Bright sky on a Saudi afternoon delivers 100,000 lux; the inside of a basement archive runs at 30 lux. That's a ratio of 3,000:1. The eye handles it through pupil change, photoreceptor adaptation, and neural gain control, but the transitions take time. Glare is what happens when adaptation can't keep up. UGR is the metric that puts a number on it.
Four ideas to carry into the next specification conversation you have.
Bright → dim. Pupils dilate quickly, photopigments regenerate. Most adaptation done in 2 minutes; full dark adaptation 30 minutes.
Dim → very dark. Rods slowly regenerate rhodopsin. Why astronomers use red lights, red doesn't bleach rhodopsin.
Calculates psychological discomfort from luminaires in your field of view. Office target: UGR ≤ 19. Drawing offices: UGR ≤ 16.
Bright source near task = scattered light in the eye = reduced contrast. Why you can't read a screen with a window behind it.
Maximum UGR per EN 12464-1 for common interior categories.
| Space | Max UGR | Notes |
|---|---|---|
| Technical drawing | 16 | Highest precision |
| Reading, writing, offices | 19 | Most office work |
| Reception, archive | 22 | Transient tasks |
| Coarse manufacturing | 25 | Workshop, factory |
| Circulation, corridors | 28 | Walk-through only |
Five real-world consequences of ignoring eye adaptation.
Driver enters tunnel from 100,000-lux desert sun. Inside is 200 lux. Eye takes 5-15 seconds to adapt. CIE 88 mandates a graded threshold zone, bright entry, then dim, to prevent the 'black hole' effect.
Drivers go from 80,000 lux to 100 lux instantly. Many can't see pedestrians for 5-10 seconds. Saudi-spec parking now uses 300+ lux entry zone, graded to 100 lux interior.
Nurses moving from bright corridor to dim ward can't see clearly for seconds. Modern designs use motion-triggered low circadian light in wards, normal light in corridors.
Lobby kept at 100 lux, foyer 30 lux, hall < 1 lux. Three-stage adaptation so customers don't trip. Bad cinemas skip the foyer step.
Veiling luminance ratio of screen to window > 1:10 = constant squinting and fatigue. Why offices use diffused windows or task-tunable luminance.
Every NLC outdoor / tunnel / parking proposal includes adaptation zones per CIE 88 and EN 12464-2. Not optional, it's how we keep people safe at entries.
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