Lumen — total light output from the source
Lumen (lm) is the SI unit of luminous flux. It integrates the photopically weighted light contribution; 1 lm = 1 cd·sr. A lumen value describes total luminous flux, not how it is distributed on the road. BIPM — SI
A light source emitting identical quantities of radiant energy in red and green would be measured as producing far fewer lumens for the red output than the green output — because our eyes are less sensitive to it, it contributes less to perceived brightness.
Distinguish LED-source flux from the flux leaving the complete luminaire. Optical losses and spectral changes depend on the actual construction; compare values at the same operating and measurement conditions.
In practice
- Check whether the quotation states LED lumens or complete-luminaire lumens. There is no universal percentage difference.
- Luminous efficacy (lm/W) measures how efficiently the luminaire converts electrical energy into light. It is calculated from luminaire lumen output divided by input wattage — not LED lumen output divided by rated LED power.
- EN 13201 uses maintained luminance for M classes and illuminance for C and P classes, with their respective uniformity requirements and other applicable criteria. Total lumens cannot establish class compliance.
Lux — illuminance on a surface
Lux (symbol: lx) is the SI unit of illuminance — the luminous flux arriving at a surface per unit area. One lux equals one lumen falling on one square metre. Because the same luminous flux spreads over a larger area as the source moves further away, illuminance decreases rapidly with distance from the source.
EN 13201 uses maintained luminance for M classes and illuminance for C and P classes, with their respective uniformity requirements and other applicable criteria. Total lumens cannot establish class compliance. EN 13201-2
A luminaire with 10,000 lm output may produce very different lux levels at road surface depending on its optic: a wide, flat distribution spread over a large area produces lower lux than a narrower, more targeted distribution concentrating the same lumens over a smaller area.
Candela — intensity in a direction
Candela (symbol: cd) is the SI unit of luminous intensity — the light output of a source in a specific direction. Unlike lumen (which integrates over all directions) or lux (which measures what arrives at a surface), candela describes how much light is being sent in a particular direction.
The photometric file describes luminous intensity by angle. Together with mounting geometry it supports illuminance calculations and, with road reflection data, luminance calculations. Glare assessment also requires the specified observer and geometry; candela alone is insufficient.
Side-by-side: lumen, lux, candela
| Quantity | Unit | What it measures | Typical use |
|---|---|---|---|
| Luminous flux | Lumen (lm) | Total light output from the source, all directions combined | Comparing total light output of sources; efficacy calculation (lm/W) |
| Illuminance | Lux (lx) | Luminous flux arriving per unit area at a surface | Road lighting calculations; EN 13201 compliance; site surveys |
| Luminous intensity | Candela (cd) | Light output in a specific direction | Photometric data files; glare analysis; directional optical design |
Luminous efficacy — lumens per watt
Luminous efficacy (lm/W) is the ratio of luminous flux to electrical power consumed. It is the lighting industry's key efficiency metric. A luminaire with 12,000 lm output consuming 80 W has an efficacy of 150 lm/W.
EN 13201-5 defines complementary indicators: PDI relates system power to illuminated area and maintained average illuminance, while AECI relates annual energy consumption to illuminated area. Compare alternatives for the same project and lighting requirements; luminaire lm/W alone does not describe installation energy performance.
Initial vs maintained: why the maintenance factor matters
Document the maintenance factor for the selected period and operating conditions. It should account for lumen maintenance, dirt and the maintenance and replacement strategy using a consistent method. LED lumen maintenance is commonly denoted LLMF; it must not be confused with luminaire dirt maintenance. ISO/CIE TS 22012:2019
A calculation showing only initial values does not demonstrate maintained performance. LM-80 component data can contribute to the lumen-maintenance basis, but do not alone establish the complete installation’s maintenance factor.
Understanding lumens leads directly to understanding lumen maintenance
What L70 and L90 actually tell you — and when each threshold matters for road lighting.
L70 or L90 alone does not determine when an installation loses compliance. That depends on the initial design margin, distribution, maintenance and all applicable class requirements.
Service Life & Reliability
What do L70 and L90 mean?
L-values, B-values, and the TM-21 projection methodology — explained for a procurement audience.
Summary
Lumen (lm) is the SI unit of luminous flux. It integrates the photopically weighted light contribution; 1 lm = 1 cd·sr. A lumen value describes total luminous flux, not how it is distributed on the road.
EN 13201 uses maintained luminance for M classes and illuminance for C and P classes, with their respective uniformity requirements and other applicable criteria. Total lumens cannot establish class compliance.