Glossary of technical lighting terms
The main basic lighting terms
There are many specialist terms and principles in lighting technology, like in any other specialist area. We use specific units to specify the characteristics of light sources and light fixtures.
The main terms are defined below.
Colour rendering (CRI or Ra)
Colour rendering requirements vary according to the application. Natural colour rendering is always the optimal option. Lower colour rendering is never an advantage.
Colour rendering is measured according to the light source's colour rendering properties. The most common way to indicate this is with the colour rendering index (Ra). To define the Ra value, the colour rendering ability of the light source in question is compared to that of a reference light source. This is done by using eight standard colour samples. The fewer colour discrepancies are detected, the higher the light source's RA value. A light source with an Ra of 100 renders the colours optimally compared to the reference light. The lower the value, the lower the colour rendering.
The Ra value should always be specified together with the colour temperature (Tf).
Examples of Ra index:
- Light bulb 100
- Halogen light bulb 100
- Daylight 100
- Full-colour fluorescent tube 85
- Special full-colour fluorescent tube 95
- Metal halogen bulb 85-92
- Monochrome fluorescent tube 52
- Mercury light bulb 50
- High-pressure sodium light bulb 20
Colour temperature (Kelvin, K)
The colour temperature of the light from a light source is defined in comparison to a black body radiator, and is presented in a chromaticity coordinate system according to the Planck temperature curve. A light bulb with a warm tone light, for example, has a colour temperature of 2,700 K. A light source resembling daylight may have a colour temperature of 6,000 K. Colour temperature should always be indicated with a chromaticity diagram.
The perceived light colour can be described relatively well by means of the colour temperature. Light colour can be divided into three main colour groups:
- Warm tone < 3,000 K
- White 3,300-5,000 K
- Daylight > 5,000 K
Light sources with the same light colour can have different colour rendering properties due to differing spectral composition.
Illuminance (Lux, lx)
Illuminance indicates how much light hits an illuminated surface.
An illuminance of 1 lux occurs when a luminous flux of 1 lumen is evenly distributed over a surface of 1 m.
Light and radiation
By light, we mean the electromagnetic radiation perceived by the eyes. Radiation within a wavelength range of 380-780 nanometers is perceived by our eyes as light. This range is only a very small part (spectrum) of the radiation's actual wavelength range.
Luminance (Candela per square metre, cd/m2)
Luminance indicates how bright the eye perceives a surface or light source to be.
Luminous efficacy (Lumen per watt, lm/W)
Luminous efficacy indicates the amount of lumen generated per watt consumed. It is also a measure of how economical a light source is.
Luminous flux (Lumen, lm)
Luminous flux means the total light radiation emitted from a light source within a person's field of vision. Luminous flux is a measure of the power of light. In some specific cases, it can also be defined in watts (W).
Luminous intensity (Candela, cd)
A light source normally emits light with varying intensity and in several directions. The intensity in a given direction is defined as luminous intensity.