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Key differences between rolling shutter and frame (global) shutter.
Article 115: Key differences between rolling shutter and frame (global) shutter.
SUMMARY:
This article describes the differences between the rolling shutter used in CMOS sensors and the frame (a.k.a. global) shutter used in CCD sensors.
APPLICABLE PRODUCTS :
All Camera Products • 
ANSWER:
Some types of CCD image sensors, such as the interline CCDs used by most PGR Imaging Products, provide an electronic shutter mechanism known as a frame (or global) shutter. In imagers that use a frame shutter, the entire imager is reset before integration to remove any residual signal in the sensor wells. The wells (pixels) then accumulate charge for some period of time, with the light collection starting and ending at exactly the same time for all pixels. At the end of the integration period (time during which light is collected), all charges are simultaneously transferred to light shielded areas of the sensor. The light shield prevents further accumulation of charge during the readout process. The charges are then shifted out of the light shielded areas of the sensor and read out.

This means that with a frame shutter sensor, the scene will be "frozen" in time, provided that the integration time is short enough i.e. there is no motion blur. An example of an image taken using a frame shutter is below.

The rolling shutter in a CMOS image sensor works differently, in that the photodiodes (pixels) do not collect light at the same time. All pixels in one row of the imager collect light during exactly the same period of time, but the time light collection starts and ends is slightly different for each row. The top row of the imager is the first one to start collecting the light and is the first one to finish collecting. The start and end of the light collection for each following row is slightly delayed. The total light collection time for each row is exactly the same, and the delay between rows is constant.

The time delay between a row being reset and a row being read is the integration time. By varying the amount of time between when the reset sweeps past a row and when the readout of the row takes place, the integration time can be controlled. Since the integration process moves through the image over some length of time, some motion blur may become apparent. An example of an image taken using a rolling shutter is below.
Rolling Shutter
Frame (Global) Shutter


The row-by-row process of light collection in a rolling shutter camera can also result in a switch noise effect, particularly at high gain settings. The image linked below shows this effect at maximum gain:

Switch Noise at Maximum Gain


RELATED ARTICLES :
KB Article 69: How is the image on my camera formed using the CCD?
KB Article 110: Key differences between imaging cameras.
KB Article 68: How is the electronic shutter on a CCD implemented?
KB Article 148: Key differences between CCD and CMOS imaging sensors.

ARTICLE INFO:
Article ID:
 115
Published:
 12:00:00 AM
Last Modified:
 8/31/2009 4:29:40 PM
Keywords:
 rolling, rolling shutter, global, global shutter, frame shutter, CMOS, CCD, photodiodes
Issue Type:
 Pre-sale


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