Also called "The seventeenth monkey". It has a front part (where IR light goes in) and a rear part (where IR light comes out).
Low level diagnostics, like "I turned on the LED on the BOM, but the other robot can't see it - is it really on?".
Cameras can be used for this, but that can be inconvenient, and not all cameras are sensitive to IR light (video cameras and cellphone cameras seem to work best).
A photo diode in series with an LED. The photo diode limits the current to about 20mA, largely regardless of the voltage. A series resistor of 2x50Ω is added for additional safety.
A switch, and the two LEDs, each with its own 200Ω series resistor in parallel.
"Does the diagnostic monkey work?"
The front part can be tested using the remote control for the projector (or probably any other IR remote). The back part can be tested using a camera (if it is sensitive to IR light). Of course, the parts can be used to test each other.
The sensitivity of the front part is a bit low. It has to be aligned quite precisely at >10cm. It might make sense to make it more sensitive at low light intensities using a transistor.