Optical tachometer?

[Jens Noriʇzsɔɥ]

The light sensor is likely too sluggish to give meaningful results. However, the new camera stop watches or even the brightness spectrum work pretty well (even more so on iDevices). IIRC, you could measure rates up to 15 Hz (60 Hz) on Android (on iOS/iPadOS*).

… and I just checked on Apple silicon: it's up to 15 Hz with a MacBook's front camera.