What is Illuminance?

Illuminance, Lux and Lumens — What's the Difference?

Lumens (lm) measure the total light emitted by a source. Illuminance (lux) measures how much of that light actually reaches a surface. One lux equals one lumen per square metre (1 lx = 1 lm/m²).

A luminaire producing 3,000 lumens will create different lux levels depending on its mounting height, beam angle, and the size of the room. This is why lighting designers use photometric software (such as Dialux or Relux) to calculate lux distributions — ensuring every point in the space meets the required illuminance.

UK Lighting Standards for Illuminance

The principal standard for interior workplace lighting in the UK is BS EN 12464-1. Key maintained illuminance values include:

• General office — 500 lux on the working plane • Meeting rooms — 500 lux • Corridors — 100 lux • Stairways — 150 lux • Classrooms — 300 lux on desks, 500 lux on whiteboards • Hospital wards — 100 lux general, 300 lux at bed head • Operating theatres — 1,000 lux general, up to 100,000 lux in the surgical field • Retail sales areas — 300 lux • Warehouses — 150–300 lux depending on activity

For exterior lighting, BS EN 12464-2 and BS 5489 apply, covering car parks, pathways, loading bays and security lighting.

Maintained vs Initial Illuminance

Standards specify 'maintained illuminance' — the minimum level that must be achieved throughout the installation's maintenance cycle. New LED luminaires produce slightly more light than the design target; over time, output gradually decreases due to LED lumen depreciation and dirt accumulation on optics.

Lighting designers apply a 'maintenance factor' (MF) to account for this. A typical MF for a clean office environment with LED luminaires is 0.8, meaning the initial illuminance is designed to be 25% above the maintained target. EOS provides L80/B10 lifetime data and maintenance factor recommendations for every product.