Glossary for detailled explanation - take your pick:
The possible switching distances for sensors depend strongly on the surface characteristics of the detecting object. To put the performances of devices of different producers on comparable basis, commonly the "Kodak Test Card" is used as reference object. This is a foto cardboard whose surface shows a defined degree of reflection. Thereby, the maximum distance that can be achieved properly by a light sensor at maximum sensibility is detected.
Wire Breakage Proof
An internal diode of a 3-wired sensor avoids the power input through the output cable and associated with this a dysfunction at wire breakage.