Luminance (cd/m²)

Luminance is the luminous intensity leaving a surface per unit area in a given direction, measured in candela per square metre (cd/m²). It is the only photometric quantity that corresponds to what the eye actually perceives as brightness, because it describes light travelling toward the observer rather than light arriving at a surface.

Luminance vs Illuminance

This is the distinction that matters, and it is the one buyers and specifiers most often collapse.

	Illuminance	Luminance
Measures	Light arriving at a surface	Light leaving a surface toward the eye
Unit	lux (lm/m²)	cd/m² (also called a nit)
Depends on the surface?	No	Yes
Answers	”Is the task lit?"	"How bright does it look?”
Used by	Interior and area standards	Road, tunnel, display and glare standards

The dependence on the surface is the whole point. Identical illuminance produces very different luminance on light concrete and dark asphalt, because the darker surface returns less of what falls on it. Two roads lit to the same lux can look completely different to a driver.

For a matte, evenly diffusing surface the relationship is:

L = (E × ρ) ÷ π

where L is luminance in cd/m², E is illuminance in lux, and ρ is the surface reflectance. White paper at 500 lx with 80% reflectance gives roughly 127 cd/m². The same 500 lx on a dark asphalt sample returns a small fraction of that.

Typical Luminance Values

Surface or source	Approximate luminance
Road surface, EN 13201 M-classes	0.3 – 2.0 cd/m²
Tunnel interior zone	1 – 10 cd/m²
White paper under 500 lx office lighting	≈ 125 cd/m²
Computer display, typical setting	200 – 400 cd/m²
Overcast sky	≈ 2,000 cd/m²
Clear blue sky	≈ 8,000 cd/m²
Bare LED die in a floodlight	Millions of cd/m²

That last row explains why glare control is a fixture-design problem. A bare, unshielded LED presents a source luminance many orders of magnitude above its surroundings, which is why an unlouvred fixture can be uncomfortable in a space that measures perfectly well on lux.

Why Road Lighting Is Designed in Luminance

Motorised-traffic classes under EN 13201 are specified in average road-surface luminance, not in lux, because a driver sees the carriageway by the light it reflects. The standard therefore combines the lighting design with a road-surface reflection table, since the same luminaires over a light concrete surface and a dark, worn asphalt produce different luminance from identical installed lumens.

Two consequences worth carrying into a tender:

• A lux specification for a main road is the wrong specification. It can be met while the class fails.
• Wet surfaces change the result. Standards address wet-road conditions separately, because water alters the reflection characteristics substantially.

Pedestrian and residential classes (P-classes) stay in illuminance, which is why a single project can legitimately mix both quantities.

How Luminance Is Measured

A luminance meter is a telescopic instrument with a defined acceptance angle, typically 1° or less. Unlike a lux meter, it is aimed at a surface from the observer’s position rather than placed on the working plane, which matters because luminance is direction-dependent: the same patch of road measured from a driver’s eye height and from directly above will not read the same.

Practical points:

• Measure from the relevant observer position, at the correct height and viewing distance.
• Note the acceptance angle, since a wider angle averages more of the scene.
• Imperial work sometimes uses the foot-lambert: 1 fL = 3.426 cd/m².
• Imaging luminance meters capture a whole field at once and are how glare evaluations of a real installation are usually done.

Luminance and Glare

Glare is fundamentally a luminance phenomenon. What causes discomfort is not how much light a fixture emits, but how bright its visible surface is relative to the background the eye has adapted to. That is why two fixtures with the same lumen output can differ enormously in comfort: spreading the same output over a larger emitting area lowers source luminance without losing a single lumen.

Indoor practice quantifies this through UGR; outdoor and sports practice uses glare rating and threshold increment limits. Both are built on luminance contrast, not on illuminance. See glare for the mechanisms and the control measures.