extreme conditions of motion or the fast flashing of light.
Global Shutter
Global shutter mode in an image sensor allows all the sensor’s pixels to start exposing and stop exposing simultaneously for the programmed exposure period during every image acquisition. After the end of the exposure time, pixel data readout begins and proceeds row by row until all pixel data has been read. This produces non-distorted images without wobble or skewing. Global shutter sensors are typically used to capture high-speed moving objects.It can be compared to conventional lens shutters in analog film cameras. Like the iris in the human eye they resemble lens aperture and are probably what you have in mind when thinking of shutters.
The shutter is to open quick as lightining when released and to shut immediately at the end of exposure time. Between open and shut, the film segment to take the image is entirely exposed all at once (global exposure).
As shown in the following figure: in global shutter mode each pixel in the sensor begins and ends the exposure simultaneously, thus a large amount of memory is needed, the whole image can be stored in the memory after the exposure is ended and can be readout gradually. The manufacturing process of the sensor is relatively complex and the price is relatively expensive, but the advantage is that it can capture high-speed moving objects without distortion, and the application is more extensive.
Global shutter cameras are used in applications such as ball tracking, industrial automation, warehouse robots, drones,traffic monitoring,gesture recognition,AR&VR etc.
Rolling Shutter
Rolling shutter mode in a camera exposes the pixel rows one after the other, with a temporal offset from one row to the next. At first, the top row of the image starts collecting the light and finishes it. Then the next row starts collecting light. This causes delay in the ending and starting time of light collection for consecutive rows. The total light collection time for each row is exactly the same.In rolling shutter mode, different lines of the array are exposed at different times as the read out ‘wave’ sweeps through the sensor, as shown in the following figure: the first line exposes first, and after a readout time, the second line starts exposure, and so on. So, each line reads out and then the next line can be read. The rolling shutter sensor each pixel unit only need two transistors to transport electron, thus less heat production, low noise. Compared to the global shutter sensor, the structure of rolling shutter sensor is more simple and low cost, but because each line is not exposed at the same time, so it will produce distortion when capturing the high-speed moving objects.
The rolling shutter camera is predominantly used for capturing slow-moving objects such as agriculture tractors, slow speed conveyors, and standalone applications like kiosks, barcode scanners, etc.
HOW TO AVOID?
If the moving speed is not so high, and the brightness is varied slowly, the problem which discussed above has little affect to the image. Usually, using a global shutter sensor instead of rolling shutter sensor is the most fundamental and effective method in high-speed applications. However, in some cost-sensitive or noise-sensitive applications, or if the user has to use a rolling shutter sensor for other reason, they can use the flash to mitigate the effects. There several aspects need to be aware of when using the sync flash feature with the rolling shutter sensor: ● Not in all the exposure time that have strobe signal output, when the exposure time is too short and the readout time is too long, all the lines have no overlap exposure, there are no strobe signal output, and the strobe does not flash ● When the time of the strobe flash is shorter than the exposure time ● When the strobe signal output time is too short (μs level), some strobe’s performance can’t meet the high-speed switch requirement, so the strobe can’t catch the strobe signal
Post time: Nov-20-2022
