2. An apparatus according to claim 1,in which said 25 signal generating means further generates a printed
region signal which identifies portions of the sheet which are printed regions according to the type of the sheet, said calculating means further calculating an output of said detecting means in accordance with the printed region signal and calculating an amount of light having interacted with the printed regions of the sheet, and wherein said comparing means further compares the amount of light having interacted with the printed regions with printed region reference signals.