Illuminance

Illuminance is the density of luminous flux arriving at a surface, measured in lux (lumens per square metre) or in foot-candles. It answers the practical question every lighting specification is really asking: how much light lands on the task?

Almost every interior and area lighting standard is written in illuminance, including EN 12464-1 for workplaces and EN 12193 for sport. When a drawing says “300 lux”, it is specifying illuminance.

The Three Qualifiers That Change the Number

A bare lux figure is incomplete. Three qualifiers decide what it actually commits anyone to.

Maintained, not initial. A specification means maintained illuminance unless it explicitly says otherwise: the level the installation still delivers at the end of its maintenance cycle, after lumen depreciation and dirt accumulation. The gap is controlled by the maintenance factor, typically 0.8 in clean interiors and 0.7 in dusty or greasy ones. An installation that only just meets its target on day one has already failed.

Average, plus uniformity. A space averaging 200 lx with 400 lx pools and 60 lx troughs performs worse than a uniform 150 lx, because the eye adapts to the bright patches and reads the dark ones as darker than they are. Standards therefore pair the average with a minimum-to-average uniformity ratio.

On which plane, and in which direction. Illuminance is measured on a surface, so the surface has to be stated. The horizontal working plane is the default (commonly 0.75 m above floor for desk work, floor level for traffic areas), but warehouse racking, shelving and inspection tasks depend on vertical illuminance on a face, which a horizontal measurement will not reveal.

Typical Maintained Targets

Space or task	Maintained illuminance
Bulk storage, low traffic	100 – 150 lx
Warehouse transit aisles, forklift traffic	150 – 200 lx
General assembly, packing	300 lx
Offices, reading and writing	500 lx
Detailed assembly, quality inspection	500 – 750 lx
Fine visual work, colour inspection	1,000 lx and above
Outdoor circulation and car parks	20 – 50 lx
Container stacking yards	20 – 50 lx

Calculating It

Two forms cover most work:

• From total flux: maintained lux = (total lumens × utilisation factor × maintenance factor) ÷ area in m². Utilisation is typically 0.4 to 0.7, covering light absorbed by walls, roof and racking.
• From beam intensity: lux = candela ÷ distance², the inverse-square law, which is how a flood light’s claim is checked at a given mounting height.

Skipping the two factors in the first form is the single most common reason a delivered installation reads roughly half its quoted figure. Our warehouse lumens per square metre guide works a full example through.

Illuminance vs Luminance

They are different quantities and they are not interchangeable.

Illuminance is light arriving at a surface and does not depend on what the surface is made of. Luminance is light leaving a surface toward the eye, measured in cd/m², and depends heavily on the surface, since a dark asphalt and a light concrete return very different amounts of identical illuminance.

The practical split: interior, industrial and area lighting are specified in illuminance; motorway and main-road classes, tunnels, displays and all glare metrics are specified in luminance. The full comparison of the underlying quantities is in lumens vs lux vs candela.

Measuring It on Site

Use a cosine-corrected, V(λ)-corrected lux meter placed on the working plane, take a grid of points rather than one reading, exclude daylight if the artificial installation is what is being verified, and allow LEDs 15 to 30 minutes to stabilise before recording. Compare the result against the maintained target, not the initial design value.