Reminds me of the Bloomberg B-Unit authentication (https://www.bloomberg.com/professional/support/b-unit/, step 5) where it shows as flashing light to enroll a token generator for their terminal.
When I got my first Indiglo watch, I realized the backlight had such fast rise/fall times (as compared to its incandescent predecessors) that it could be used to transmit data. I imagined watches with photocells, held face-to-face, flickering their backlights at each other to "synchronize watches" before a special-ops mission or something.
A few years later, Timex DataLink came out, indeed using a photocell to receive data, but not using the backlight to transmit it! And in all the years since, nobody ever took advantage of that.
But here we seem to be, with phones, a million times more powerful and a million times less power-efficient.