PeterH:
..Is it correct that you're using the dark/light contrast to find the position of a boundary between dark and light parts of an image?
That's right.
How accurately can you position the sensor? Is it feasible to scan the sensor across the boundary? If so, I suspect you could get a much more precise position for the boundary by interpolating the position of the '50%' brightness point from a sequence of brightness readings in different positions rather than needing a single brightness reading to give you a hugely precise measurement of the proportion of dark/light areas within the image.
Unfortunately 'not really' possible given that a big problem would be the inertia in the hardware used to accomplish a scan. The material is moving at speeds of 20 fpm minimum (can be much faster) but in the direction that everything being equal (which of course cannot be so) will give a constant reading over the time taken to examine the analogue pin i.e. along the threshold of the two colours. Hence there is really just one shot possible and it has to be the best effort. In the current guise, a chart plots the result along the X axis as time passes.
I hasten to add: I have lashed together a temporary complete system to test the reliability of using a moving target and the result is indistinguishable from a completely stationary image and so I have no worries with a moving target in principle. I should also mention: The duration that a consistent image will be available for under the sensor is as long as a piece of string in that it could start to be very different within say 1mm movement (prorata time elapsed and speed).
Thanks a lot PeterH