PeterH:
It sounds as if, as well as being at the limit of how accurately it's feasible to measure an analog signal, you're also at the limit of how frequently you can take those measurements. This is not encouraging, and you may be constraining yourself into an intractable problem. I still feel that using a single brightness reading requires that reading to have unrealistically fine resolution and interpolating from multiple readings would enable you to get a better resolution.One way to relax the precision requirements would be to reduce the size of the scanned area. That would mean you have even stricter requirements to get the scanner positioned in the right place and I don't know whether that's feasible.
Is it possible to give a more general explanation of what you're trying to achieve and what overall system this solution has to fit within? It may be possible to apply some lateral thinking, such as applying a vernier technique using multiple sensors, but without any idea what the thing is you're trying to measure the position of, it's hard to know where to start.
As often is the case, this project is relative to how things are currently which is without any feedback or automatic control at all. It would be a shame for the final prognosis to be way under ambition but I don't expect that to happen...
The light 'spot' from these eyes looks something like 4mm x 1.2mm (fuzzy edges) and my experiments have placed the spot so that 4mm x 0.6mm is on one colour and 4mm x 0.6mm is on the other and a 0.5mm image movement would not in principle change this. Unfortunately, turning the sensor through 90 degrees would transform the available sampling 'window' but wouldn't be possible most of the time because the images will not always extend far enough, which is a prerequsite of a linear greyscale to position formula. Another unfortunate byproduct is a worse position resolution relative to greyscale (albeit potentially linear) for any image regardless (we can't win).
You mentioned the accuracy of sensor placement in an earlier post and I think it is worth mentioning that it is not important to put the sensor exactly half on half off the image and this is because only a 'piece of string' reference is required i.e. the inital value will become the 'ideal' and any wander is an error. Ultimately a mV:position ratio will be used to provide user friendly info and of course for any automatic correction (another project using steppers perhaps - I'm excited).
N.B. The current guise takes the first n readings and provides a plot of both extremes and the mean average over which the ADC output is ploted.
I am pleased to say that I have ordered the 1115 chip/breakout that Lefty suggested because I have to get my hands dirty with electronics per se and think it will be instructive to observe the outcome for our project using our hardware in our environment etc. I hasten to add, this project suits me down to the ground - I've always wanted to marry up programming and machinery and get into electronics proper. I'm trying to learn DesignSpark too (now V4) with varying success.
Thanks peterH