More than most know about lighting

Glossary of important lighting terms

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 luminaries and to standardize 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 as brightness. 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

Unit of measurement: lumen [lm]. All the radiated power emitted by a light source and perceived by the eye is called luminous flux .

Diagram 1

Luminous intensity I is a measure of the luminous flux emitted in solid angle .

Luminous intensity I

Unit of measurement: candela [cd]. Generally speaking, a light source emits its luminous flux in different directions and at different intensities. The visible radiant intensity in a particular direction is called luminous intensity I.

Illuminance E

Unit of measurement: lux [lx]. Illuminance E is the ratio between the luminous flux and the area to be illuminated.

An illuminance of 1lx occurs when a luminous flux of 1lm is evenly distributed over an area of 1 square meter.

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 great an impression of brightness is created in the brain.

Luminous efficacy

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

Colour temperature

Unit of measurement: kelvin [K].
The colour temperature of a light source is defined in comparision 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 2700 K, whereas a daylight fluorescent lamp has a colour temperature of 6000 k.

Light colour

The light colour of a lamp can be neatly defined in terms of colour temperature. There are three main categories here:

• Warm <3300 K
• Intermediate 3300 to 5000 K
• Daylight > 5000 K

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

Colour 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 measurement 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 and the light source under test. The smaller the discrepancy, 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 between the luminous flux emitted by the luminaire and the luminous flux of the lamp (r lamps) installed in the luminaire.

For detailed information on indoor lighting with artificial light click here: DIN 5035.