Let's assume it's supersonic (kuh-RACK-ack), 800 m.p.h.
800 * 5280 * 12 = 50,688,000 in / h
50,688,000 / 60 / 60 = 14080 in / sec --> 72 usec / in.
1 inch is pretty long, let's say 1/4 in.
So, its theoretical time in the window (a plane) would be 18 usec.
I looked at the specs for a couple of phototransistors and their rise and fall times were 5 usec each, each. So, 5 usec will pass before the photo-t output changes state, but if we assume the worst, 2X (??), 10usec in would still leave us with 8 usec of presence.
And that's > than the 0.13usec (min) as stated by CrossRoads.
And that doesn't include the processing time to trigger an LED or whatever is supposed to happen as a result.
So, velocity101, what's the "end game"?
** What needs to happen as a result of this detection event? **
What's the design goal?
You'll have a row of emitters shining into a row of detectors (AND'ed together or something) and any beam-break (!) results an event?