Three quantities describe every lighting product, and quotations routinely mix them up:
- Lumens measure how much light a fixture emits in total. A property of the product.
- Lux measures how much of that light lands on a surface, per square metre. A property of the installation.
- Candela measure how intense the light is in one particular direction. A property of the beam.
| Lumens (lm) | Lux (lx) | Candela (cd) | |
|---|---|---|---|
| Measures | Total light emitted | Light landing on a surface | Light intensity in one direction |
| Property of | The product | The installation | The beam |
| Changes with distance? | No | Yes | No |
| Where you find it | Catalogues and datasheets | The design brief and the site meter | The IES file and polar curve |
| Answers | ”How much light does it make?" | "Is the floor bright enough?" | "How far will the beam throw?” |
The confusion is expensive because only one of them — lux — is what the building actually needs, and it is the only one no supplier can put in a catalogue. Lux depends on mounting height, spacing, room surfaces and dirt. Two fixtures with identical lumen figures can produce completely different lux on the same floor, because the one with better optics sends its lumens where they are needed instead of into the roof structure.
This guide explains what each number means, how to convert between them, and the three questions that stop a lumens-only quotation from turning into a badly lit building.
Key Takeaways
- Lumens are output, lux is delivery, candela is direction. Only candela describes a beam, and only lux describes what a worker sees.
- Lux = lumens ÷ area, but only in the ideal case. Real installations lose 20–40% to utilisation and dirt, and any calculation ignoring that will overpromise.
- Candela converts to lux with the inverse-square law: lux = candela ÷ distance². This is how you check a flood light’s claim before buying.
- “Chip lumens” summed from LED datasheets are typically 10–20% higher than measured fixture lumens. Insist on the whole-fixture LM-79 figure.
- 1 foot-candle = 10.76 lux. US specifications in foot-candles and European ones in lux are describing the same quantity.
Lumens: How Much Light Leaves the Fixture
The lumen is total luminous flux — all the visible light a source emits, in every direction, added up. It says nothing about where that light goes.
This is why a lumen figure alone cannot specify a fixture. A 20,000 lm high bay with a 120° optic and a 20,000 lm high bay with a 60° optic emit exactly the same amount of light, and produce very different floors: the narrow one concentrates it into a smaller, brighter circle, the wide one spreads it thinner over more area. Which is correct depends entirely on your ceiling height and spacing — not on the lumen number they share.
Two traps live inside the lumen figure itself.
Chip lumens versus fixture lumens. The easiest way to inflate a specification is to take the LED manufacturer’s datasheet output per chip, multiply by the number of chips, and print the result. That number ignores everything that happens after the chip: the lens absorbs some light, the reflector absorbs some, the diffuser absorbs more, and the LEDs run hotter in a real fixture than in a lab test, which reduces output further. Measured whole-fixture output is typically 10–20% below the summed chip figure, and occasionally much worse.
The defence is to ask for the LM-79 report — a whole-fixture measurement, made with the fixture assembled and powered as it will actually be sold. If a supplier can only produce a chip datasheet, you have a claim rather than a measurement.
Efficacy hides the same problem. Luminous efficacy in lumens per watt is only meaningful if both halves are measured on the same complete fixture. Chip efficacy divided by fixture wattage is not a real number, and it is how implausible efficacy claims get built.
Lux: How Much Light Lands on the Surface
Lux is illuminance — lumens per square metre arriving at a surface. One lux is one lumen spread over one square metre.
Every lighting standard specifies in lux, because lux is what determines whether a task can be performed: 150 lux for a warehouse storage aisle, 300 lux for general assembly, 500 lux for detailed inspection, 750 lux and up for fine visual work.
The basic relationship is simple:
Lux = total lumens ÷ area in m²
Ten thousand lumens spread evenly across 50 m² gives 200 lux. That arithmetic is exact and also optimistic, because it assumes every emitted lumen reaches the floor. In a real building it does not:
- Some light hits walls, roof steel and racking and is absorbed. The fraction that arrives is the utilisation factor, typically 0.4–0.7 depending on room shape and surface reflectances.
- Fixtures dim as they age and get dirty. The maintenance factor covers this, typically 0.8 in clean spaces and 0.7 in dusty or greasy ones.
So the honest form is:
Maintained lux = (total lumens × utilisation × maintenance) ÷ area
Twenty thousand lumens into a 100 m² bay at 0.6 utilisation and 0.8 maintenance gives 96 maintained lux, not the 200 lux the naive division suggests. That factor-of-two gap is the single most common reason a delivered installation disappoints against its quotation. Our warehouse lumens per square metre guide works the method through with real numbers.
