Camera Cycle
A camera cycle is composed of two consecutive phases: the exposure phase and the readout phase.
Exposure phase
The exposure phase is the period of time during which the photocells of the imaging sensor integrate electric charges induced by the incoming photons.
For cameras having an electronic shutter, the exposure phase begins with a pixel reset action that clears all the sensor photocells. For permanent exposure cameras, i.e. cameras having no (or not using) the electronic shutter, the exposure phase begins immediately after the completion of the previous exposure phase.
For all types of cameras, the exposure phase terminates with a “pixel transfer” action. The accumulated charges in the photocell are transferred to the storage area for further readout. This action clears the photocells and new charge integration begins immediately.
Cameras having an electronic shutter have the capability to reset the pixels asynchronously and initiate a new exposure on request. These cameras are named asynchronous reset cameras.
Having the capability of controlling the time of the start of exposure (pixel reset) and the time of the end of exposure (pixel transfer) gives full control on:
- The timing of each image capture
- The sensitivity of the imaging sensor by selecting the exposure time
Readout phase
The readout phase is the period of time during which the total amount of electrical charges accumulated by each pixel is measured and delivered to the imaging sensor output.
The readout phase is not controlled by the frame grabber:
- It is automatically initiated after each pixel transfer.
- Its duration is fixed; it is determined by the amount of pixel data to be transferred and by the readout structure of the sensor (one or more taps, tap output data rate).
Some sensors provide the capability to select one or more region of interest (ROI) speeding up the readout since less data needs to be transferred.