Lighting technical definitions

Size/Concept

Symbol

Unit Formula Explanation
Light flux

φ

Lumen (lm)

  The lumen value is the total amount of light from a light source, defined as the amount of light perceived by the human eye.
Light strength

I

Candela(cd)

I = φ/ω

Light strength is the intensity of light in a given direction, defined as a lumen value per solid angle.
Light intensity

E

Lux (LX)

E = φ/A

Light intensity is defined as the lumen value which reaches a surface (A) pr. m².
Luminance

L

(cd/m²)

L = I/A (L=I/A cosφ)

Luminance/brightness is defined as the amount of light reflected from a point or surface in a specific direction.
Luminance difference  
  Indicates the transition between light and dark surfaces. Substantial differences can be disturbing in a concentrated working environment.
Colour temperature  

Kelvin (K)

CIE 17.4

Colour temperature describes the impact of colour and gives a warm or cold light < 4000K = warm light; > 4000K = cold light. Colour temperature is defined as the colour a black body becomes at a given temperature.
Colour rendering

Ra

Ra-index

CIE 17.4

Colour rendering measures the light source’s ability to render colours correctly and is graded from 0-100. 8 colours act as controls against a pre-defined light source.
Luminous efficacy

H

 Lm/W

Η = φ/P

The luminous efficacy of a light source is defined as the light source’s lumen value against energy consumption. When the consumption of other components in the luminaire is included (ballast, emergency lighting etc.) we call this the “system efficacy”.
Uniformity    

Emin / Emid Lmin / Lmid

The relationship between the lowest value and the average value. relates to both light intensity and luminance.
Glare    

CIE-31

Unpleasant visual circumstances which reduce the ability to see details. Normally divided into unpleasant glare (UGR/NB) and Sight restrictive glare (TI/GR).
Cut-of angle       The angle between a horizontal axis through the luminaire and the line of sight where the light source becomes visible.
Luminarie efficiency

η

%

  The efficiency of the luminaire is a percentage of how much light escapes the luminaire. The efficiency of the luminaire is measured at 25ºC and compared with the light source, which is also measured at 25ºC.
Reflection coefficient

ρ

%

φind / φud

A percentage of how much light is reflected from a surface.
Reflective surface semi-diffuse surface Diffuse surface       A reflective surface reflects light at an opposite angle to the source. A semidiffuse surface reflects most of the light in the same way as a reflective one, but some of the light is also diffused in other directions. A diffuse surface has a max reflected light strength 90º of the surface in which I = cosφ x Imaks.
Average lifetime  

Hours (h)

  The average lifetime is defined as 50% of the time that a majority of the components work for.
Service lifetime  

Hours (h)

  Service lifetime is the point in time where 80% of the total amount of light remains.
Economic lifetime  

Hours (h)

  The economic lifetime is the point at which 70% of the total available light from a unit remains.

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