Lux

• 1. Introduction

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  The lux is the unit scientists use to measure illuminance, and the scale it covers runs from the faint trace of a moonless, overcast night sky to over 32,000, the light striking a surface in direct sunlight. Both figures carry the same label: lux, borrowed from the Latin word for light. One lux equals one lumen of luminous flux spread across a single square metre of surface, so the same beam of light grows dimmer or brighter simply by changing how much area it covers. Photometry, the study of light as the human eye perceives it, treats this unit as its core measurement, weighting every wavelength by how strongly people actually see it rather than by its raw physical power. What turns a unit this small into the standard for describing everything from starlight to a factory floor?

• 2. The Math Of Illuminance

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  One lux equals one lumen per square metre, written as 1 lx equals 1 lm per m2, or equivalently 1 cd times sr per m2. Luminous flux, measured in lumens, is the total amount of visible light a source emits; illuminance measures how concentrated that light becomes once it lands on a surface. A flux of 1,000 lumens spread evenly across one square metre produces an illuminance of 1,000 lux, but the same 1,000 lumens spread across ten square metres produces only 100 lux, since the identical light is now diluted over ten times the area. A home kitchen lit to 500 lux might need only a single fluorescent fixture with an output of 12,000, while a factory floor covering dozens of times that area would need dozens of matching fixtures to reach the same brightness. Like other SI units, lux takes standard prefixes, so 1,000 lux becomes one kilolux, a shorthand that turns up again once the numbers climb toward daylight.

• 3. From Moonlight To Midday

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  A moonless night lit only by airglow measures just 0.002 lux, barely brighter than true darkness. A quarter moon raises that to about 0.01 lux, and a full moon can climb as high as 0.05 to 0.3 lux depending on conditions. Civil twilight fades into true night at around 3.4 lux, the point defined as the dark limit of civil twilight under a clear sky. Public spaces with dark surroundings measure 20 to 50 lux, close to the 50 lux the source records for a family living room in Australia in 1998, while an office hallway or toilet runs to about 80 lux. A very dark overcast day reaches about 100 lux, and a railway station platform sits brighter still. Office lighting typically reaches up to 500 lux, a range that also covers the brightness of a sunrise or sunset on a clear day. An overcast day, or a typical television studio, measures close to 1,000 lux, and full daylight without direct sun climbs to 10,000-25,000 lux. None of these readings stays fixed, though: the same light source can produce very different numbers depending on the angle at which it strikes a surface.

• 4. Angle And Distance

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  The Sun itself obeys this rule at the largest scale: an unobscured sun delivers up to 100 kilolux at the Earth's surface, though the exact figure shifts with the time of year and the state of the atmosphere, tied to a solar illuminance constant of 128000. That same relationship explains something as small as a pocket flashlight: aimed straight at a wall, it lights a small, bright patch, but angled away from perpendicular, it spreads the same light over a larger area, dimming it in the process. The dimming follows the cosine of the angle between the incoming ray and the surface's normal line, a rule that holds for any point source and any surface it strikes. Adding up the contribution of every point on every light source, weighted by its own angle and distance, is how lighting engineers calculate the total illuminance falling on a real surface.

• 5. Lux Versus Watts

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  Lux has a radiometric cousin: the watt per square metre, which measures irradiance rather than illuminance. Radiometric units weigh every wavelength of light equally, treating raw physical power as the only thing that matters, while photometric units like lux weight each wavelength by how strongly the human eye actually responds to it, a weighting called the luminosity function. There is no single formula for converting between the two; the conversion factor changes with every wavelength, so it depends entirely on the spectral makeup of the light being measured. The luminosity function peaks at 555 nanometres, a green wavelength the eye's image-forming visual system responds to more strongly than any other. At that wavelength, one watt per square metre of monochromatic light produces 683.002 lux, the theoretical ceiling for lumens per watt, and the irradiance needed to register just one lux there is about 1.464 milliwatts per square metre. White light sources fall well short of that ceiling, because a source that looks white cannot rely on green light alone; it needs a mix of red and blue wavelengths the eye is far less sensitive to. The ratio between a source's actual lumens per watt and the 683.002 theoretical maximum is called luminous efficiency, and a typical incandescent bulb reaches only about 2 percent. Individual human eyes vary slightly in their own luminosity functions, which is why photometric units are defined against an agreed standard built from measurements across many people's eyes rather than any one person's vision.

• 6. Cameras And Older Units

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  Video camera specifications, such as those for camcorders and surveillance cameras, often list a minimum illuminance in lux, the darkest light level at which the device can still capture a usable image; a lower lux rating signals better performance in near darkness. Still cameras skip this specification because longer exposure times can compensate for low light, an option video generally lacks once the frame rate sets a hard limit on how long each exposure can run. Older, non-SI units persist alongside lux. The foot-candle, still used in English and American measurements, equals about 10.764 lux, the illuminance a one-candela source casts on a surface exactly one foot away; some call it a metre-candle informally, though that name falls outside SI standards. One phot equals 10 kilolux, and one nox equals one millilux, defined at a light color of 2042 or 2046 kelvin, a figure that was formerly set at 2360 kelvin. Even Unicode carries a legacy symbol for the abbreviation lx, a holdover meant for old Asian code pages that modern style guides advise against using in new documents.

• 7. Starlight By The Numbers

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  A star of apparent magnitude 0 delivers 2.54 microlux outside Earth's atmosphere, dropping to 2.08 microlux, 82 percent of that value, once a clear sky is factored in, a scale astronomers use to define how bright any star looks from the ground. A magnitude 6 star, at the edge of what the naked eye can detect, provides just 8.3 nanolux. A single candela of light, a kilometre away, produces about one microlux, roughly the same illuminance as a magnitude 1 star. Illuminance itself can be redefined for other purposes: the melanopic function, built around the melanopsin response rather than ordinary vision, yields what is called melanopic equivalent daylight illuminance, still expressed in lux, and researchers have proposed comparable versions of the unit for measuring light as other animal species perceive it.