I'm studying the datasheet of Sony ILX511 linear CCD (PDF). On the Clock Timing Diagram, first 32 or 33 (depending on the mode) clock cycles give out dummy pixels, and only 18 of them are Optical black, which, as I understand, means they are covered and can be used as reference dark signal.
When I watched the output of this CCD with an oscilloscope, I saw that, although the dummy pixels indeed don't depend on illuminance of the sensor, they have consistent bias from the dark level. Moreover, this bias depends on the pixel number. The first dummy pixel shows a large dip in "measured illuminance" (if we could interpret it as such—actually simply higher voltage), and the rest dummy pixels show something like an exponential decrease of the bias (with a different sign than the first pixel), with the bias getting to zero when optical black pixels begin.
What is the purpose of these non-black dummy pixels? Are they actually useful for something, or is this simply an artifact of the way the CCD functions?
