Interesting links, thanks! So it's mostly an IR lamp, an IR sensor with an optical filter and the air to test in between. I don't understand from the article ho

Interesting links, thanks! So it's mostly an IR lamp, an IR sensor with an optical filter and the air to test in between. I don't understand from the article how long the waveguide actually is. Is this just a cylinder that is a few mm long bent around a corner, or is this some hollow tube much longer wound around the center of the sensor many times?