Focus and signal to noise measurement routines in input scanners

Abstract

Imaging quality is measured in an electronic input scanner using the scanner imaging processing electronics. A contrast transfer function (CTF) of an optical system in an electronic input scanner is measured to determine focus quality by scanning a test pattern of lines, extending generally parallel with the slow scan direction, with no relative motion between the scanning element and the test pattern. The output of the scanning process is directed through the device thresholding operation. A series of successive scan lines are produced in this manner, each line produced with a successively incremented threshold level applied. The output of the thresholding device is examined at a plurality of locations to determine maximum and minimum transition points associated with the variation of threshold levels. Upon detection of the maximum and minimum, CTF is calculated using the threshold transition values. Signal to noise ratio is measured similarly by scanning a test pattern having a uniform gray image. A series of successive scan lines are produced, each line with a successively incremented threshold level applied. Noise causes producing a non-uniform scan line. The noise is measured by examining the number of lines where a non-uniform output is produced. Upon detection of the first and last non-uniform lines, signal to noise ratio can be calculated. For both tests, the scan lines produced may be printed in a standard binary printer allowing a qualitative examination of the results.