How about a row of LEDs above the photodetectors spaced so the half power points of the radiated beams overlap, a diffuser, and an optical slit. Then shield from direct ambient relative to the detectors. Would require less power. IR to match the sensitivity of the probably phototransistor detectors would be ideal but you could see red ones. Once considered that and a fast processor on a precise 4 foot beam just for fun, never could justify the more precision/accuracy for my purposes. And something like Bluetooth so I could easily read the results on the bench.
