An IR sensor consists of two parts, the emitter circuit, and the receiver circuit. This is collectively known as a photo-coupler or an optocoupler. The emitter is an IR led and the detector is an IR photodiode. The IR photodiode is sensitive to the ir light emitted by an ir led. The photo-diodes resistance and output voltage change in proportion to the IR light received. This is the underlying working principle of the IR sensor. The type of incidence can be direct incidence or indirect incidence. Indirect incidence, the IR led is placed in front of a photodiode with no obstacle in between. In indirect incidence, both the diodes are placed side by side with an opaque object in front of the sensor. The light from the IR led hits the opaque surface and reflects back to the photodiode.