Why does the thru-beam fork sensor switch to dark mode?

The thru-beam fork sensor operates based on a pair of components: a transmitter (emitter) that sends out a beam of light and a receiver that detects this beam. When the light beam is uninterrupted, the sensor is in light mode and indicates that a target is present in the sensing area.

When the sensor switches to dark mode, this typically occurs because the receiver is no longer detecting the light signal from the emitter. This scenario can happen when there is a blockage (such as an object interrupting the beam), but specifically, a lack of the emitter signal itself can cause the sensor to switch to dark mode. In this case, the absence of a signal suggests that the light communication is disrupted, leading the sensor to determine that the environment is dark, hence activating dark mode.

The situation does not directly relate to power supply issues, ambient light interference, or faulty wiring impacting the communication between the emitter and receiver in the context of being in dark mode. Instead, it specifically emphasizes the failure of the signal from the emitter, which is the reason this choice is the most accurate in the context of the question.