Proximity Sensor is a hardware level sensor to detect nearby proximity within the handset.

The sole purpose of having a such proximity sensor is for the screen to automatically turn off when the handset is positioned near the head when during on-call or taking the handset away from the head to see the on-screen status of the call.

This saves both power and prevents unintended touch events on the display.

In some handsets, the sensor is usually near by the ear-piece of the handset, if that was covered with your thumb or some object, the screen should dim and turn off (this is different to the screen time-out found in Settings > Display)

Further readings:

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