Lumen, watt, lux: understanding the units before buying
Comparing two luminaires on their power in watts alone leads to costly sizing errors. Each lighting unit answers a specific question, and here is which one to look at depending on the actual need.
6 min read
Two product sheets lie side by side on the meeting table. The first states 150 watts, the second 100 watts, and the conclusion follows: the first would give more light. Electrical power is precisely the figure that does not tell you that.
Yet four units are enough to make a serious comparison between two devices. Each answers a different question, and confusing them produces installations that are undersized or needlessly expensive.
The watt measures consumption, not a quantity of light
The watt expresses the electrical power drawn. It indicates what the installation will cost to run and what the cable will have to carry, nothing else.
In the days of incandescent sources, the relationship between watts and light was stable, which created a lasting reflex. With LED sources, two devices of the same power can emit very different quantities of light depending on the quality of the components and the optics.
The power to consider is that of the complete luminaire, driver included, not that of the LEDs alone. This distinction is checked on the data sheet: the relevant figure is the total power drawn by the system.
The lumen: the flux actually emitted by the luminaire
The lumen measures luminous flux, that is, the quantity of light produced. It is the figure to compare between two devices intended for the same use.
One precaution is needed. Some sheets state the flux of the bare LEDs, measured before the optic and the diffuser. The flux that actually leaves the device is always lower, the difference being described by the luminaire efficiency. The value to use is the flux leaving the luminaire, measured according to the EN 13032 standard.
The flux also changes over time. Serious manufacturers publish a pair of values such as L80B10 or L90B10, which describes the proportion of flux retained after a given number of hours and the share of devices that fall short of it. It is this figure, and not a stated lifetime on its own, that is used to calculate a maintenance factor.
The lux: what reaches the working plane
The lux measures illuminance, that is, the flux received per unit area. One lux corresponds to one lumen per square metre. It is the only unit that standards use to set requirements, because it is the only one that describes what the user actually sees.
The step from lumen to lux depends on the mounting height, the beam angle, the reflectance of the walls and the maintenance factor chosen. Two installations using the same luminaires can therefore produce very different illuminances.
Standard values are always expressed in lux over an identified area. The EN 12464-1 standard sets, for example, 500 lux for ordinary office tasks and 300 lux for sports halls and gymnasiums. Reliable sizing is checked by a photometric study in false colours, which shows the distribution of lux over the whole room.
| Unit | What it measures | Question it answers |
| --- | --- | --- |
| Watt (W) | Electrical power drawn | How much it consumes |
| Lumen (lm) | Luminous flux emitted | How much light the device produces |
| Lux (lx) | Illuminance received on a surface | How much light reaches the workstation |
| Candela (cd) | Luminous intensity in one direction | Where the light is sent |
| Lumen per watt (lm/W) | Luminous efficacy | What efficiency for the consumption |
Luminous efficacy and why it is compared under equal conditions
Luminous efficacy relates the flux emitted to the power drawn, in lumens per watt. It is used to compare two devices of the same family, provided both values come from the same measurement conventions.
Three frequent discrepancies distort the comparison. The flux of bare LEDs set against the flux of the complete luminaire. The power of the LEDs set against system power, driver included. Finally, the ambient measurement temperature, often 25 degrees Celsius in the laboratory, whereas the luminaire will operate under a much hotter roof structure.
The European ecodesign regulation applicable to light sources requires the publication of a common set of comparable information, and the energy label ranks products on a scale from A to G. These benchmarks make initial sorting easier, without removing the need to read the full data sheet.
Candela and beam: understanding photometric curves
The candela measures luminous intensity in a given direction. Two luminaires with the same flux in lumens can send that light in completely different ways: one as a wide spread over a floor, the other as a narrow beam towards a precise point.
This distribution is read on the photometric curve, shown as a polar diagram. The beam angle usually published corresponds to the opening within which intensity remains at least half of the maximum intensity.
For any serious calculation, the data to request is the photometric file in IES or LDT format. It is this file, and not the beam angle stated in degrees, that feeds simulation software. A supplier who does not provide this file makes it impossible to check its claims.
The choice of beam then follows from the height. The higher the luminaire, the narrower the beam must be to keep uniformity on the ground, a principle that governs the choice of LED floodlights as well as that of industrial high bays.
Colour rendering and colour temperature
Two distinct notions are regularly confused. Colour temperature, expressed in kelvins, describes the shade of the white light: warm around 2700 kelvins, neutral around 4000 kelvins, cool above 5000 kelvins.
Colour rendering, expressed by the Ra or CRI index on a scale up to 100, describes how faithfully the colours of objects appear. The EN 12464-1 standard requires Ra of 80 or more in most workplaces, and higher values for visual inspection stations.
Two further items are worth requesting. The R9 value, which specifically describes the rendering of saturated reds, is often poor even when the overall Ra remains good. Shade consistency between devices, expressed in SDCM, ensures that a line of LED panels installed on the same day will not show visible drift from one device to the next.
The right questions to ask a supplier
- What is the flux leaving the complete luminaire, and not that of the LEDs alone.
- What is the power drawn by the system, driver included.
- Is the IES or LDT photometric file available.
- What are the L and B flux maintenance values, and over what duration.
- What Ra index, R9 value and SDCM are stated.
- At what ambient temperature are the performances given.
- Can the driver be replaced separately, and with what supply lead time.
These seven questions are asked in writing and kept on file. They are enough to rule out incomplete product sheets before any request for prices.
The detailed data sheets of each family of luminaires can be consulted from the catalogue of professional luminaires.
