Exposure time is a period of the shutter from open to close. During the period, the light exposing on the chip’s photosensitive array and Photoelectric effect occurs, then photoelectric charges are produced. By the A/D transformation, the grayscale value of each pixel is displayed. Under a certain light intensity, the longer the shutter is open, the longer the exposure time, the brighter the image. Long exposure time can show the trajectory of slow moving objects on an image, and short exposure time can record things more accurately.
With the development of technology, the film gradually gave way to the chip, and the control method of the shutter also changes from mechanical control to electric control gradually. When using the electric control mode, a new exposure begins when the charge of the photoelectric unit is completely emptied, and when the charge of the photoelectric unit is transferred out, the exposure ends.
For the global-shutter CCD
sensor, the camera starts and stops exposure of all pixels in an array simultaneously. But for the rolling-shutter
CMOS sensors, the camera only exposure one line at a time, then to the second line, etc., they read out one line at a time, by the time they read the next line, the object has moved.