Full Well Capacity

Full well capacity defines the amount of charge an individual pixel can hold before saturating. Saturation must be avoided in high-performance CCD imaging because it diminishes the quantitative ability of the CCD and produces image smearing due to a phenomenon known as blooming.

Full well is dependent upon the pixel size of the CCD, whether or not multi-pinned-phase (MPP) mode is used, and the operating voltages used on the CCD. Larger full wells are found on large-pixel devices. MPP mode reduces full well since a large gate potential is not applied to the CCD electrodes during integration. This has the intended effect of reducing dark current, but it can suffer the penalty of reduced full well. This tradeoff should be considered when selecting a particular CCD for high-performance imaging applications.

Photometrics provides a test report with every camera listing the full well for that particular CCD. This value has been measured at the factory and the camera gain has been adjusted so that the full range of the ADC matches the single-pixel linear full well capacity of the CCD at 1x gain. Only the linear range of the full well capacity is used since this is where the CCD functions as a radiometric detector and produces quantitative results. For this reason, full well capacities reported for Photometrics cameras may be lower than those found in CCD manufacturers data sheets.