[Groxx]

I wish I could find more things talking about this, but there's this classic demo of the technique: https://cs.nyu.edu/~jhan/ledtouch/index.html

It has always felt under-utilized to me. Maybe it needs power-sensing hardware that's too sensitive / too fast for most scenarios / costs?

[JohnFen]

It's actually not that hard to use LEDs in this way. I think it's not done a lot because in applications that need a light sensor, a purpose-built light sensor works a whole lot better, and it's rare that space is such a premium that having one is an issue.

Glasses may be a use case where space is at a premium, though, so it could make sense there.