Method for shading an optical sensing element of an optical sensor. Reference values are sequentially obtained of the output signal of the sensing element while relatively moving the optical sensor including the sensing element over a shading reference target. An edge detection filter is applied to each of the reference values obtained in step a) in determining if each of the reference values is or is not from an optical defect on the shading calibration strip. An average is calculated of the reference values obtained while excluding each of the reference values determined to be from any optical defect in calculating the average. The output signal of the sensing element is calibrated to the shading reference target using at least the calculated average.