What is a through-beam sensor?

What is a through-beam sensor? 

Basically in this sensor, light passes to the receiving element and this element immediately produces an electrical output. This means the transmitter and receiver will be at different ends. The sensor will give a different output when there is an object in it; And the sensor will give a different output when there is no object in it. The output can be NO or NC, and PNP or NPN.

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