Lux meters measure illuminance, photons arriving at a sensor. But what your eye actually sees is luminance, photons leaving a surface. A white wall at 500 lux has different luminance than a black wall at 500 lux. Glare standards, contrast ratios, and tunnel lighting all use luminance, not illuminance. Lighting professionals must speak both.
Four ideas to carry into the next specification conversation you have.
Arriving photons. Lux meter measurement. Standard for working planes (EN 12464-1).
Departing photons toward viewer. What the eye actually senses. Standard for glare and contrast.
Approximate formula for diffuse surfaces. 500 lux × 0.8 white wall ≈ 127 cd/m². Same lux, black wall = 16 cd/m².
Because the eye responds to luminance. EN 1838 emergency, CIE 88 tunnel, EN 12464-1:2021 wall lux all reference luminance.
The four key photometric measures every lighting engineer must know.
| Quantity | Unit | Meaning |
|---|---|---|
| Luminous flux | lumen (lm) | Total light output |
| Luminous intensity | candela (cd) | Output per solid angle |
| Illuminance | lux (lx) = lm/m² | Arriving at a surface |
| Luminance | cd/m² | Leaving a surface to viewer |
| Reflectance ρ | 0-1 dimensionless | Fraction of light reflected |
| L (diffuse) ≈ E·ρ/π | lux→cd/m² | Conversion formula |
Five engineering decisions where luminance vs illuminance matter.
CIE 88 specifies luminance on the road surface in cd/m², not lux on the ceiling. Because the driver's eye sees the road, not the lamp.
150 lux on walls (illuminance) means different luminance depending on wall reflectance. Designers now report both, for circadian and perception.
Glare = (source cd/m²)² / (background cd/m²). You can't compute UGR from illuminance, you need fixture luminous output and the surrounding luminance ratio.
Computer monitors target 250-400 cd/m². Phone screens 600+ cd/m². OLED contrast 1,000,000:1 luminance ratio. Different physics from room illumination.
Stadiums for HDR specify cd/m² on the pitch in 4 directions, not just lux. Because the camera sees luminance through its lens, the same as the eye.
Every NLC photometric report includes illuminance grid, wall luminance, ceiling luminance, and UGR observer table. Because lux alone is half the story.
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