Somewhere in each of your eyes is a spot that's completely blind — the place where the optic nerve leaves the back of the eye, with no light-sensing cells on it at all. You never notice, because your brain quietly fills the gap with a guess.
I built this while trying to understand that, and how vision fades from sharp in the middle to rough at the edges. It's a demonstration, not a test. It can help you find roughly where your own blind spot sits, and let you watch faint dots appear at the edges of what you can see.
It can't tell you whether your eyes are healthy, and it won't pretend to. If anything about your sight ever worries you, an eye doctor is the only real answer.
Where the optic nerve exits, there's nothing to catch light. The gap is real; you just never see it.
Hold any credit or ID card flat against the screen and drag until the widths line up. That's how the page works out how big a millimetre is on your particular display.
Use a tape if you can. A rough guess here throws off every angle, so it's worth thirty seconds.
Stare dead at the cross and don't let your eye wander. Every so often a dot appears somewhere off to the side — press space the moment you notice one. When a dot lands on your blind spot, you simply won't see it, even though it's right there. That's the demo finding the gap.
Same idea, now spread across your side vision. Keep your eye on the cross the whole time. Dots appear here and there, faint and quick — press space each time you catch one. A few ticks come with no dot at all, so don't guess.
Anything you miss gets shown once more at the end, since missing something once barely means anything.
A few honest notes. This shows one brightness of dot and asks whether you saw it — real testing measures the dimmest light you can detect at each spot, across a range an ordinary screen can't produce, which is exactly why faint problems won't show up here. There's no database of normal eyes behind it, so it can only ever compare you to yourself. And there's a knack to it: your first couple of runs will be your worst. None of that makes it a test. If your vision ever changes, or something feels off, or you just want to be sure — see an eye doctor. That's the part this can't do.