dya@unccvax.UUCP (York David Anthony @ W06BF-TV, Concord, NC) (09/14/88)
Many people have written to ask about the so-called "Kell" factor particularly about the statement that a "ccd array has a K-factor of 1.0." This is my response (I thought it would be of general interest): The Kell factor for electron-scanned pickup tubes and CRTs comes from the fact that there is a degradation due to the energy distribution of the scanning spot. The ideal scanning spot would cause a modulation transfer function (MTF) which was unity up to the Nyquist limit for the number of theoretical pixels or lines in use, and zero thereafter. However, the scanning spot has a Gaussian distribution. The Kell factor attempts to account for the falling amplitude response with an increase in spatial frequency, and is actually pretty poor at characterising video systems. Nevertheless, the Kell factor is a good first approximation in visual system response to square wave gratings and television systems. In vertical resolution, the Kell factor also is a "fudge factor" designed to describe the degradation due to vertical interlacing. In the vacuum tube pickup tube, lateral leakage between frames (where there are rows of undriven-to-cathode-potential regions due to interlace) causes a loss of vertical spatial resolution. Aliasing components with the interlace structure degrade the apparent vertical resolution even more. What I meant about the CCD array having a Kell factor nearly 1.0 is that each individual sensing element could be (and generally is) rectangular, and thus, a sine wave grating at or near the Nyquist limit for the CCD array in use would have an amplitude response of 1.0 because the scanning "spot" is synthetic and is shaped damn nearly perfectly. Networks outside the pickup array can provide the necessary shaping to ensure perfect sin(x)/(x) shaping with alternate zeroes on adjacent lines, thus giving perfect interlace. Horizontally, the perfect sampling is provided by the electrical and optical isolation of adjacent elements. In this regard, the CCD array is a perfect pickup device. The interested student should probably read "Image Quality: A Comparison of Photographic and Television Systems" by Doctor Otto Schade. This gives a much more precise and theoretically correct treatment of scanning spot energy distribution and its effect on TV spatial resolution. It was reprinted in the entirety in the June, 1987 SMPTE Journal; it was also published by RCA in 1975. Hope this helps... York David Anthony DataSpan, Inc