Some characteristics of a TDI scanner

Abstract

We develop methods to characterize the noise, positional accuracy, and color shift for a camera type scanner. These methods have been applied to several similar scanners based on an experimental Time Delay Integration chip developed at IBM Research. The scanner is further discussed in the paper by Hon-Sum Wong, et al, in this session. To measure noise, we scan a field twice under identical conditions. We then consider the noise to be the difference in these measurements, computed pixel by pixel. All data is measured with a 12 bit a/d converter (0-4095). For a typical dark current correction we find a standard deviation of 1.97. For a white field, we find the standard deviation of the measurements to be 3.65 out of the total range of 4095 (this is .089%). To measure color shift and positional accuracy we scan a transmissive test image which consists of an aluminized glass plate in which 6 rows of 6 diamonds per row, all on a square grid, were engraved. The position of a diamond may be accurately obtained for each color scan by summing the gray values in the various rows and in the various columns of an area containing one diamond. We find color shifts of up to .5 pixel. The position accuracy is good to .2 pixel if the slight rotation of the TDI device relative to the direction of transport is taken into account.