These infrared photodiodes are a proven light sensor in BEAM technology.
IR emitter and receiver in one package for close-range optical detection (sold individually)
This sensor uses infrared technology to detect an object at 5mm by determining the difference between low reflective conditions (i.e. black) and high reflective conditions (i.e. white).
The Pololu IR beacon is a compact board that is used in pairs to allow robots to locate each other.
When the QTI sensor is over a dark surface, the reflectivity is very low; when the QTI is over a light surface, the reflectivity is very high and will cause a different reading from the sensor.