Reflective Presence Optical Sensor
This system is based on the principle that light will impinge on 2 different sensors under different conditions.
When there is no object present, the Light beam will reflect from the reflective surface on the far side of the object path, through the second lens, to sensor 1.
When an object is in the path, the Light is reflected through a much shorter path to sensor 2.
Ambient Light is suppressed because the transducible event is the relative light levels of sensors 1 and 2. As long as the light level detected by sensor 2 is much higher than the light level detected by sensor 1, there is an object present in the optical path.