What is the photoelectic sensors?！

Photoelectric Sensors detect objects, changes in surface conditions, and other items through a variety of optical properties.
A Photoelectric Sensor consists primarily of an Emitter for emitting light and a Receiver for receiving light. When emitted light is interrupted or reflected by the sensing object, it changes the amount of light that arrives at the Receiver. The Receiver detects this change and converts it to an electrical output. The light source for the majority of Photoelectric Sensors is infrared or visible light (generally red, or green/blue for identifying colors).
Photoelectric Sensors are classified as shown in the figure below

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