Guaranteed line of sight is, I'm afraid, not an option. What I'm wanting here is to transfer data by infrared but only receive it when the line of sight condition is met. The reason I hope to avoid reflections is because they could cause confusion and allow the passing of data when the detector is not in line of sight of the emitter, which I don't want. Reflections will always be much weaker than direct light, so I should be able to avoid them as long as I can set (and tinker with) a minimum sensitivity on the detector. That's why I'm wondering what the circuitry inside those TSOP devices actually is, because if I can build something similar I should be able to modify it where necessary. Any tips for how I could go about finding diagrams of it or similar circuits?