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MultiCam Boards Documentation > MultiCam Documentation > MultiCam User Guide > General Topics > Line-Scan Inspection Topics > Exposure Issues > Permanent Exposure

Permanent Exposure

In the permanent exposure situation, the CCD sensor is never brought into the pixel reset condition.

The line-scan camera hosting the sensor experiences a periodical cycle paced by successive instants we will refer to as "trigger pulses". The trigger pulses occur at the line frequency. Each trigger pulse causes a transfer gate event followed by a readout process.

The trigger pulses are issued by the frame grabber according to rules that will become clear later.

The following figure shows the operational sequence at some line frequency.

Between two successive trigger pulses, the sensor integrates the light. The sensitivity of the camera depends on the line period separating the pulses. The lower the line frequency, the higher the response to a given scene.

The following figure shows the operation sequence at some slower line frequency.

It is seen that the pixel response is higher.

Suppose that we are willing to adjust the aspect ratio by varying the line frequency in order to keep it adequately coupled to the web speed. With the permanently exposed sensor, we are faced to some response variation when the web speed is changing.