Dimensions, units

Luminous flux

The luminous flux ERCOsymphi is a measure for the amount of light of a light source.
= lumen (lm)

Luminous flux describes the total light power emitted by a light source. As a rule, this radiant power could be expressed as emitted energy in the unit of watts. However, this method is inadequate for describing the optical effect of a light source, since the emitted radiation is recorded without discrimination over the entire frequency range and the different spectral sensitivity of the eye is not considered. The inclusion of the spectral sensitivity of the eye results in the quantity termed lumen. A radiant flux of 1W emitted at the maximum extent of spectral optical sensitivity (photopic, 555 nm) gives a luminous flux of 683 lm. Conversely, the same radiant flux emitted at frequency ranges of lower sensitivity (as per the V (λ) curve) results in correspondingly smaller luminous fluxes.

Luminous efficacy

η = F / P

η = lm / W

The luminous efficacy describes the efficacy of a lamp. It is expressed as the ratio of the emitted luminous flux in lumen and the power used in watts. The theoretically attainable maximum value assuming complete conversion of energy at 555 nm would be 683 lm/W. The luminous efficacies that can actually be attained vary depending on the lamp, but always remain far below this ideal value.

Light intensity


An ideal, point light source radiates its luminous flux evenly in all directions in the room, with its light intensity being equal in all directions. In practice, however, there is always an uneven spatial distribution of luminous flux, partly due to the lamp design and partly due to the manner in which the luminaire is formed. The Candela, as the unit of light intensity, is the basic unit of lighting engineering from which all other lighting engineering values are derived.


The spatial distribution of the light intensity of a light source results in a three-dimensional body of light intensity distribution. A section through this light intensity body will give the light intensity distribution curve, which describes the light intensity distribution in one plane. The light intensity is, usually displayed in a polar co-ordinate system as a function of the emission angle. To enable direct comparison of the light intensity distribution of different light sources, the values are expressed in relation to 1000 lm luminous flux. With rotationally symmetrical luminaires, a single light intensity distribution curve is sufficient to describe the luminaire. Axially symmetrical luminaires need two curves, although, these can usually be represented on one diagram.


The illuminance is a measure for the luminous flux density on a surface. It is defined as the ratio of the luminous flux incident on a surface to the size of that surface. The illuminance is not tied to a real surface, it can be determined anywhere in the room. The illuminance can be derived from the light intensity. Whereby, the illuminance reduces by the square of the distance from the light source (inverse square law).


The illuminance is a measure for the luminous flux density on a surface. It is defined as the ratio of the luminous flux incident on a surface to the size of that surface. The illuminance is not tied to a real surface, it can be determined anywhere in the room. The illuminance can be derived from the light intensity. Whereby, the illuminance reduces by the square of the distance from the light source (inverse square law).


The product of the illuminance and the exposure time for which the surface is illuminated is called the exposure.


Whereas illuminance expresses the luminous power incident on a surface, the luminance describes the light given off by this surface. This light can be given off by the surface itself (e.g. when considering luminance of lamps and luminaires). Luminance is defined as the ratio of light intensity and the area projected perpendicularly to the emission direction. The light can also be reflected or transmitted by the surface however. For diffuse reflecting (matt) and diffuse transmitting (murky) materials, the luminance can be calculated from the illuminance and the reflectance or transmittance . Brightness correlates with luminance; although, the actual impression of brightness is still influenced by how well the eyes have adapted, by the surrounding contrast levels and by the information content of the viewed surface.

Colour of light


Light colour is the colour of the light emitted by a lamp. Light colour can be expressed using x,y coordinates as chromacity coordinates in a standard colorimetric system, or, for white light colours, it can also be given as the colour temperature TF . In the CIE standard colorimetric system, the colour of light is calculated from the spectral constitution and represented in a continuous, two-dimensional diagram. The hue is defined via the chromaticity co-ordinates of the spectral colour and via the saturation level. The design of the diagram features a coloured area that contains every possible real colour. The coloured area is encompassed by a curve on which the chromaticity locations of the completely saturated spectral colours lie. At the centre of the area is the point of least saturation, which is designated as a white or uncoloured point. All levels of saturation of one colour can now be found on the straight lines between the uncoloured point and the chromaticity location in question. Similarly, all mixtures of two colours are likewise to be found on a straight line between the two chromaticity locations in question.

Closest colour temperature

Planck's curve contains the chromaticity locations of Planck's radiation of all temperatures. Since the chromaticity location of a light source often lies near to the curve, starting from the curve of Planck's radiator, a host of straight lines of the closest colour temperatures is added. With their help, even those light colours that are not on this line can be identified by the closest colour temperature. On temperature radiators, the closest colour temperature corresponds to something approaching the actual temperature of the lamp filament. On discharge lamps, the closest colour temperature is stated.

Main groups colour temperatures

In addition, white colours of light are divided into three main groups: the warm white range (ww) with the closest colour temperatures below 4000 K, the neutral white range (nw) between 4000 and 5000 K and the daylight white range (dw) with the closest colour temperatures over 5000 K. The same colours of light may have different spectral distributions and a correspondingly different colour rendition.

Colour rendition

Colour rendition

Colour rendition refers to the quality of the reproduction of colours under a given illumination. The degree of colour distortion is indicated using the colour rendition index Ra and/or the colour rendition grading system. A comparative light source with continuous spectrum serves as a reference light source, whether this be a temperature radiator of comparable colour temperature or the daylight.

Ranges of the colour rendition index Ra for different lamp types

To enable the colour rendition of a light source to be determined, the chromatic effects of a scale of eight body colours viewed under the type of illumination being scrutinised and also under the reference illumination are calculated and related to each other. The resulting quality of colour rendition is expressed in colour rendition indices; these can relate both to the general colour rendition (Ra) as an average value or to the rendition of individual colours. The maximum index of 100 signifies ideal colour rendition as experienced with incandescent lamp light or daylight. Lower values refer to a correspondingly worse colour rendition. Linear spectra of light lead to good colour rendition. Linear spectra in general lead to a worse rendition. Multiline spectra are composed of several different linear spectra and improve the colour rendition.

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