standard sensing object - Sensors Glossary

What is 'standard sensing object'? What does 'standard sensing object' mean?

The standard sensing object for both Through-beam Sensors and Retro-reflective Sensors is an opaque rod with a diameter larger than the length of a diagonal line of the optical system. In general, the diameter of the standard sensing object is the length of the diagonal line of the Emitter/Receiver lens for Through-beam Sensors, and the length of a diagonal line of the Reflector for Retro-reflective Sensors. For Diffuse-reflective Sensors, the standard sensing object is a sheet of white paper larger than the diameter of the emitted beam.

Photoelectric Sensors