A through beam arrangement consists of a receiver located within the line-of-sight of the transmitter. In this mode, an object is detected when the light beam is blocked from getting to the receiver from the transmitter. They work by detecting an object breaking a laser beam - but you usually have to set up a laser on one end and a sensor on the other. Unless your sensor is big, it can easily get out of alignment.

A through beam arrangement consists of a receiver located within the line-of-sight of the transmitter. In this mode, an object is detected when the light beam is blocked from getting to the receiver from the transmitter. They work by detecting an object breaking a laser beam - but you usually have to set up a laser on one end and a sensor on the other. Unless your sensor is big, it can easily get out of alignment.