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Example of a Masked CCD Operating in an Unusual Mode
This system was built to work as part of a system performing a Cavity Ring-Down Spectroscopic (CRD) examination of Atmospheric Pollutants.
In this case, the CCD is required to detect the tiny fraction of exponentially-decaying light which is transmitted through one of the mirrors, and to operate in a mode which permits the decay constant to be determined. The resulting spectrum temporal resolution is around 2 microseconds.
 
The two images above show the CCD with a very precisely-aligned mask fitted to its surface, which leaves just 5 horizontal rows of the CCD uncovered. The CCD operation, is as follows:
- The CCD starts integrating upon receiving a trigger but ALSO the image is progressively shifted downwards during integration
- At the end of the integration the image is shifted to the store section
- Upon receipt of the next trigger the image is shifted up to precisely the same position as last time and the integration starts again
- Repeat a user-specified number of times (typically 1000) to build spectrum up
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