the queue on Fri, 5 Apr 2002 00:40:07 +0200, and nailed this to the
^ > the queue on Mon, 11 Mar 2002 17:21:48 +0100, and nailed this to the
^ > shed door:
^ > ^ One of the most important reasons is that CCD scanners have a linear
^ > ^ gamma response.
^ > Except that they don't. I've investigated several different makes and
^ > models of scanners in the last couple of years, and invariably they
^ > have an intrinsic gamma in the range 1.05 - 1.1. WHY, is a question I
^ > am still unable to answer. At the moment, I am looking into
^ > differential and integral non-linearity of the ADC as a possible
^ > cause, but I find it difficult to believe that this could account for
^ > such a substantial departure from linearity.
^ Interesting. After the "conversion" of photons into electrons, the electrons
^ are just collected and read-out. So that would point in the direction of a
^ non-linearity being introduced in the ADC. It can hardly be intentional,
^ because what's the benefit? On the other hand, not all CCDs are created
^ equal. Some have optional anti-blooming functionality, which could introduce
^ deviations (depending on how it's implemented).
I looked at anti-blooming as a possibile explanation. If it's there at
all, it takes the "gamma" the other way (I mean, a sub-linear response
to increasing intensity, rather than the super-linear which I have
^ Would be interesting to know which CCDs/scanners do this gamma "correction",
^ because it could perhaps increase the accuracy of programs like VueScan if
^ factored into the calculations.
And I would be interested to know if it is absent in higher-price
scanners. The ones I have measured it in so far:
HP 6100C/ScanJet 4C, 6200C, 6300C
Microtek ScanMaker X6USL, X12 USL
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