Bypass rolling shutter

With a rolling shutter image sensor, the lines of the sensor are read out one after the other. With moving objects or a moving camera, this leads to image distortions and sometimes even to very bizarre artifacts. With global reset release mode, it is often possible to circumvent this disadvantage of rolling shutter sensors by precisely controlling the illumination.

The interaction of light control and camera mode

The image sensor in Global Reset Mode

With the rolling shutter sensor, the reading of the sensor lines is started and ended one after the other, i.e. sequentially. If the rolling shutter sensor is operated in global reset release mode, however, the exposure starts at the same time for all lines of the sensor, as with the global shutter. The sensor is immediately open on all lines from the trigger signal. Especially when the camera is operated with flashed illumination, this is an advantage, because the delay before full opening is not necessary, the exposure of all lines starts immediately with the trigger signal and the flash can be fired without delay on the trigger event. The artifacts caused by sequential reading of the rolling shutter sensor do not appear as a result, since all pixels are exposed simultaneously during the flash time.

Special features of the Global Reset Release

However, the reading of the lines does not end at the same time. The lines at the end of the readout process are exposed to light for longer than those read out at the beginning and are brighter accordingly. To avoid this progression of brightness, it is important that only the flash illuminates the object and exposure stops before this final readout with partly open sensor.

The field of application of the global reset release is the flashed lighting, if the flash duration is chosen shorter or equal to the exposure time. The flash starts with the trigger signal in dark surroundings, constant (extraneous) light should be avoided. The exposure time is controlled by the length of the flash. Since there is complete darkness except for the flash, all pixels of the sensor are exposed equally. During the complete readout after the end of the flash, the environment is in darkness again, which avoids a brightness gradient. The next exposure does not take place until the complete readout process (in complete darkness) is finished.

Exposure and flash start simultaneously on all lines. Flash can be used during the nominal exposure time of the sensor (entire sensor open). The flash time determines the exposure time.

Precision is crucial

Since the exposure time is controlled by the flash, the timing of the flash is of utmost importance. A jitter of the strobe controller will become noticeable in brightness fluctuations in the image. Therefore, we recommend using a strobe controller with very high precision or using a pulse width modulated (PWM) strobe controller. These lighting controllers are built into the 4I/O system of our cameras as standard.