Lumen — total light output from the source
Lumen (symbol: lm) is the SI unit of luminous flux — the total quantity of light emitted by a source, weighted by the sensitivity of the human eye. That second part is important: lumen is not a pure physical energy measurement. It is weighted by the photopic luminosity function, which reflects that the human visual system is most sensitive to green-yellow light (peak at around 555 nm) and progressively less sensitive towards red and blue.
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.
In a street luminaire data sheet, the headline lumen figure is almost always the luminaire lumen output — the total flux leaving the luminaire, after accounting for losses through the optics, diffusers, and housing. This is lower than the LED lumen output, which is measured before those losses.
In practice
- LED manufacturers typically quote LED lumen output. Luminaire manufacturers quote luminaire lumen output. The two numbers describe the same product but can differ by 15–30% — always check which one is stated.
- 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 road lighting calculations use maintained lux values — lux being derived from the luminaire lumen output and the specific photometric distribution of that luminaire over the road surface.
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.
In road lighting, lux (or its maintained equivalent — maintained illuminance, Em) is what EN 13201 specifies. The standard defines minimum maintained illuminance values and maximum/minimum uniformity ratios for different road lighting classes. These values are calculated in a lighting design tool using the photometric data file (typically an IES or LDT file) supplied by the luminaire manufacturer.
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 full angular distribution of a luminaire's intensity — its candela values in every direction — is contained in the photometric data file. Road lighting design software uses this data to calculate lux values at road surface under specific mounting conditions. For photobiology and glare analysis, the candela value in the direction of the observer is the relevant figure.
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.
For energy performance in road lighting, EN 13201-5 uses the Power Density Indicator (PDI) and Annual Energy Consumption (AEC) metrics. Both rely on maintained lux values at road surface — not on lumen output alone. A luminaire with high lm/W but a poor optic may produce the same road surface lux as a luminaire with lower lm/W and a tightly controlled beam, while consuming more energy to do so.
Initial vs maintained: why the maintenance factor matters
Road lighting standards specify maintained values — the minimum acceptable performance over the lifetime of the installation, not the initial performance when the luminaire is new. The gap between initial and maintained is described by the maintenance factor (MF), which accounts for LED lumen depreciation (captured in the LMF — Lamp Maintenance Factor), luminaire dirt accumulation, and driver degradation.
If a luminaire is specified to an initial lux level without applying an appropriate maintenance factor, the installation will fail its road lighting class well before end of the planned service period. Specifying to maintained values — and verifying that the claimed maintenance factor is supported by LM-80 data and field measurement rather than assumed — is the correct approach.
Understanding lumens leads directly to understanding lumen maintenance
What L70 and L90 actually tell you — and when each threshold matters for road lighting.
Lumen maintenance thresholds translate directly into when an installation loses compliance with EN 13201 — and when maintenance or replacement becomes necessary.
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 describes total light output from a source. Lux describes illuminance on a surface — the quantity that road lighting standards specify. Candela describes intensity in a direction — the quantity that photometric data files report. All three are necessary to specify, design, and verify a road lighting installation correctly.
High lumen output from an LED source does not guarantee adequate illuminance at road surface — the optical distribution, mounting height, spacing, and maintenance factor all determine that. A complete specification asks for maintained lux values to EN 13201, supported by a photometric data file and the maintenance factor used to produce the calculations.