Science reference · cited · not medical advice
Outdoor light levels: how bright is bright enough?
"Go outside for your eyes" only makes sense once you see the numbers: daylight is not a bit brighter than indoor light — it's tens to hundreds of times brighter, even under cloud. This page is the reference table.
Typical illuminance, from moonlight to noon
| Setting | Typical illuminance | Vs. a classroom |
|---|---|---|
| Full moon, clear night | ~0.1–0.3 lux | — |
| Living room, evening lighting | ~50–150 lux | 0.2× |
| Classroom / office, well lit | ~300–500 lux | 1× (baseline) |
| Bright retail / workshop | ~750–1,500 lux | ~2–3× |
| Heavily overcast day, outdoors | ~1,000–5,000 lux | ~3–10× |
| Bright overcast day, outdoors | ~5,000–25,000 lux | ~10–50× |
| Open shade on a clear day | ~10,000–25,000 lux | ~25–50× |
| Direct sunlight, midday | ~30,000–100,000+ lux | ~100–250× |
Ranges are standard photometry values; exact readings vary with season, latitude, time of day and meter placement. Free lux-meter phone apps are rough but fine for comparisons.
The threshold research points at
The Taiwanese school trial that measured light directly found protection associated with time at ≥1,000 lux — a level indoor lighting rarely reaches and any outdoor daytime setting clears easily (Wu et al., Ophthalmology, 2018). Objective light-logging studies likewise show children's eye growth tracking daily bright-light exposure, with myopic children getting measurably less of it (Read et al., IOVS, 2015) (Read et al., Optom Vis Sci, 2014). The leading mechanism is light-stimulated retinal dopamine slowing axial eye growth (Lingham et al., Br J Ophthalmol, 2020).
What this means in practice
- Cloudy counts. A grey playground still delivers several classrooms’ worth of light.
- Shade counts. Open shade on a clear day is brighter than almost any interior.
- A brighter lamp does not count. No normal fixture reaches outdoor levels; the fix is a door, not a bulb.
- Windows help less than they look like they should. Right beside a bright window you may reach low-outdoor levels; a few steps in, illuminance collapses back to ordinary indoor numbers.
- Time is the dose. The school programs target roughly two hours a day — accumulated, not consecutive.
Track it with a class using the classroom outdoor tracker, or put the habit in writing with the family screen agreement.
References
- Wu PC, Chen CT, Lin KK, et al. Myopia prevention and outdoor light intensity in a school-based cluster randomized trial. Ophthalmology. 2018;125(8):1239–1250. doi:10.1016/j.ophtha.2017.12.011
- Read SA, Collins MJ, Vincent SJ. Light exposure and eye growth in childhood. Invest Ophthalmol Vis Sci. 2015. doi:10.1167/iovs.14-15978
- Read SA, Collins MJ, Vincent SJ. Light exposure and physical activity in myopic and emmetropic children. Optom Vis Sci. 2014. doi:10.1097/opx.0000000000000160
- Lingham G, Mackey DA, Lucas R, Yazar S. How does spending time outdoors protect against myopia? A review. Br J Ophthalmol. 2020;104(5):593–599. doi:10.1136/bjophthalmol-2019-314675
Not medical advice. Illuminance ranges are for orientation; for research-grade numbers, read the instrumented studies above.