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Light, how it's measured and how to choose the right lighting

  • A graphical representation of 1 steradian. The sphere has radius r; the circular patch has an area on the sphere of A=r2. The solid angle is θ=A/r2 so in this case θ=1. The entire sphere has a solid angle of 4π sr ≈ 12.56637 sr.
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The Measure of Light:

Illuminance is a measure of how much luminous flux is spread over a given area. One can think of luminous flux as a measure of the total "amount" of visible light present, and the illuminance is a measure of the intensity of illumination on a surface. A given amount of light will illuminate a surface more dimly if it is spread over a larger area, so illuminance is inversely proportional to area.

The lumen (symbol: lm) is the (SI derived unit) of luminous flux - a measure of the power of light perceived by the human eye. 
Luminous flux
 differs from radiant flux in that luminous flux measurements (such as lumens) are intended to reflect the varying sensitivity of the human eye to different wavelengths of light, while radiant flux measurements (such as watts) indicate the total power of light emitted.

The lumen is defined in relation to the candela as 1 lm = 1 cd·sr 

As a full sphere has a solid angle of 4·π steradians, a light source that uniformly radiates one candela in all directions has a total luminous flux of 1 cd·4π sr = 4π ≈ 12.57 lumens.The illustration on the page header shows how steradians are represented.
Luminous flux is usually measured in
 lumens. One lux is equal to one lumen per square metre:1 lx = 1 lm/m2 = 1 cd·sr
To get an idea of the light energy represented, when measured in Lux, the table below demonstrates typical examples:-



10−4 lux

Total starlight, overcast sky

0.002 lux

Moonless clear night sky with airglow

0.01 lux

Quarter moon

0.27 lux

Full moon on a clear night

1 lux

Full moon overhead at tropical latitudes

3.4 lux

Dark limit of city twilight under a clear sky

50 lux

Family living room

80 lux


100 lux

Very dark overcast day

320–500 lux

Office lighting

400 lux

Sunrise or sunset on a clear day.

1,000 lux

Overcast day;  typical TV studio lighting

10,000–25,000 lux

Full daylight (not direct sun)

32,000–130,000 lux

Direct sunlight


















Radiometry  measures electromagnetic radiation, including visible light but note that light is also measured using photometry techniques, which measures brightness as perceived by the human eye, rather than by absolute power. Photometry is thus quite an arbitary guide and where useful for photographers, it is not much use to scientists.


Spectroradiometry is the measurement of absolute radiometric quantities, at very specific wavelengths. Just as with radio frequency, light of differing colours represents a wide range of wavelenghts and frequencies, usually measured in nano-metres, for convenience. We are not considering this aspect here. Suffice it that, across the visible light spectrum (e.g. rainbow), red light has the longest wave length and violet, the shortest (and therefore the highest frequency)

SI radiometry units



SI unit



Radiant energy





Radiant flux




radiant energy per unit time, also called radiant power

Radiant intensity


watt per steradian


power per unit solid angle



watt per steradian per square metre


power per unit solid angle per unit projected source area. called intensity in some other fields of study.


E, I

watt per square metre


power incident on a surface, sometimes confusingly called " intensity".

Radiant exitance / Radiant emittance


watt per square metre


power emitted from a surface.


J or Jλ

watt per square metre


emitted plus reflected power leaving a surface

Spectral radiance

Lλ or Lν

watt per steradian per cubic metre  or, watt per steradian per square metre per hertz

W·sr−1·m−3 or W·sr−1·m−2·Hz−1

commonly measured in W·sr−1·m−2·nm−1

Spectral irradiance

Eλ or Eν

watt per metre3 or watt per square metre per hertz

W·m−3 or W·m−2·Hz−1

commonly measured in W·m−2·nm−1 or 10-22W·m-2·Hz-1, known as a Solar Flux Unit (SFU)   [SI Radiometry units 1]


The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1683 watt per steradian.

The definition describes how to produce a light source that (by definition) emits one candela. Such a source could then be used to calibrate instruments designed to measure luminous intensity.

The candela is sometimes still called by the old name candle, such as in foot-candle and the modern definition of candlepower. The foot-candle is equal to one lumen per square foot. Since1948, candlepower has been replaced by the international unit known as the candela. One old candlepower unit is about 0.981 candela. Less scientifically, modern candlepower now equates directly (1:1) to the number of candelas.

The steradian (symbol: sr) is the SI unit of solid angle. It is used to describe two-dimensional angular spans in three-dimensional space, analogous to the way in which the radian describes angles in a plane. The steradian, like the radian, is dimensionless because 1 sr = m2·m−2 = 1. It is useful, however, to distinguish between dimensionless quantities of different nature, so in practice the symbol "sr" is used where appropriate, rather than the derived unit "1" or no unit at all. For example, radiant intensity can be measured in watts per steradian (W·sr−1).


SI photometry units



SI unit



Luminous energy


lumen second


units are sometimes called talbots

Luminous flux


lumen (= cd·sr)


also called luminous power

Luminous intensity


candela (= lm/sr)


an SI base unit



candela per square metre


units are sometimes called "nits"



lux (= lm/m2)


Used for light incident on a surface

Luminous emittance


lux (= lm/m2)


Used for light emitted from a surface

Luminous efficacy


lumen per watt


ratio of luminous flux to radiant flux

See also SI · Photometry · Radiometry



















Hyperlinks on this page lead to relevant descriptions on Wikipedia; which source we gratefully acknowledge for its clear descriptions.

Product Videos

Spectrometer Demonstration (04:26)
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