Well, I dunno about CCDs directly. But as an old spacecraft hand, we used to fly many rather high-specification A-to-Ds. These were always "guaranteed linear", "guaranteed monotone", "guaranteed quantization", etc., of course. I can tell you that even many expensive A2Ds are frequently none of those things, especially in the low order couple of bits. When we were finally able to fly microprocessors, starting in the mid 70s, we were finally able to program/correct some of the telemetry encoding in within the satellite and also in the ground support equipment to make up for the f*****g A2Ds. I've always been a bit skeptical of CCD-based photography because of the inherent (non)linearity and (non)monotonicity of a process which captures data on a CCD, which is essentially a light-driven A2D (i.e., "light in - digital out"). That says nothing about the inherent noise floor of inexpensive CCDs, which *will* affect the linearity and monotonicity of the output --- simple algebra and finite sampling times, people, even if you have software which "purifies" the colors and "masks" the noise. IMHO, the CCD problem is necessarily more dicey than more usual (Well, maybe not so "more usual" today!) "voltage in - digital out" A2Ds. Put another way: You do see very significant differences in results from digital cameras being set at different "ISO"s, yes? All that's going on is that the sampling times are varied and noise cancellation algorithms are applied to the raw data. But would one actually imagine that one's image encoding is "linear", just because someone did a statistical noise cancellation on the data? Non-sequitir. That ain't the way electronics works. (All of this is to say nothing at all about the linearity/monitonicity of display devices. Another rant for another day.) But, sorry. In the interest of disclosure, I've never actually instrumented all this. So the whole argument is totally subject to refutation by someone who has actually done the research, say, by evaluating CCDs in a cryostat in an RF-shielded room, then repeating in ambient over a statistically significant period / number of cycles. My gut says I'm willing to bet anyone a quarter that most of the production devices are pretty raunchy at least in the low order 2 bits or so. - Don Feinberg ducque@xxxxxxxxxxxxxx -------------------------------------- There is always talk about the linear response of electronic imaging devices such as CCDs, etc. while the pitfalls on non-linear response of films are denigrated. I'd like to know whether digital systems truly, completely and irrevocably provide straight line response throughout their range. --------------------------------------
