My guess is that the variations say more about the lack of any enclosure so that light variations play a part.
I would suggest a CED with the infra-red lighting and sensors in a portable "box" which keeps all other light out. Board on front with tunnel to poke barrel in and tunnel at back.
Only issue is the need for battery power for the infrared illuminators.
First versions could be cumbersome to transport but there are enough engineers on the circuit to come up with better configurations.