Lux (lx)
Lux is the SI unit of illuminance: one lumen of luminous flux spread over one square metre of surface. It is the number lighting specifications are written in — a warehouse aisle may require 200 lx maintained, a road surface 20–30 lx average. Because lux measures light arriving at a surface, it depends on distance and geometry: doubling mounting height roughly quarters the illuminance from the same fixture.
The Formula
Two equations cover almost every practical calculation:
- From total flux:
lux = lumens ÷ area in m². Ten thousand lumens spread evenly over 50 m² gives 200 lx. - From beam intensity:
lux = candela ÷ distance². A fixture with 34,444 cd peak intensity delivers about 538 lx directly beneath it at 8 m, and 239 lx at 12 m.
The first form is optimistic in a real building, because it assumes every emitted lumen reaches the surface. A working calculation multiplies by a utilisation factor (typically 0.4–0.7, for light absorbed by walls and roof) and a maintenance factor (0.7–0.8, for ageing and dirt). That is the difference between initial and maintained lux, and standards are always written in maintained terms.
How Bright Is 10, 100 or 500 Lux?
Illuminance is hard to intuit because the eye adapts. These anchors make the scale concrete:
| Lux | Roughly equivalent to | Typical use |
|---|---|---|
| 0.25 lx | Full moon, clear night | Below any design level |
| 1 lx | Deep twilight | Minimum perceptible orientation |
| 10 lx | Late twilight | Quiet residential footpath, low-traffic amenity area |
| 20 lx | — | Residential street lighting, car park circulation |
| 50 lx | Domestic living room, evening | Corridors, stairwells at night, low-risk storage |
| 100 lx | Very overcast daylight | Bulk storage aisles, plant rooms, infrequent traffic |
| 150–200 lx | — | Warehouse transit aisles, general storage, forklift routes |
| 300 lx | — | General assembly, packing, machine shops |
| 400 lx | Overcast dawn | Open-plan office, reading and writing tasks |
| 500 lx | — | Detailed assembly, quality inspection, drawing work |
| 700–1,000 lx | Bright overcast day | Fine visual work, colour inspection, electronics assembly |
| 1,600 lx | Bright overcast, midday | Precision inspection and broadcast-grade areas |
| 10,000–25,000 lx | Full daylight, no direct sun | Outdoors — not a design target |
| 32,000–100,000 lx | Direct sunlight | Outdoors — not a design target |
Two things this table makes obvious. First, indoor lighting is a tiny fraction of daylight — a 500 lx office is roughly 1% of direct sun, and the eye barely notices because it adapts logarithmically. Second, the interesting range for industrial specification is narrow: nearly all of it lives between 100 and 500 lx.
How Lux Is Measured
A lux meter is a photodiode with two corrections that separate an instrument from a phone app:
- V(λ) correction — a filter shaping the sensor’s response to match the human eye’s sensitivity curve, which peaks in the green. Without it, readings are unreliable under coloured or narrow-band sources.
- Cosine correction — a diffuser so light arriving at a shallow angle is weighted correctly. Without it, wide-beam and multi-fixture installations read low.
Practical measurement discipline:
- Measure at the working plane — normally 0.75 m above floor for desk work, floor level for traffic areas — not at head height.
- Take a grid of points, not one reading. Standards specify average illuminance and uniformity, and a single point under a fixture tells you neither.
- Exclude daylight: measure at night, or with blinds closed, if the target is the artificial installation.
- Let the fixtures stabilise (LEDs typically 15–30 minutes) before recording, since output drops as the junction warms.
- Compare against the maintained target. A new installation should read comfortably above its maintained figure — if it only just meets it on day one, it will fall short within a year.
Lux, Lumens, Candela and Foot-candles
| Quantity | Unit | What it measures | Property of |
|---|---|---|---|
| Luminous flux | lumen (lm) | Total light emitted | The product |
| Illuminance | lux (lx) | Light arriving per m² | The installation |
| Luminous intensity | candela (cd) | Light in one direction | The beam |
| Luminance | cd/m² | Light reflected toward the eye | The surface as seen |
1 foot-candle = 10.76 lux. A US specification asking for 30 fc is asking for 323 lx — the same quantity in imperial units. See foot-candle.
“How many lux in a lumen?” is a question without a fixed answer: it depends entirely on the area the lumens land on. One lumen over one square metre is 1 lx; the same lumen concentrated onto 0.1 m² is 10 lx. Controlling that relationship is precisely what optics exist to do — see lumens vs lux vs candela for the full explanation.
Where Lux Is Not the Right Metric
Road lighting classes under EN 13201 are specified in luminance (cd/m²), not lux, because what a driver sees is light reflected off the road surface rather than light arriving at it. A wet or dark asphalt surface returns far less to the eye than a dry light one at identical lux. Interior and area lighting stay in lux; motorway and main-road classes move to luminance.
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