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Glossary

Emmetropization

Defined in topic 24

The feedback loop that grows the eyeball until its length matches its focusing power, using blur on the retina as its guide. Starved of bright daylight in childhood, it overshoots.

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Where this comes from

In the reference topic that owns this term

Reference topic 24, The Indoor Eye, states the mismatch as a card: what the system evolved to expect, what it gets instead, and what happens in the gap.

Mismatch card

What it expects

A childhood of bright outdoor light, thousands of lux for hours a day, with things to look at near and far.

What it gets

Years of classrooms at a few hundred lux, and 87% of time spent inside a building.

What happens in the gap

The eye overgrows, distance blurs for good, and a stretched retina carries lifelong risk.

The full card, and the topic behind it

What that chapter recommends

3 of the 5 graded recommendations in Reference topic 24, strongest evidence first. How the grades work.

  1. Get children outdoors about two hours a day, with a floor of one

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    Two hours was Taiwan's school target and is widely endorsed. China's plan specifies one to two, and the US National Academies at least one. Shade and cloud count: benefit appeared from 1,000 lux.

    Read it in full

  2. Get eyes checked

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    Vision screening at least once between ages 3 and 5, an eye examination before first grade, and sooner if a child squints at distance.

    Read it in full

  3. If a child is already myopic and getting worse, ask about myopia control

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    The US Food and Drug Administration authorized the first myopia-control spectacle lenses in September 2025, for children 6 to 12, citing 71% less progression over two years.

    Read it in full

Related terms

The argument behind this term, its studies and its notes are in Reference topic 24 of the reference library. The book gives the practical version. Get the book.