1. Apparatus for determining the fitness of a document by determining its degree of soiling, comprising: (A) sensor means for providing a reflectance signal representing the intensity of light reflected from a sensed element of the face of the document; and (B) signal processing means responsive to the reflectance signals from the sensor corresponding to sensed elements on the face of the document to provide a "fit" or an "unfit" signal for a document, depending upon the degree of soiling, the signal processing means including: (1) a first comparator for comparing the level of the signal from the sensor means with a first predetermined threshold; (2) means for integrating the signal only over periods in which the signal level exceeds the first predetermined threshold, to provide a total reflectance signal indicative of the total reflectance of the brighter areas of the face of the document; (3) means for providing a duration signal representing the total time that the reflectance signal level exceeds the first threshold; and (4) means for comparing the total reflectance signal and the duration signal to provide a "fit" or "unfit" output signal.