A site meeting on a renovated office floor. The occupants complain about the light, the project owner asks for more luminaires to be added, and yet the lux meter survey shows 520 lux on the work surfaces. The word missing from the debate is luminance: it is luminance, not illuminance, that the eye perceives when it looks at a luminaire or a screen.
Four quantities describe a lighting installation. They do not measure the same thing, are not expressed in the same unit and are not recorded with the same instrument. Confusing them leads to treating a comfort problem with a power answer.
Four quantities, four units
Luminous flux characterises the source alone: it is the total quantity of light it emits, in all directions, expressed in lumens. A 10,000-lumen luminaire emits this flux wherever it is placed.
Luminous intensity describes the distribution of this flux in a given direction, in candelas. This is what the photometric curve of a luminaire describes, and what distinguishes a narrow-beam optic from a wide-beam optic with the same flux.
Illuminance is what a surface receives, in lux, that is one lumen per square metre. It depends on the luminaire, but also on the distance and the angle. It is the quantity the lux meter records and the one the EN 12464-1 standard sets by type of task.
Quantity | Unit | What it describes | Instrument |
|---|---|---|---|
Luminous flux | lumen (lm) | What the source emits in total | Integrating sphere, in the laboratory |
Luminous intensity | candela (cd) | The flux emitted in one direction | Goniophotometer, in the laboratory |
Illuminance | lux (lx) | What a surface receives | Lux meter, on site |
Luminance | candela per square metre (cd/m²) | What a surface sends back towards the eye | Luminance meter or imaging photometer |
Luminance, the only quantity the eye perceives
The eye sees neither lumens nor lux. It perceives the light sent back by the surfaces in its field of view: the work surface, the partition, the ceiling, the window, and of course the luminaire itself. This light sent back per unit of apparent area is luminance.
On a diffusing surface, it is deduced from the illuminance and the reflectance. A light work surface, with a reflectance of 0.7, lit at 500 lux, sends back a luminance of about 110 candelas per square metre. The same illuminance on a dark work surface, with a reflectance of 0.2, drops to around 30.
Two workstations compliant in lux can therefore offer very different comfort. This quantity also explains the name of our design office: luminance is what remains once you have finished counting the watts.
Why a room at 500 lux can remain uncomfortable
Three classic situations are read in luminance and not in illuminance. The first is discomfort glare: a very bright luminaire in a dark environment creates a contrast that the UGR rating quantifies, with a usual threshold of 19 in offices according to the EN 12464-1 standard.
The second is reflection on screens. The standard sets limits on the average luminance of luminaires seen at high elevation angles, of the order of 3,000 candelas per square metre for ordinary screens and 1,500 for the most sensitive screens. A luminaire compliant in flux can fail on this criterion.
The third is the dark ceiling. A room whose surfaces absorb light produces a cave-like impression, even at the regulatory illuminance level. The answer lies in the reflectances of the surfaces, not in adding luminaires.
Where luminance is directly regulated
In indoor lighting, the EN 12464-1 standard governs illuminance and, in parallel, imposes limits on glare and on the luminance of luminaires. In sports lighting, the EN 12193 standard adds control of the apparent luminance of the sources seen from the playing area.
In road lighting, the change is more radical: the EN 13201 standard defines the classes of motorised roads in road surface luminance, expressed in candelas per square metre, and not in lux. It is the light sent back by the surface towards the driver that makes the performance, which makes the type of surfacing an integral part of the calculation.
Our page dedicated to lighting compliance and standards details the texts that apply according to the type of room.
What this changes in a project
Four design decisions follow directly from this distinction.
- Choose the reflectances of the surfaces before choosing the luminaires, because they determine the luminance obtained for the same illuminance.
- Check the apparent luminance of the fittings in rooms with screens, and not only their UGR declared in a standard configuration.
- Treat contrasts between zones, as too marked a ratio between the task and its immediate surroundings is more tiring than a low level.
- For traffic routes, reason in road surface luminance from the programming stage.
These four points are checked in simulation, with the real photometric files and the reflectances of the room. That is the purpose of a photometric study, whose false colour rendering maps illuminance and luminance on the same plane.
Vocabulary confusions that cost a lot
Three phrasings come up regularly in tenders and produce disappointing installations. The first consists of expressing a need in watts: power describes consumption, not a lighting result. Two luminaires of the same power can differ by a large factor in useful flux.
The second consists of asking for a number of lumens for a room. The flux of a luminaire says nothing about what reaches the work surface, since everything depends on the distribution, the height and the surfaces.
The third consists of writing a level in lux where the regulations require a luminance, which is the case for motorised traffic routes. A tender written in lux for this type of project makes the offers impossible to compare and the check impossible to carry out.
Expressing a need correctly
A usable specification sets four things: the maintained illuminance level by zone, the minimum uniformity, the applicable glare limit, and the reflectances used for the calculation. The first three come from the standards, the fourth determines the luminance obtained.
Adding the maintenance factor used and its justification avoids the classic gap between a brilliant acceptance and an installation that falls short after three years.
Reading product data sheets with the right units
On a data sheet, flux in lumens, efficacy in lumens per watt and maximum intensity in candelas do not replace one another. Our guide to reading an LED luminaire data sheet goes over these benchmarks line by line.
On the product side, ranges are compared at equivalent output flux and distribution, as explained in the analysis of luminous efficacy in lumens per watt published by Rexylum.
If a room compliant in lux keeps causing problems, have the luminance surveyed on site: write to us from the contact page with the plan of the room and the measurements already taken.
