What is the "function reserve" of a photoelectric retroreflective sensor?
what is the "function reserve"? 


The "function reserve" of optical sensors (also known as the "operating margin" and "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.

Examples of the additional losses which can occur over the lifetime of sensor configuration include dust build-up on transmitter and/or receiver, and gradual alignment errors due to wear on the mechanical components onto which the optical transmitter and/or receiver may be attached.

Some optical sensors (e.g. used in factories) have additional status lights to indicate that their "function reserve" is above some preset level (e.g. a status light may indicate "function reserve >1.1" i.e. at least 110% of the minimum optical power needed, is being received). In other words, with that status light lit, the receiver isn't only just able to detect the transmitter, but is able to detect the transmitter with some power to spare.

Therefore if the receiver sensitivity slowly reduces (e.g. due to dust), or the transmitter output power slowly reduces (e.g. due to dust or misalignment), there is still some margin before the signal detection would fail to operate.  When that additional status light is no longer lit, this gives early warning that the optical link has deteriorated, even if the optical link still seems to be operating. That allows the maintenance staff to detect and schedule repairs, before a complete failure occurs.