An imaging system having a pixel-based imaging device is calibrated to take account of factors such as the non-linear response of a pixel cell and/or differences between pixel cells. A table is defined which includes an entry for each pixel cell. This is accessed for applying the conversion function determined for the pixel cell to a value output from the pixel cell to compute a corrected value representative of the radiation incident on the pixel cell. The conversion function can include a response function for each pixel cell which represents a non-linear relationship of pixel output values to pre-defined externally varied pixel input values e.g. reset voltage value (VR) and a scale parameter for each pixel cell which represents non-uniformity between pixel cells. Alternatively, the conversion function for each pixel cell can represent respective pixel output values in response to a series of different amounts of radiation applied to the imaging device.
