为什么需要对比度比例

When you read text, your eye is comparing how light or dark the letters are against the background. That gap is contrast. 👀

One designer says a color looks fine, another squints. To settle it, accessibility uses a contrast ratio, a single measurable number anyone can check.

The ratio runs from 1:1 to 21:1. Black on white is the maximum, 21:1, while the same color on itself is 1:1 and totally invisible.

The math is built on luminance, the perceived brightness of a color. Two very different hues can still clash if their brightness is too close.

Contrast rules exist because vision varies. Low-vision users, older eyes, and tired readers all need a bigger gap than a young designer on a bright screen.

Sunlight glare, cheap monitors, and dimmed phones all eat into contrast. A ratio that survives those conditions keeps text readable everywhere.

WCAG picked 4.5:1 for normal text as a research-backed floor. It roughly accounts for moderate low vision without forcing pure black and white.

Large text is easier to read, so the bar drops to 3:1. Thick, big letters tolerate a smaller brightness gap than tiny body copy does.

A ratio is always about two colors together. A gray is never readable or unreadable alone, only against the surface it sits on.

The same idea extends to UI components. Button borders, icons, and form outlines also need enough contrast so users can find them.

These two colors form roughly a 4.6:1 ratio, just clearing the normal-text bar for comfortable reading.

Let us test why the ratio exists at all.

You learned that a contrast ratio turns readability into one honest number, built on luminance, so text works for every eye. Next: hitting the AA thresholds. ✅