Hey Phil, think there would there be any value in this?
Put two photodiodes in series. The node between them is like a voltage divider, so run it to a FET op-amp. Now, one photodiode is the input, and the other is servoed with an LED to match the current, which is what the op-amp is for. It also doubles as an logarithmic converter, so you get log(light) is an output voltage.
Seems to me you were recently asking about stupid-low-current CCS's, maybe this was already suggested... if not, you read it here first, it's in the public domain, you can't patent it! :-)
Tim