Method of discriminating sheet

Claim 1

In a method of dicriminating a sheet comprising passing one-by-one a plurality of sheets of the same type before at least one photodetector, said photodetector producing printing pattern signals or respective sheets thus passed before the photodetector, and determining the defect of the sheet by comparing said printing pattern signals with stored printing pattern signal, wherein the improvement comprises: generating pulses while each sheet is passing before said photodetector and counting the number of pulses for determining the length of each sheet; calculating and storing, in a first, reference storage means, a first comparison and discrimination value based on counted pulses and the printing pattern signals produced by said photodetector for one of said sheets; calculating and storing, in a second storage means, a second comparison and discrimination value based on counted pulses and the printing pattern signals produced by said photodetector for the next succeeding sheet which passes before said photodetector; comparing said second comparison and discrimination value with said first comparison and discrimination value; and when the second comparison and discrimination value is normal as determined by comparing said second comparison and discrimination value with said first comparison and discrimination value, erasing the first comparison and discrimination value from said first, reference storage means and transferring said second comparison and discrimination value from said second storage means to said first, reference storage means; whereby the comparison and discrimination value of a preceding sheet in said first, reference storage means is replaced by the comparison and discrimination value for the next succeeding normal sheet and each succeeding sheet passing before the photodetector is measured for normalcy by the comparison and discrimination value of said preceding sheet.