[Ivan’s] friend built a proximity sensor to switch his LED bench lighting off ... to give distance feedback base on how much of the light is reflected back to the detector.

[mircemk] shows how to create a simple non-contact proximity sensor using little more than an Arduino Nano board, and a convenient software library intended to measure the value of capacitors.

The current iPad does have an ambient light sensor, but the proximity sensor would further refine its environmental awareness in respect to accessories.