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Please read moreQuantitatives
Units and formulas
| Type | Value | Symbol | Formula | Name | Unit |
| Radiation value | Luminous flux | F | F = I � W | Lumen | [lm] |
| Sender-side value | Luminous intensity Luminance |
I L |
I=F/W L=I/A |
Candela Candela pro square meter |
[cd] [cd/m2] |
| Recipient-side value | Iluminance | E | E=F/A | Lux | [lux] |
Luminous intensity is the luminous flux of a light source in a particular direction and not dependent on the size of the recipient. It can be indicated by a vector.
If we connect the ends of all the luminous intensity vectors lying in one plane for a light source we obtain the luminous intensity distribution curve.
The luminous flux indicates the radiated power emitted by a light source in all directions. This radiation is evaluated according to the sensitivity of the human eye. All other photometric values are derived from this basic value.
Illuminance is purely a physical measurement value. It is the ratio between the luminous flux and the area to be illuminated, irrespective of the reflectance of the surface. An illuminance of 1 lux occurs when a luminous flux of 1 lm falls evenly on an area of 1 m2. Illuminance reduces with the square of the distance between the light source and the surface.
Luminance indicates the luminous intensity of a light source or an illuminated surface, referred to its observed area. For humans, light is not visible until radiation enters the eye. Luminance is the only variable that can be perceived by humans.
| Light source | Average luminance [cd/m2] |
| Sun at high noon | 1,6 � 109 |
| Xenon short-arc lamp | 1,5 � 108 - 2,7 � 109 |
| Metal halide short arc lamps HMI, HTI | 5 � 107 - 1 � 108 |
| Metal halide lamps HQI | 5,3 � 106 |
| Incandescent lamps clear | 2 � 106 - 2 � 107 |
| Incandescent lamps frosted | 5 � 104 - 4 � 105 |
| Low-pressure sodium lamp | 7,5 � 104 |
| Fluorescent lamp / Compact fluorescent lamp | 1,2 � 104 - 1,4 � 104 |
| White illuminated cloud | 1 � 104 |
| Candle | 7.500 |
| Clear Sky | 3.000 - 5.000 |
| Moon | 2.500 |
| Glow discharge lamp | 200 - 600 |
| Nightsky | 10-3 |
Illuminance is a value that is easy to calculate. Luminance, on the other hand, depends on the reflection properties and the reflectance of the materials illuminated and these variables are often not known. Lighting designers therefore use illuminance for planning indoor lighting systems.
