Function reserve mentioned in the data sheet of optical sensor.
The function reserve of optical sensors (operating margin or functional reserve) is the ratio of how much extra power the optical receiver is receiving, compared to the minimum power it needs to detect the transmitter (i.e. the minimum power it needs to function, hence the term function(al) reserve).
Therefore the larger the distance between a given optical transmitter and receiver, the lower the functional reserve, since there are greater optical losses and the less additional losses can be accepted before the transmitter is no longer detected.
For example:
