Area-scan Exposure and Readout

Let us consider the case of an area-scan camera equipped with a CCD sensor of the interline transfer or frame transfer type. It is useful to consider two successive conditions: frame exposure, followed by frame readout.

Additionally, the area-scan CCD sensor can be temporarily set in a reset condition. In this condition, the light has no effect on the sensitive area of the sensor. Any electrical charge that could be contained in the photosites is cleared.

At some instant, the sensor leaves the reset condition to enter into the exposure condition (also known as integration). During this time, every photosite builds an electrical charge growing at a rate proportional to the light intensity it receives. The longer the exposure time, the larger the electrical charge.

Consequently, increasing the exposure time is a mean to increase the light sensitivity of the camera.

The end of the exposure time is marked by a special event usually referred to as the "transfer gate". At this instant, the individual electrical charges built by the photosites of the entire frame are set aside and made ready for transport towards the CCD sensor output.

Simultaneously, the photosites get emptied of any electrical charge. This sets the photosites in the same state as the reset feature does.

The transfer gate duration is short, virtually instantaneous for interline transfer type area-scan CCD sensors.

After the transfer gate, the CCD sensor enters the readout period. This takes a fixed amount of time to extract the individual electrical charges of the entire frame set aside at the transfer gate instant. The charges are converted to a voltage, serially conveyed outside the CCD sensor and made available at the camera output as a video signal.

Operational sequence