The technology and practical problems of pixel-synchronous CCD data acquisition for optical metrology applications

Abstract

The geometrical accuracy of modern CCD sensors is typically better than 1/100 of the pixel period. For this reason CCDs are almost ideal tools for many optical metrology applications. Traditional video framegrabbers are ill suited for these applications because framegrabbers cannot acquire the video signals synchronously with the CCD's pixel clock. A universal CCD data acquisition system was developed, capable of digitizing video signals from almost any CCD camera, up to a speed of 20 MHz with either 8 or 10 bits of accuracy. Practical problems had to be solved for applications with standard CCIR CCD video cameras (560v x 600-800h resolution, 50 fields per second), and scientific high-resolution CCDs (1024v x 1024h resolution, 10-40 frames per second). The experimentally determined accuracy of the position of an object, measured with a CCD image sensor using 10-bit pixel- synchronous digitization is better than 1/100 of the pixel period, obtained by averaging many measurements. This experimental accuracy is not as good as the theoretical calculation of the precision indicates. The main reason for this are: pixel response non-uniformity, fabrication tolerances of the CCD sensor, non-ideal sampling of the video signal, and residual timing jitter in the pixel-synchronous sampling.