A method and an apparatus to determine a class of a medium on which an image is formed. The method includes emitting light to the medium; sensing the light affected by the medium; collecting a first predetermined number of features which are represented by a relationship between a parameter and an intensity...http://www.google.com.au/patents/US7145160?utm_source=gb-gplus-sharePatent US7145160 - Method and apparatus to discriminate the class of medium to form image
