I am making an infrared break beam to detect insects, using an IR emitter LED and detector. The insects fall into a channel and then eventually cross the beam, which travels the length of the channel.
The insects are quite small, so I want to make the beam as narrow as possible; otherwise an insect could block part of the beam and go undetected. The infrared light from the LED has some spread. I was using a pinhole to screen the light into a beam.
Through trial and error with 3D printing I found a pinhole profile that works: pinholes of diameter 1.3mm and depth of 3mm on both the emitter and detector can detect an insect with a cross section of 3mm by 1mm.
However, I want to make the beam even smaller, to detect an insect with a cross section of 0.5mm by 0.5mm, and I would like to find a more rigorous way to define a pinhole profile. I am not sure what effect the pinhole diameter and depth have on the size or spread of the beam. I have three questions:
1) How do I choose a pinhole diameter? If a laser goes into a slit, there is diffraction, but is this a problem for an extended source like an LED?
2) How do I choose a pinhole depth? A thicker depth could reduce false readings from IR light reflected from the side of the channel.
3) Do I need pinholes on both the emitter and detector? I had some challenges with IR reflections from walls when the emitter did not have a pinhole, but that might be avoided if the pinhole is made sufficiently deep.
Right now the insect trap is being 3D printed, but eventually it will be manufactured (eg injection molded). I would like to avoid lasers or lenses for size and cost restrictions. I am not interested in focusing the LED, and I understand that I will only be accessing a fraction of the LED's output light.