Lux
SI derived unit · quantity: illuminance
01 · Definition
The lux is the illuminance of a surface on each square metre of which a luminous flux of one lumen falls. The quantity describes not the source but the place: how much light reached the table, the page, the floor.
The lux is the only photometric unit regulated by law: how much light must fall on a working table, an operating table and a school blackboard is written down in health regulations.
Hence the main property of the unit: one and the same lamp gives completely different illuminance depending on how high it hangs and where it points. The flux falls off as the square of the distance, and a lamp raised from one metre to two lights the table four times more weakly. The second factor is the angle of incidence: light arriving obliquely is smeared over a larger area, and the cosine of that angle enters the calculation directly. That is why lighting design counts not lamps but lux on the working surface, and why standards specify the value at eighty centimetres above the floor.
Lighting standards are not a recommendation but a binding requirement: three hundred lux for an office, five hundred for drafting work, twenty thousand in the surgical field. Measurements are taken with a lux meter at the height of the working surface, not under the ceiling.
On an overcast day under the open sky there can be five thousand lux, right at the window two thousand, and four metres away from it only fifty: a fortyfold difference across the length of a room. The eye hardly notices this, because it adapts logarithmically, and subjectively the room seems evenly lit. Building codes therefore work not with lux directly but with the daylight factor — the ratio of the light at a point to the light under the open sky, expressed as a percentage.
02 · Conversion
Enter an illuminance — the sheet will break it down by unit
The foot-candle is the same quantity referred to a square foot: a factor of almost eleven. The phot and the nox are units of the CGS system, met with in older lighting literature.
The last row of the list is not a conversion but an estimate: if the light falls on a matte white surface that reflects almost everything, the luminance of that surface in candelas per square metre is roughly three times smaller than the illuminance in lux. The exact factor is the reflectance divided by pi, so for a grey wall it is four times smaller, and for black velvet it goes to almost nothing. A lux meter measures incident light while the eye sees reflected light — hence the difference in how equally lit rooms feel.
A white sheet at five hundred lux has a luminance of about a hundred and sixty candelas per square metre — roughly like a laptop screen indoors.
| Unit | Name | Value | Where it is met |
|---|---|---|---|
| lx | lux, the SI unit | 500 | lighting standards, illumination engineering |
| klx | kilolux | 0.5 | daylight, sunshine |
| mlx | millilux | 500000 | astronomy, twilight |
| lm/m² | lumen per square metre | 500 | calculation from luminaire flux |
| fc | foot-candle | 46.45 | USA, Canada, lighting standards |
| cd/m² | the luminance of a white surface | 159.15 | an estimate of what the eye sees |
| ph | phot, CGS system | 0.05 | lighting engineering before the 1960s |
| nx | nox, a thousandth of a lux | 500000 | night illuminance, blackout calculations |
| metre-candle | the old name of the lux | 500 | literature before 1948 |
| candle-foot | the Russian translation of foot-candle | 46.45 | literature before the 1960s |
| lm/ft² | lumen per square foot | 46.45 | the foot-candle written through flux |
| lx | lux | 500 | the present-day unit |
The cosine factor works for a flat surface: a tilted page receives less than a horizontal table.
03 · Orders of magnitude
from a starry sky to an operating roomhundreds of lux: home, office, classroom
04 · Measuring instruments
What illuminance is measured with
A silicon photodiode with a correcting filter: without it the instrument would see infrared radiation that the eye does not notice. The filter matches the sensitivity to the luminosity curve, and the whole accuracy class of the instrument depends on how well that match is made.
A milky dome over the photodetector makes the instrument respond to light from any direction exactly as a flat surface would. Without such correction a lux meter understates the contribution of side light and, in a room with a window, is wrong by a factor of two.
The photographic relative of the lux meter: it measures the same light but reports not lux but a pair, «shutter speed — aperture». The scale of exposure values is logarithmic, and each step means a doubling of illuminance — a habit that came into photography from lighting engineering.
The dot next to the front camera is the same photodiode, the one that controls screen brightness. Lux-meter apps use exactly it, but the sensor has no cosine correction, the screen glass distorts the spectrum, and the error easily reaches thirty per cent.
Measurements on a grid give a map of illuminance and a uniformity ratio: a single value under the lamp says nothing about what is happening in the corner. Without such a map lighting acceptance does not pass.
For plants lux are useless: the luminosity curve describes the human eye, not photosynthesis. What is counted is the flux of photons in micromoles per square metre per second, and a separate sensor measures it.
05 · Writing rules
Both letters lower-case
The symbol of the unit is two lower-case Latin letters, lx, set in upright type. The unit is named not after a person but after the Latin word for «light», so there are no capitals in it. In English the plural takes no s: five hundred lux.
The symbol of the quantity of illuminance is an italic E. The commonest mistake in catalogues is to ascribe lux to a lamp: a lamp has luminous flux in lumens, while lux appear only on a surface and depend on distance. Conversely, for a floodlight or a torch the luminous intensity in candelas is given, and sometimes the illuminance at a stated distance — and then the distance must stand next to the number. Submultiple prefixes are used freely: millilux occurs in astronomy, kilolux in descriptions of daylight.
06 · Neighbouring units
Four photometric quantities describe the path of light from source to eye: the candela is the strength of the source, the lumen is all of its flux, the lux is what reached the surface, and the candela per metre is the luminance the eye sees.
Of the Simetrium passports next to the lux stand lumen and candela from the definition, foot-candle as the Anglo-American counterpart, steradian from the solid angle, and watt, to which luminous flux is tied by the luminosity curve.
07 · Historical section
The light that came to be regulated
The standard was a spermaceti candle of a certain thickness, burning at a certain rate, and illuminance was expressed in foot-candles. Reproducibility was poor: the flame depended on the air, the wick and the weather in the laboratory.
The International Congress of Electricians adopted the metric photometric units: the lux, the lumen and the phot. The name is taken from the Latin lux — light: one of the few in the system that come straight from a word rather than from a surname, like the lumen, the radian and the katal.
The lighting commission averaged the sensitivity of the human eye across the spectrum and obtained a curve with its maximum at 555 nanometres. From that moment photometric quantities ceased to be purely physical: an averaged observer is built into them.
The candle standard was finally replaced by a definition through radiant power: a monochromatic source at a frequency of 540 terahertz gives 683 lumens per watt. The lux, as a unit derived from the candela, received the same exact anchoring.
A unit with a human inside
The lux is a rare case of a physical unit whose definition contains biology. Luminous flux is obtained by weighting the power spectrum with the luminosity curve, and that curve was derived in the 1920s from experiments on a few dozen observers who compared the brightness of flickering colours. The curve was averaged, standardised and has barely changed since, although it is known that in real people the peak of sensitivity wanders, and that with age the lens yellows and passes ever less of the blue part of the spectrum. Hence a practical consequence: two sources with the same illuminance in lux can look different if their spectra differ — an LED with a dip in the cyan region and an incandescent lamp will give the same number on the meter and a different feeling in the room. For this reason standards always pair illuminance with requirements on colour temperature and colour rendering: lux alone are not enough to describe light.
The eye deceives: between dusk and noon the difference is a millionfold
A person reads confidently at fifty lux and at five thousand, without noticing a hundredfold difference: the pupil narrows and the brain adjusts perception. That is precisely why lighting standards have to be set by a number rather than by eye — the meter sees what a person feels only as «bright». Hence too the common mistake when choosing lamps for the home: the room seems well lit, while on the working table there is not even half of the required three hundred lux.
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