Glossary of Technical 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.

Color Rendering (CRI or Ra)

Color rendering requirements vary according to the application. Natural color rendering is always the optimal option. Lower color rendering is never an advantage.

Color rendering is measured according to the light source’s color rendering properties. The most common way to indicate this is with the color rendering index (Ra). To define the Ra value, the color rendering ability of the light source in question is compared to that of a reference light source. This is done by using eight standard color samples. The fewer color discrepancies are detected, the higher the light source’s RA value. A light source with an Ra of 100 renders the colors optimally compared to the reference light. The lower the value, the lower the color rendering. The Ra value should always be specified together with the color temperature (Tf).

Examples of Ra index:

  • Light bulb 100
  • Halogen light bulb 100
  • Daylight 100
  • Full-color fluorescent tube 85
  • Special full-color fluorescent tube 95
  • Metal halogen bulb 85-92
  • Monochrome fluorescent tube 52
  • Mercury light bulb 50
  • High-pressure sodium light bulb 20


Color temperature (Kelvin, K)

The color 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 color temperature of 2,700 K. A light source resembling daylight may have a color temperature of 6,000 K. Color temperature should always be indicated with a chromaticity diagram.

Light color

The perceived light color can be described relatively well by means of the color temperature. Light color can be divided into three main color groups:

  • Warm tone < 3,000 K
  • White 3,300-5,000 K
  • Daylight > 5,000 K

Light sources with the same light color can have different color 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 meter, 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.