Basic Terms in Light Technology

We clarify some of the common terms related to lighting and light technology.

Technical Lighting Glossary

Technical aspects of light are described using a number of common terms. Similarly, the qualities of light sources and fittings are described using established units.

Let us give you a quick guide regarding our most important technical lighting terms.

Light intensity (Lux, lx)

Light intensity refers to how much light falls in a given lit-up area.
A light intensity of 1 lux occurs when the flow of light of 1 lumen is evenly distributed on an area of 1m.

Color temperature (Kelvin, K)

The color temperature of light from a light source is defined by comparing it to a “blackbeam” and is accounted for in a color coordination system according to the Planks temperature curve. A light bulb with warm light, for example, might have a color temperature of 2,700 K. A daylight-like light source might have a color temperature of around 6,000 K.

Color temperature is always stated in combination with color rendering. Light sources that don’t administer light through heat, which is nowadays the most common way, use the term correlated color temperature (CCT), where the Kelvin value is calculated graphically to get an idea of how light will be perceived. This is the case for LED, fluorescent light, metal halogen and high pressure natrium light sources, to name a few examples.

Color rendering (Ra or CRI)

The term Ra (Rendering average) is commonly used in Sweden, while the term CRI (Color Rendering Index) is used internationally.

Depending on the use there are different requirements for color rendering. A natural color rendering is always advantageous. Poor color rendering can never increase value.

The way color rendering is measured is by checking the color rendering qualities of the light source. The most common way to state this is through the color rendering index Ra.

The color rendering index Ra is the unit by which how well the compared light fits with a reference light is measured. To get the Ra value, the ability of the light in question to render colors is compared to the ability of the reference light. 8 standardized color samples are used for this. An additional 6 fuller standardized colors have also been brought forward, to achieve a more accurate measurement. In this case, the term used is Ra14. The less deviation from the reference light, the higher the Ra number of the source light. A light source with a Ra value of 100 optimally renders colors compared to the reference light. The lower the number, the worse the color rendering is.

Color rendering measured in Ra is always specified in combination with the color temperature Tf.

Some examples of the Ra index:

  • Light bulb 100
  • Halogen light bulb 100
  • Daylight 100
  • Full color fluorescent rods 85
  • Full color fluorescent rods special 95
  • Metal halogen lamps 85-92
  • Single color fluorescent rods 52
  • Quicksilver lamps 50
  • High pressure natrium lamps 20

Light flow (Lumen, lm)

For light flow, the total radiation of light from a light source inside the visible area is stated. The light flow is a lighting effect that in special cases also can be stated using the term watt (W).

Light color

The perceived color of light can be described relative to the color temperature. The color groups can be divided into three primary groups in this regard:

  • Warm white < 3 000 K
  • Neutral white 3 000-5 000 K
  • Cold white or Daylight > 5 000 K

Despite having the same light color, different light sources can have different rendering capabilities due to differences in the spectral composition.

Light exchange (Lumen per watt, lm/W)

The light exchange indicates in lumen how much light is produced per watt spent. Consequently, this becomes a measurement for the financial qualities of a light source.

Light and radiation

By light, we mean electromagnetic radiation perceived by the eye. The eye is having a light spectrum experience as a result of this radiation. In this case we are talking about a radiation of a wavelength range of 380-780 nanometer. This range of light is a very small part of the radiation spectrum.

Light strength (Candela, cd)

Normally, a light source radiates its light at various degrees of strength and in multiple directions. The intensity in a given direction is denoted as the strength of the light.

Luminance (Candela per square meter, cs/m2)

The luminance is the experienced brightness of the light on any given surface or in the light source itself, as perceived by the eye.


Flickering might be perceived by the eye, or at higher frequencies not directly by the eye but still appear annoying. Flickering is a problem often associated with lighting powered at too low a frequency, but it can also appear as a strobe effect during machine work.

Flickering should be avoided as it can cause problems like tired eyes and headaches. Today, most lighting is powered by high frequency units, so called HF units, which makes the risk of flickering very low. When dimming light, the risk of flickering increases.

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