A new definition of picture degradation error caused by the sampling-transferring process in image scanners is described. The aim of this new error definition is to characterize the whole image transfer process. A method is shown based on calculation ofthis similarity error (which could also be called spectral distortion error) to estimate the possible character recognition error rate related to the optical transfer functions and sampling performance of the scanner. The increase of the probable reading error can be sufficiently estimated by the similarity error. Theoretical and experimental results are in good accordance. The different character types, scanners, and printer outputs can be characterized by this method, as well.