I don't knoe of a reference source fot this, but I tried building one about a year ago. I used photo-transistors hooked up to a comparator circuit. I found that the sensitivity variation from one phototransistor to the next was far too sloppy for the circuit to work. The only way I could get it to function on the bench was to put a trimmer on the bias of each transistor to adjust gain, but then I had the thermal non-linearities of four trimmers to worry about. I use a CCD camera now, so I didn't invest any more effort in the circuit, but there are probably ways to make it work.
- Matt