One more property of lux is worth knowing: average lux is not the whole story. A scheme averaging 200 lux with 400 lux under each fixture and 60 lux between them is worse than a uniform 150 lux scheme. That is what uniformity ratio measures, and it is why standards specify both.
Candela: How Intense the Beam Is
Candela is luminous intensity — light per unit solid angle, in one specific direction. It is the only one of the three that describes the shape of the light.
The relationship to lumens: one candela radiating into one steradian produces one lumen. A source that emits equally in all directions produces 4π ≈ 12.57 lumens per candela. Focus that same source into a narrow cone and the lumens stay the same while the candela in the beam direction rise sharply — which is exactly what a reflector or lens does.
That is why candela, not lumens, is the meaningful number for a flood light, a spot, or anything with a defined beam angle. Two 200 W floods with identical lumens but 30° and 90° beams have peak intensities differing by roughly an order of magnitude, and they light completely different scenes.
Converting Candela to Lux
Candela becomes useful the moment you apply the inverse-square law:
Lux = candela ÷ distance²
(Distance in metres, measured along the beam. Off-axis you multiply by the cosine of the angle, which is why the centre of a beam is always brighter than its edge.)
This is the calculation that lets you audit a flood light claim before you buy it. If a supplier claims 500 lux at 10 m, they are claiming a peak intensity of 500 × 100 = 50,000 cd. Ask for the candela figure and the polar curve, and check that the arithmetic closes.
| Peak intensity | Lux at 6 m | Lux at 8 m | Lux at 10 m | Lux at 12 m |
|---|---|---|---|---|
| 10,000 cd | 278 | 156 | 100 | 69 |
| 20,000 cd | 556 | 313 | 200 | 139 |
| 34,444 cd | 957 | 538 | 344 | 239 |
| 50,000 cd | 1,389 | 781 | 500 | 347 |
Lux vs Candela: The Two That Collide Most Often
Of the three pairings, this is the one that causes the most argument in a quotation, because both of them sound like an answer to “how bright is it?” They are not the same answer, and the difference is where you stand when you ask.
Candela is measured at the fixture. Lux is measured where you are. Candela describes the beam leaving the source in a given direction and does not care how far away anything is. Lux describes light arriving on a surface and changes the moment that surface moves. The two are the same information at opposite ends of a distance, which is exactly why the inverse-square law connects them.
That gives a one-line test that settles most disputes:
If the number changes when you raise the pole, it is lux. If it does not, it is candela.
Which is why datasheets quote candela and design briefs quote lux. A manufacturer can state candela honestly because it is a fixed property of the product as built. Nobody can print lux in a catalogue, because lux is not a property of the fixture at all — it belongs to the installation, and it is not decided until someone chooses a mounting height, a spacing and an aiming angle.
The trap this creates. A claim like “1,200 lux at 5 metres” reads as a lux specification, but it is a candela claim wearing a disguise: it is really asserting 1,200 × 25 = 30,000 cd on the beam axis. That is a perfectly reasonable thing to assert — and a perfectly checkable one. Ask for the candela figure and the polar curve, and confirm the arithmetic closes. Suppliers quoting lux at a convenient distance without a candela figure behind it are quoting the single best point in the scene and letting you assume it is the average.
One more distinction worth holding onto: candela describes intensity travelling in a direction, and luminance — a fourth quantity, and the one that actually corresponds to perceived brightness — describes intensity per unit of emitting or reflecting area. A small bright source and a large dim one can share a candela figure and look nothing alike.
A Worked Example From a Real Report
Abstract definitions land better against a measurement. A 200 W-class flood light from our range, measured on our HPG1800 distributed goniophotometer, returned:
| Quantity | Measured value | What it tells you |
|---|---|---|
| Input power | 201.3 W | What the meter bills |
| Total luminous flux | 35,309 lm | Lumens — total output of the whole fixture |
| Luminaire efficacy | 175.4 lm/W | Output per watt, measured on the complete fixture |
| Peak intensity | 34,444 cd | Candela — intensity on the beam axis |
| Beam angle (50%) | 60.1° H × 59.3° V | Where those candela fall away to half |
Now the three quantities connect. The fixture emits 35,309 lumens in total. On the beam axis it reaches 34,444 candela, which at an 8 m mounting height delivers 34,444 ÷ 64 = 538 lux at the centre of the pool. At 12 m the same fixture delivers 239 lux at centre — the light did not get weaker, the distance got longer.
Note what the beam angle does to the average. The 60° cone at 8 m throws a pool roughly 9 m across, and illuminance falls from 538 lux at the centre toward the edge — so the average across that pool is well below the centre figure. A quotation citing centre lux without the beam angle is quoting the best point in the scene. The full 16-page report is downloadable on the LED flood lights page, alongside the reports for our 150 W and 300 W units.
