As with any technical or scientific discipline, lighting technology has its own terminology. Special terms and concepts are used to define the characteristics of lamps and luminaires and to standardise the units of measurement. The most important of these are described here.

Light and radiation

  Light is taken to mean the electromagnetic radiation that the human eye perceives, in other words that part of the spectrum that can be seen. This is the radiation between 360 and 830nm, a tiny fraction of the known spectrum of electromagnetic radiation.

Luminous flux £X
  Unit of measurement : lumen (lm).
All the visible radiated power emitted by a light source and perceived by the eye is called luminous flux £X.

Luminous intensity I

Unit of measurement : can-dela(cd).

Generally speaking, a light source emits its luminous flux £X in different directions and at different intensities. The visible radiant intensity in a particular direction is called luminous intensity I.

Illuminance E

Polar diagram

Unit of measurement : lux (lx).

Illuminance E is the ratio between the luminous flux and the illuminated area. An illuminance of 1 lx occurs when a luminous flux of 1 lm is evenly distributed over an area of 1 square metre.

Luminance L

Unit of measurement : candelas per square metre (cd/m2)

The luminance L of a light source or an illuminated area is a measure of its luminous intensity per unit area of illuminated surface.

The most important photometric formulae :

  I , Luminous intensity[cd] = Luminous flux in solid angle / Unit solid angle
  E , Illuminance[lx] = Luminous flux falling on area[lm] / Illuminated area[¢T]
  = Luminous intensity[cd] / (Distance in metres2[m] )2
  L , Luminance[cd/m2]= Luminous intensity[cd] / Viewed luminous area[m2]
  Luminous efficacy[lm/W]= Generated luminous flux[lm] / Electrical power consumed[W]

Chromaticity diagram to DIN 5033


Extract from the chromaticity diagram showing the Planckian curve


Daylight spectrum of a BIOLUX fluorescent lamp.
The radiation is very evenly distributed over the entire visible range.

Luminous efficacy,
  Unit of measurement : lumens per watt[lm/W].
Luminous efficacy indicates the efficency with which the electrical power consumed is converted into light.

Color Temperature
  Unit of measurement ( Kelvin, K )

The colour temperature of a light source is defined in comparison with a "black body radiator: and plotted on what is known as the "Planckian curve". The higher the temperature of this "black body radiator" the greater the blue component in the spectrum and the smaller the red component. An incandescent lamp with a warm white light, for example, has a colour temperature of 2700K, whereas a daylight fluorescent lamp has a colour temperature of 6000K.

Light color
  The light colour of a lamp can be neatly defined in terms of colour temperature. There are three main categories here :
Warm < 3300K
Intermediate 3300 to 5000K
Daylight > 5000K.

Despite having the same light colour, lamps may have very different colour rendering properties owing to the spectral composition of their light.

Color rendering
  As a rule, artificial light should enable the human eye to perceive colours correctly, as it would in natural daylight. Obviously, this depends to some extent on the location and purpose for which light is required. The criterion here is the colour rendering property of a light source. This is expressed as a "general colour rendering index"(Ra).

The colour rendering index is a measure of the correspondence between the colour of an object (its "self-luminous colour") and its appearance under a reference light source. To determine the Ra values, eight test colours defined in accordance with DIN 6169 are illuminated with the reference light source of the same colour temperature as the source under test and the test light source. The smaller the discrepancy, between the two sources, the better the colour rendering property of the lamp being tested.

A light source with an Ra value of 100 displays all colours exactly as they appear under the reference light source. The lower the Ra value, the worse the colours are rendered.

Luminaire efficiency
  Luminaire efficiency (also known as light output ratio) is an important criterion in gauging the energy efficiency of a luminaire. This is the ratio of the luminous flux emitted by the luminaire to the luminous flux of the lamp (or lamps) installed in the luminaire.
Information data from Osram.


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