Fleetie wrote:

Regarding the other poster not being about to see the flashing: I've had
a technique for detecting very rapidly-flashing lights since I was about
10 and starting in electronics with 555 timers making bulbs and LEDs
flash at varying rates. You kind of "flick" your eyeball as quick as you
can between looking up, above the flashing light, and then down, looking
below it. What you see is a broken (dashed, or dotted) trace that gets
left on the retina, which persistence of vision allows you to see, and
analyse. So you not only get to estimate the RATE (Hz) of flashing, but
also, the mark-space ratio, or duty cycle.

Just wave a hand around in front of it.