The second type of optical sensor, here’s Vishay app note on how do they operate and their considerations when using one of these on your projects.
These sensors integrate an infrared emitter and photo detector in a single package. The most common types of optical sensors are transmissive and reflective sensors. Both types detect the presence of an object without any mechanical or electrical contact. The output signal of the sensor is used to control various functions of an application
Reflective sensors incorporate an infrared emitter and photo detector adjacent to each other. When an object is in the sensing area, the emitted light is reflected back towards the photo detector, the amount of light energy reaching the detector increases. This change in light energy or photo current is similarly usedan input signal in the
application.