Totally agree @friolator. I think in this case the sensor would be placed before/after the gate and position just above the middle of the film so that it detects a constant surface, similar to how a mouse would.
We decided not to go this route for now since we didn’t have enough evidence that it would work at higher speeds and did not have time to test it.
I would love it if someone did a test with a small loop of film, though!