What this check actually measures

Two things, one after the other.

👁️ Acuity — using the Landolt C, the international standard optotype. You see a ring with a gap on one side and pick the direction of the gap. The ring gets smaller at every level.
🎨 Colour — the same principle as an Ishihara plate. A digit is hidden in a disc of coloured dots. The background and the digit are matched for brightness and differ only in hue, and every dot's lightness is jittered from the same distribution, so you cannot find the digit by brightness. Only hue discrimination reveals it. The hue gap narrows from 40° down to 5°.

You get three hearts per stage. A wrong answer costs one heart and you retry the same difficulty; run out and the check ends with whatever you reached.

⚠️ Read this first — it is not a medical test

Measuring vision through a web browser has fundamental limits.

Screen size and pixel density differ between devices. The same "20px" is physically a different size on a phone and a monitor.
Viewing distance cannot be controlled. A real eye exam fixes the distance (typically 6m or 40cm).
Screen brightness and ambient light change the result substantially.

That is why this check does not give you an acuity figure like 20/20. It only reports how many stages you cleared. The most useful way to read it is as a change over time on the same device in the same posture.

If something feels wrong with your eyes, see an optometrist or ophthalmologist.

Getting a meaningful reading

Hold the screen about an arm's length away (40cm) and do not move during the check.
Work in a well-lit room, with screen brightness a little above your usual setting.
If you wear glasses or contacts, keep them on — this measures corrected vision.
To compare eyes, cover one and run it twice. A large gap between the two is worth noting.
Look at the colour plates straight on. Viewing an LCD at an angle shifts the colours.

Reading your result

Acuity has 10 stages.

Stages 1–4 — only large marks resolved. Check your lighting and distance before reading anything into it
Stages 5–7 — typical range
Stage 8+ — resolving fine detail well

Colour has 7 stages, and the last one is a hue gap of just 5°. Most plates are drawn from the red–green range, because the most common colour vision deficiency (deutan/protan) is exactly the difficulty telling those apart.

Stages 1–2 — 40° and 30° are obviously different colours to most people. Getting stuck here is worth a look at your display profile, and if it repeats, a proper colour vision test
Stages 3–5 — typical range
Stage 6+ — resolving 8° and 5° differences, a sharp eye for colour

Looking after your eyes

The 20-20-20 rule — every 20 minutes, look at something 20 feet away for 20 seconds. Widely recommended for giving your focusing muscles a break during screen work.
Blink deliberately — blink rate drops sharply when you concentrate on a screen, which is what dries your eyes out.
Match screen brightness to the room — a bright screen in a dark room is the hardest combination on your eyes.
Put the screen slightly below eye level — your eyelids stay lower and your eyes dry out less.

Frequently asked

Q. My result changes every time.
A. That is normal. Fatigue, lighting, screen brightness and viewing distance all move it. Look for a trend under the same conditions rather than a single number.

Q. My phone and my laptop give different results.
A. Expected — the screens differ in size and pixel density. Compare each device against itself.

Q. Why no acuity score like 20/20?
A. A real figure requires knowing the physical screen size and the exact viewing distance. The web cannot control either, and we would rather report stages honestly than hand you a number that is wrong.

If you want to train colour discrimination further, try the colour spotting game — same principle, built as a game.

※ This is not a medical diagnosis and does not replace an eye examination. Seek professional advice immediately for visual field loss, sudden changes in vision, or a rapid change in colour perception.