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Light Glossary

Luminous flux Φ

Unit: Lumen [lm]

The luminous flux – measured in units called Lumen [lm] – is the radiant power emitted by the light source in all directions.

Luminous efficacy ŋ

Unit: Lumen per Watt [lm/W]

In order to produce artificial light an electricity supply is required. The luminous efficacy – with the ratio Lumen/Watt [lm/W] indicates how efficient the electrical energy is converted into light. The higher the lu minous efficacy, the more economical the light source. Some examples: A classic light bulb produces approximately 1000 hours of light with a maximum luminous efficacy of 14 lm/W. A halogen lamp with 12 volts achieves almost double the luminous efficacy with 26 lm/W and only needs to be changed after approximately 4000 hours. A fluorescent light with 54 W has a luminous efficacy of 82 lm/W.

Luminous intensity I

Unit: Candela [cd]

A source of light emits its luminous flux, i.e. its radiant power in varying degrees of strength and in different directions. When the luminous flux radiates in a particular direction it is known as luminous intensity I. If one connects the luminous intensity of the different directions a luminous intensity distribution curve occurs. Light distribution is an important criteria for choosing a light or a lamp. One must decide between direct, indirect or uniform light distribution. Direct means, that the light shines in a straight direction, for example on to a writing table. An indirect light distribution results when the light from a lamp is reflected from light coloured walls and ceilings. With a uniform distribution, the light shines evenly in all directions.

Light density L

Unit: Candela per m2 [cd/m2]

Every shining or illuminated object leaves behind a definite impression of brightness. The luminance measured in Candela per square meter [cd/m2] expresses this impression of brightness. Luminance ist the only measurement of lighting technology that is visible to the eye. It is better suited to describe the lighting situation in a room than the indication of the luminous power in Lux, which, however, is much easier to measure and calculate.

Luminous power E

Unit: Lux [lx]

The luminous power indicated as Lux [lx] is a measurement for light falling on an illuminated surface (luminous flux). It is calculated as follow:
E = Φ ÷ A
Φ = Luminous flux in Lumen (lm)
A = Surface [m²]
For working areas and interiors there are European levels for the luminous intensity, which can be found in the standard DIN EN 12464. For working areas this standard must be complied with and they therefore serve as a guideline for the planning and layout of lighting systems.

Light colour K

Unit: Kelvin [K]

The colour of light is determined by the light temperature, which is measured in Kelvin [K]. There are three main groups: warm white light (ww) below 3300 K, neutral white light (nw) ranging from 3300 to 5000 K and daylight white light (tw) above 5000 K. Neutral white light is produced for example by fluorescent lamps and because of its functional light effect it is preferred in workshops and industrial surroundings. In living areas mainly bulbs and halogen lamps with warm white light are used to create a cosy atmosphere. The light colour says nothing about the colour reproduction properties of the light source!
The colour temperature does not give any information on the luminous intensity. Luminaires with 4000K produce a light which often appears brighter to the human eye. In sanitary and white rooms (e.g. bathrooms, laboratories, etc.), the luminaires with 4000K are often used to give surfaces a cleaner appearance. 

Colour rendering index Ra

Depending on the surroundings, artificial light should facilitate an accurately correct colour perception of illuminated objects (as with natural daylight). The colour reproduction index Ra or the colour reproduction level serves as a guideline. The highest value of Ra is 100. A light source with Ra = 100 allows all the colours in our surroundings to appear natural. The lower on the scale, the poorer the colours of the illuminated objects will render.