the field · the eyes

How often you blink

At rest people blink around 15 to 20 times a minute. Reading a screen it drops to roughly a third of that, the tear film breaks up between blinks, and that is most of what gets called digital eye strain. It is also the one thing about your eyes a webcam can measure honestly without a trained model, so it is the only thing this claims.

Something to follow

A target that drifts and changes distance continuously — nothing steps, nothing flashes. The quickest thing on it completes a cycle every 13 seconds. Follow it with your eyes and let your head stay still. Two minutes is plenty.

What this is not

It is a blink counter and a moving dot. It is not a diagnosis, not a treatment, and not vision therapy — that is a clinical practice with protocols and an optometrist attached. It does not measure gaze, fixation or pupil size, because a laptop camera without a trained landmark model cannot do those honestly, and saying otherwise would be the easiest lie on this page.

Nothing here flashes. The first sketch of this page did — fast colour flashes to provoke a response. Two things ruled it out. The pupil light reflex takes 200 to 500 ms to begin and about a second to finish, so a pupil cannot follow anything quicker than roughly 1 Hz and fast flashing measures nothing at all. And flashing is how you trigger a photosensitive seizure; the recognised ceiling is three per second, lower for saturated red. The fastest oscillation here is 0.080 Hz — about 38× under that ceiling, and it is continuous rather than stepped, so there is no flash to count. The safe rate turned out to be the only rate that could have worked.

Blink detection here is deliberately crude: contrast in a band where your eyes sit collapses when the lids close. Glasses, low light, a moving head or a beard in the box will all confuse it. Treat a single reading as a hint and the direction of change across a session as the signal.