[fragmede]

Is a micro with a PLC (or just a fast enough adc+CPU these days) connected to a photodiode fast enough to do it in software?

Actually it seems modern devices use a CCD and then process the image.

[convolvatron]

i think you'd have a hard time determining the direction from just one sample

[fragmede]

for the photodiode you'd use to perpendicular sensors