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Four basic physical units of light
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Radiometry and Photometry
While radiometry is the measurement of power produced by a source of electromagnetic radiation photometry is the effect of electromagnetic radiation to the human eye or in other words in a visual system.
It is distinct from radiometry, which is the science of measurement of light in terms of absolute power; rather, in photometry, the radiant power at each wavelength is weighted by the luminosity function that models human brightness sensitivity.
Luminous flux
Unit of measurement: lumen [lm]
All the radiated power emitted by a light source and perceived by the eye is called luminous flux. The unit of measurement is the lumen (lm).
The visible radiated power of a light source is not expressed in watts but in lumens since the sensitivity of the human eye differs according to the particular wavelength.
1 lumen is the luminous flux radiated to 1 sr angle by point like light source having 1 candela luminous intensity.
Luminous Intensity (I)
Unit of measurement: candela [cd]
A light source emits its luminous flux in different directions (solid angle ; the unit of measurement is the steradian sr) and at different intensities. The visible radiant intensity in a particular direction is called luminous intensity (I). The unit of measurement is the candela (cd).
  
The sun is a natural incandescent source (at about 5800 K on the surface), so is a candle flame. The most common man-made source is the tungsten filament light bulb at about 2854 K.
Illuminance E
Unit of measurement: lux [lx]
Illuminance E indicates the degree to which an area is illuminated. It is the ratio between luminous flux and the area to be illuminated. The unit of measurement is the lux (lx). An illuminance of 1 lx occurs when a luminous flux of 1 lm is evenly distributed over an area of 1 square meter. In practice, however, it is unlikely that the luminous flux will be so evenly distributed over the illuminated area that all the points in this area will have the same illuminance value.
 
For humans, light is not visible until radiation enters the eye. Luminance is the only variable that can be perceived by humans.
The table below shows some general illuminance values.
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Light source |
Average luminance [cd/m2] |
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Sun at high noon |
1,6 ~ 109 |
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Xenon short-arc lamp |
1,5 ~ 108 - 2,7 ~ 109 |
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Metal halide short arc lamps HMI, HTI |
5 ~ 107 – 1~ 108 |
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Metal halide lamps HQI |
5,3 ~ 106 |
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Incandescent lamps clear |
2 ~ 106 – 2~ 107 |
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Incandescent lamps frosted |
5 ~ 104 - 4 ~ 105 |
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Low-pressure sodium lamp |
7,5 ~ 104 |
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Fluorescent lamp / Compact fluorescent lamp |
1,2 ~ 104 - 1,4~104 |
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White illuminated cloud |
1 ~ 104 |
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Candle |
7.500 |
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Clear Sky |
3.000 - 5.000 |
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Moon |
2.500 |
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Glow discharge lamp |
200 - 600 |
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Nightsky |
10-3 |
Luminance L
Unit of measurement: candelas per square meter [cd/m2]
The luminance L of a light source or an illuminated area is a measure of how much the eye is stimulated and therefore of how great an impression of brightness is created in the brain. Let�s assume we are looking at an illuminated (or self-luminous) area from a particular direction. The luminous intensity of this area divided by its size apparent to our eyes is its luminance L. It is measured in candelas per square meter (cd/m2).
 
Summary of luminous flux, luminous intensity, illuminance and luminance.
To summarize all this, the intensity (I) of the source is candelas, the flux () transmitted through space is lumens, the light illuminating a surface (E) is lux (lumen/ m2), and the light reflected from an area, the luminance (L) is candela/m2.
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