1. A method for determining an offset correction level for a light sensitive sensor used to record the intensity of exposure light reflected from the surface of a document, comprising the steps of: measuring the sensor response when the sensor is exposed to light reflected from a first target having a first, non-zero reflectance level; measuring the sensor response when the sensor is exposed to light reflected from a second target, the second target having a second reflectance level greater than the first reflectance level; and calculating, only as a function of the sensor response to the first target and the sensor response to the second target and the reflectances of the first and second targets, the response of the sensor to light reflected from a zero reflectance target, and thereby enabling the calculated response to be used as the offset correction level for the sensor.