Foot-Candles, and Other Units You Will Meet
| Unit | Measures | Convert |
|---|---|---|
| Lux (lx) | Illuminance, metric | 1 lx = 1 lm/m² |
| Foot-candle (fc) | Illuminance, imperial | 1 fc = 10.76 lx |
| Candela (cd) | Intensity | lux = cd ÷ m² |
| Luminance (cd/m²) | Brightness of a surface as seen | Used in road lighting |
One quantity that is not on this list is colour temperature. Kelvin describes how the light looks, not how much of it there is, and cooler light only appears brighter at identical lux — see LED colour temperature. | Lumen (lm) | Total flux | — |
Two practical notes. A US specification asking for 30 foot-candles is asking for 323 lux — the same requirement in different clothes. And luminance is not illuminance: road lighting standards such as EN 13201 specify the luminance of the road surface in cd/m², because what a driver sees is light reflected off the tarmac, not light arriving at it. Our roadway lighting design guide covers that distinction where it matters most.
Three Questions That Protect a Quotation
1. “Is this figure whole-fixture measured, or summed from chip datasheets?” Ask for the LM-79 report. The gap between the two is typically 10–20%, and it lands directly on your lux result.
2. “What is the peak candela and the beam angle?” Without these, a lumen figure cannot be converted into anything the building will experience. For any product with a beam — flood, spot, street, high bay — these two numbers are the specification.
3. “Can I have the IES file?” The IES file is the full intensity distribution in machine-readable form. With it, a designer can calculate delivered lux at your actual heights and spacings in DIALux or Relux. Without it, every lux claim is unverifiable. How to read an IES file walks through the file line by line, including the four checks that expose a fabricated one. A supplier who cannot produce IES files for the exact SKU quoted is asking you to take the photometry on trust.
The Bottom Line
Lumens tell you what a fixture emits. Candela tell you where it sends it. Lux tells you what arrives — and lux is the only one of the three that appears in a lighting standard, because it is the only one that describes what a person can actually see well enough to work.
Buy on lumens alone and you are buying a number that no standard asks for. Ask instead for measured fixture lumens, peak candela, beam angle and the IES file, and any competent designer can tell you the delivered lux before a single fixture ships.
Send us your mounting heights and the lux level you need to hold, and our engineers return the calculation with the IES files and measured photometric reports behind it. For beam-driven applications, the flood light selection guide covers how beam angle and mounting height interact once the units make sense.
FAQ
What is the difference between lumens and lux? Lumens measure the total light a fixture emits; lux measures how much of that light lands on a surface, per square metre. Lumens are a property of the product, lux is a property of the installation. The same fixture produces different lux at different mounting heights and spacings, which is why standards specify lux and catalogues quote lumens.
How do I convert lumens to lux? Divide total lumens by the area in square metres — but only after applying a utilisation factor for light absorbed by walls and roof (typically 0.4–0.7) and a maintenance factor for ageing and dirt (0.7–0.8). Twenty thousand lumens over 100 m² is 200 lux in theory and closer to 96 maintained lux in a realistic industrial calculation.
What is candela and when does it matter? Candela measures luminous intensity in a single direction, so it describes the beam rather than the total output. It matters for any product with a defined beam angle — flood lights, spots, street lights, high bays. Two fixtures with identical lumens but different beam angles have very different candela and light completely different scenes.
How do I calculate lux from candela? Use the inverse-square law: lux = candela ÷ distance in metres squared. A fixture with 34,444 cd peak intensity delivers about 538 lux directly beneath it at 8 m, and about 239 lux at 12 m. Off the beam axis, multiply by the cosine of the angle — which is why the centre of a light pool is always brighter than its edge.
How many lux is 1 foot-candle? One foot-candle equals 10.76 lux. A US specification calling for 30 foot-candles is asking for roughly 323 lux. The two units measure exactly the same quantity — illuminance — in imperial and metric terms.
Why do fixture lumens differ from the LED chip lumens? Because light is lost between the chip and the outside world: lenses, reflectors and diffusers absorb some of it, and LEDs run hotter inside an assembled fixture than in a laboratory chip test, which reduces output further. Measured whole-fixture output is typically 10–20% below the summed chip figure. Always specify against an LM-79 whole-fixture measurement.
Is a higher lumen figure always better? No. More lumens in the wrong distribution produce glare and wasted energy rather than a better-lit space. A fixture with 20% fewer lumens and optics matched to your ceiling height and spacing will often deliver higher maintained lux on the working plane, and at lower connected load, than a higher-lumen fixture with the wrong beam.