Luminance — What Your Eye Actually Sees

Illuminance and luminance are related but they are not the same, and the distinction matters more than most people realise.

Illuminance — measured in lux — describes the light landing on a surface. It is an input measure. Luminance — measured in candela per square metre — describes the light leaving a surface toward the observer. It is an output measure. It is, specifically, what your eye actually sees.

Here is why that difference is critical. A white wall and a dark charcoal wall can receive exactly the same lux level from the same source. But they will look completely different, because their reflectance values determine how much of that arriving light is sent back toward the eye. The white wall looks bright. The dark wall looks dim. Same illuminance. Radically different luminance.

This principle has several practical consequences. First, it explains why paint colour is a lighting decision. When a designer selects a deep, saturated wall colour, they are reducing the luminance of every surface it covers — and increasing the number of luminaires required to make the space feel adequately bright.

Second, it explains glare. A bare LED source has an extremely small emitting surface and an extremely high luminance — often exceeding 1,000,000 cd/m². Even at low wattage, a naked LED appears blinding. The solution is not reducing the lumen output. It is distributing that output over a larger surface area, which reduces luminance to a comfortable level.

Third, it is the reason luminance limits — not lux targets — are the correct metric for visual comfort in workplaces and healthcare environments. The WELL Building Standard and EN 12464-1 both include luminance guidance precisely because what matters is not the light you put in, but the light the eye receives.

Lux is what you specify. Luminance is what people experience.