For the quantitative measurement of light, a special set of concepts and units has been adopted that bear no direct relationship to those used in other domains of physical science. This is in contrast with measuring practice in other wavelength regions of the electromagnetic spectrum which is generally based on the familiar concepts of energy and power, and therefore use the SI units of joules and watts.
The principal reason for this is that a lighting unit must not only take into account the energy content of the radiation but also the spectral distribution of the sensitivity of the human eye, which varies greatly with wavelength.
Luminous flux
Luminous flux – is the total amount of light radiated by a light source per second. A more familiar term would be “light output". It is expressed in lumens (lm).
Illuminance
Illuminance – is the quantitative expression for the luminous flux incident on unit area of a surface. A more familiar term would be “lighting level”. Illuminance is expressed in lux (lx), one lux equals one lumen per square metre (lm/m²). [other units are – metrecandle, phot, nox] In Imperial units the unit is the foot-candle which equals lumen per square foot (lm/ft²).
Luminous intensity
Luminous intensity – is the luminous flux radiated by a light source in a specific direction. Luminous intensity is expressed in candelas (cd).
Luminance
Luminance – is the quantitative expression for the amount of light reflected by a surface in a specific direction. A more familiar word is “brightness”, although this term must, strictly speaking, be reserved to describe the subjective impression of luminance on the eye. The luminance of a surface is determined by the illuminance on the surface in question and its reflective properties. Luminance is expressed in candelas per square metre (cd/m²), referred to as the nit. [other units are – lambert, stilb, apostilb, blondel, skot]. In Imperial units the unit is the foot-lambert, which is candelas per square foot (cd/ft²)
Luminous efficacy
Luminous efficacy – is the ratio between luminous flux and power dissipation, and is expressed in lumens per watt (lm/W). Each lamp type has a different luminous efficacy.
Examples of Illuminance
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Summer, under a cloudless sky, in the open 100,000 Lux
Summer, under a cloudless sky, under a tree 10,000 Lux
In the open, under a heavily overcast sky 5,000 Lux
Indoors by the window, shaded, clear day 2,000 Lux
Indoors away from the window 300 Lux
Full moon, in the open, on a clear night 0.25 Lux
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The target illuminance for an interior space depends on the specific visual tasks carried out in the space and can be anything from hundreds to thousands of Lux.
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100 Lux interiors used occasionally with visual tasks confined to movement and for only limited perception of detail.
200 Lux interiors occupied for long periods, or for visual tasks requiring some perception of detail.
500 Lux moderately difficult visual tasks, perhaps involving colour judgement.
1000 Lux very difficult visual tasks.
2000 Lux exceptionally difficult visual tasks
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[see “CIBSE, Code for Interior Lighting” for more details]
