Light is visible radiation between 380nm and 780nm. Beyond this range the radiation is not visible for the human eye. It is radiation in infrared, ultraviolet or radio frequency. The visible light range is called spectrum. The spectrum of a light source contains blue, green, yellow, red and all the colours in between. The more colours represented in the spectrum, the more colors are visible under the light source. The less colours in the spectrum, the poorer the light source’s colour reproduction (see also colour rendering index CRI and spectrum).

Luminous Flux – lm

Luminous flux is measuring the light power emitted by a light source. Luminous flux is describing the total visible light power that is radiated into all directions. It is appreciated by ? and measured in lumen (lm). The instrument for measuring luminous flux is an Ulbricht sphere.

Light Intensity – I – cd

Light intensity is measuring the luminous flux of the light source that is emitted into a certain angle or direction. It is appreciated by I and measured in candela (cd). It is describing the light intensity distribution curve of a luminaire or light bulb (see Light Intensity Distribution Curve LDC). The instrument for measuring light intensity is a Goniometer.

Light Intensity Distribution Curve (LDC)

The Light Intensity Distribution Curve is describing the spatial radiation of a light source or luminaire. It is mostly illustrated as graphic, showing the curves of the light intensity. The instrument for measuring light intensity distribution curve is a Goniometer.

Illuminance – E – lx

Illuminance is measuring the luminous flux of a light source that is reaching a certain surface. It is measured in lm per m² called lux (lx) and appreciated by E. 1lx is measured if 1lm is illuminating equally an area of 1m². The instrument for measuring the illuminance on a surface is a Luxmeter. Illuminance is not an indicator for brightness! A dark and bright colored room may have the same illuminance level on the floor but the dark one will always appear darker.

Horizontal Illuminance – E– lx

Horizontal Illuminance is describing the illuminance level on horizontal and nearly horizontal spaces like office desks or keyboards.

Vertical Illuminance – Ev – lx

Vertical Illuminance is describing the illuminance level on vertical and nearly vertical areas like signboards or advertisement boards.

Zylindric Illuminance – Ez – lx

Zylindrical Illuminance is measuring vertical illuminance levels of figures, objects and human faces. Several measurements points are considered. Objects and especially faces are important to illuminate equally without hard shadows.

Luminance – L – cd/m²

Luminance is used for describing brightness. It is measuring the luminous flux from an illuminated or self-lighting surface at the observer’s eye. Luminance L is measured in candela per square meter cd/m². The instrument for measuring luminance is a luminance camera.

Light Efficiency – lm/W

Light Efficiency is describing the effectiveness of a light source regarding the electrical energy that is converted into visible light. How much electrical power (W) has been converted into luminous flux (lm).   Some examples:
Incandescent lamps (home application): 12 lm/W Halogen lamp: 20 lm/W CFL: 60 lm/W Fluorescent lamp: 90 lm/W LED: 120 lm/W


Glare is tiring the eye and makes it hard to see clearly in certain spaces. Glare is reducing the visual performance and visual comfort of the eye. There are differences in direct and indirect glare. Direct glare is caused by luminaires or surfaces with too high luminances. If the light sources have no housing for example or if luminaires are mounted wrongly, direct glare can be the result. Indirect glare is caused by specular reflection at mirrors or highly shiny surfaces like monitors, high gloss paper or by wet streets. It is highly important to avoid any type of glare as the visibility is dramatically reduced and the possibility for accidents is exalted. Glare can be measured by UGR system for indoor applications or as TI rates for outdoor use.

(Correlated) Color Temperature – CCT – K

Each light source is radiating light in a certain light color. The light color is defined by the spectrum of the light source and in comparison to an ideal black body radiator. The black body radiator is emitting visible light when heated up. The more heated up, the cooler the color temperature. The lower the temperature, the warmer the light color of the radiator. The temperature range of a black body radiator is called black body line. Due to production processes and material, the color temperature of a light source does not lie on the black body curve. The light color is then defined as correlated color temperature and is an approximation of the color temperature that is not located at the black body curve but corresponds to lines crossing the black body curve indicated as CCT and measured in Kelvin K as well. Instrument for measuring the color temperature is a luminance camera. The software is recording the spectrum and calculating the color temperature from that information. A typical range of color temperatures for products of:

2700K – 3200K WW Warm White
4000K – 4500K NW Natural White
5000K – 5500K DW Day White
6500K – 7500K CW Cool White

Colour Rendering Index – CRI

The CRI is describing the coluor rendering index of a light source. The better the light source represents colours, the better the index. Sunlight and incandescent bulbs offer a CRI 100 (best rate). They have a wide spectrum and represent all colours. The CRI of a LED varies between 70 up to 95, the one of a fluorescent tube lies between 60 and 98. It is important to watch the CRI when choosing light sources for a project. Faces should be illuminated with a high CRI because the skin colour should look healthy. Restaurant owners want their dishes to look delicious and all colours of the food should be represented. Warehouses store goods only and machines or people run through the aisles and check IR code for packing and sending them further. These areas don’t need a good colour rendering index as the task is just to choose objects that are not differentiated by their colour, a CRI of 70 would be enough. Instrument for measuring the colour rendering index is a luminance camera. The software is recording the spectrum and calculating the colour rendering index from that information.


Light is the visible range from 380nm till 780nm, called spectrum. The more colours are represented in the spectrum, the better the CRI of a light source. Each light source has a different spectral distribution of radiation. Temperature emitters like incandescent lamps have a continuous spectrum. High as well as low intensity discharge lamps represent a spectrum according to their filling gas and tube cover slush. Instrument for measuring the spectrum is a luminance camera. The software is recording the different wavelengths and illustrates the spectrum from that information.

Radiation Angle

The radiation angle is the angle of a luminaire in which it is emitting the light from the light source. It depends on the construction and optical design of the luminaire. The smaller the radiation angle, the smaller the illuminated surface but the more focused the light on the object. Spots are typical luminaires with small radiation angles to highlight surfaces and objects. Small radiation angles are used for accent lighting. The bigger the radiation angle, the wider the illuminated surface and the more equally illuminated. Highbay luminaires as well as office luminaires have the goal to illuminate a wide area equally with a low number of luminaires. Big radiation angles are used for general lighting purposes. Instrument for measuring the radiation angle is a Goniometer.

Flicker and Stroboscopic Effects

Flicker is disturbing and can lead to physiological effects like headache. Stroboscopic Effects can result in dangerous situations when changing the visual impression of the movement of rotating machine parts. Lighting systems should be planned to avoid these effects.