Abstract: The present invention is a method of compensating for the offset present in image signals produced by an image processing apparatus, and more particularly to accurately characterizing the offset, gain and higher order responses of image sensing elements to enable correction thereof in an image sensing device or scanner. The method utilizes intermediate (grey) reflectance targets to measure the response of each sensor element and then, using the measured responses, fits a characteristic curve to the responses to calculate the response of the sensor element, including a zero reflectance situation, to characterize an accurate offset level, gain, and higher order response for the sensor. Moreover, additional reflectance targets and response may be employed so as to enable the characterization of image sensing elements exhibiting non-linear responses to linearly increasing light intensities.

Abstract: A photosensor for measuring the reflectivity of a surface is calibrated for consistent gain and offset. The reflectivities of a plurality of regions of a test surface are measured. An average of the measured reflectivities is calculated, and a threshold value related to the average is derived therefrom. A revised average of the measured reflectivities is then calculated, the revised average being an average of measured reflectivities only of regions having reflectivities of a predetermined relationship to the threshold value. Subsequent signals from the photosensor are then revised according to a function related to the revised average.