Encyclopédie d’éclairage

Encyclopédie d’éclairage

Explanations of important lighting terms


Advantages Offered by LEDs

Luminous efficacy of different light sources:Incandescent light bulbs emit 10-20 lm/W, mercury vapour lamps 30-60 lm/W, fluorescent lamps generate 65-110 lm/W and LEDs produce 110-130 lm/W.
Longevity:LEDs last 50 times longer than incandescent sources.
Maintenance:Thanks to their long operating lives, LEDs are virtually maintenance-free.
Light quality:LEDs generate white non-flickering, homogeneous light.
Light distribution:LED light collimates and reflects precisely, boosting efficiency.
Soiling/Insect protection:LEDs are IR and UV radiation free, rendering them unattractive to nocturnally active insects.
Mercury-free:LEDs are environment-friendly and dispose of safely.
Size:LEDs are compact and variable.
Robustness:LEDs are resistant to vibration.

CE mark

The CE marking signifies that the product conforms to all the essential requirements of all relevant EU directives. It is applied to the luminaire, packaging or accompanying documents by the manufacturer or importer. It is not the mark of any particular approval or testing institute.
- EU directive on electromagnetic compatibility, according to which products since 01.01.96 that fall within its scope are required to have a CE mark.
- EU low voltage directive, according to which products since 01.01.97 that fall within its scope are required to have a CE mark.

Colour rendering

With respect to the application location and visual task, artificial light should render colours as accurately as possible, i.e. as they appear in daylight. The criterion here is the colour rendering property of a light source. This is expressed as a "general colour rendering index" (Ra). The colour rendering index is a measure of the correspondence between the colour of an object (its "self-luminous colour") and its appearance under a reference light source.
Colour rendering qualityColour rendering gradeColour rendering
index Ra
Type of lamp (sample)
Excellent1 A> 90- tungsten-halogen lamps
- de luxe fluorescent lamp
- metal-halide lamps

1 B80 - 89- three fold double tube fluorescent lamps
- metal-halide cold white or warm white lamps
Good2 A70 - 79- basic fluorescent lamps, Grade 10 and 25

2 B60 - 69- basic fluorescent lamps, Grade 30
Adequate340 - 59- high-pressure mercury-vapour lamps
Unsatisfactory4< 39- high-pressure and low-pressure sodium discharge lamps
The chromaticity of eight (or 14) test colours standardized in DIN 6169 that occur when they are illuminated by the light source being tested are compared with the same test colour when illuminated by the reference light source. The smaller the difference, the better the colour rendering quality of the lamp being tested. A light source with an Ra value of 100 shows all the colours perfectly, as in the case of the reference light source. The lower the Ra value, the poorer the colour rendering.

Colour Rendering Index (CRI)

The Colour Rendering Index is a measure of the degree of colour shift that objects undergo when illuminated by a lamp, compared with their colour when illuminated by a reference source of comparable correlated colour temperature (CCT). The higher the CRI value, the more accurate is the reproduction of all colours and therefore the object.

ENEC mark

The ENEC mark (European Norms Electrical Certification) is a European approval and certification marking for lighting fixtures and electrical components used in fixtures. It certifies that a product complies to current applicable European codes and signifies that inspection by the respective testing institute has taken place. The ENEC marking appears together with the identification number of the national testing institute and is often displayed with the institute’s logo.

F mark

Indicates luminaires that are suitable for mounting to normally combustible surfaces.


E symbol
Unit of measure: Lux (lx)
Illuminance (usually “E” in formulas) indicates the amount of luminous flux from a light source falling on a given surface. It is the quotient of the luminous flux divided by the area of the surface. Because illuminance is relatively easy to measure or calculate, it is frequently used in planning. Illuminance does not provide clear conclusions about subjective brightness

Ingress protection symbols

Ingress protection IPx1
Luminaires displaying this symbol are protected against ingression of dripping water.
Ingress protection IPx3
Protection against spray water (e.g. rain).
Ingress protection IPx4
Symbol for spray water protection.
Ingress protection IP5x
Dustproof luminaires.
Ingress protection IP54
Dust and spray proof luminaires.
Ingress protection IP55
Luminaires that are protected against dust and directed water.
Ingress protection IP65
Dust and jet-proof luminaires.

Isolux plot

Isolux plots are graphic representations of the distribution of illuminance on a given surface. The isolux lines depict curves of equal illuminance and can be used as parameters for rough planning. Each Isolux plot corresponds to a luminaire’s respective location and light spot height (or mounting height). We use our DiaLux lighting calculation programme for precise planning.


Light emitting diode
A LED is a semiconductor component that emits light when voltage is applied to it.
In contrast to incandescent lamps, which are thermal light sources, LEDs emit monochromatic light in a limited range. Unlike conventional light sources, a large proportion of their energy is not lost due to colour filtration, making them more efficient.


Light is electromagnetic radiation that is visible to the human eye. The entire frequency range stretches from low energy waves such as radio waves, to high-energy x and gamma rays. The spectrum of visible light is relatively small and lies between 370 nm (violet) and 750 nm (red).

Light colour

Colour temperature is a relatively good way of defining light colour. We distinguish between three main groups: warm white < 3300 K; neutral white 3300 - 5000 K; daylight > 5000 K. It is, however, important to note that due to the spectral composition of light, light sources of the same colour can vary widely in the colour rendering of light emitted.


Symbol: L
Unit of measure: Candelas per square metre (cd/m²)
Luminance (usually “L” in formulas) is the amount of visible light leaving a point on a surface in a given direction. This "surface" can be a physical surface or an imaginary plane, and the light leaving the surface can be due to reflection, transmission, and/or emission. Luminance describes the impression of brightness the viewer experiences. The relationship between luminance and brightness is non-linear and very complex.

Luminous Efficacy

Luminous efficacy is the most commonly used measure of the efficiency with which a light source provides visible light from electricity. it is the ratio of luminous flux to power and appears as lumens per watt (lm/W).

Luminous flux

Unit of measure: Lumen (lm)
Luminous flux describes the total photometrically weighted radiant flux (power) emitted by a light source. It is evaluated according to the spectral sensitivity of the eye. Oversimplified, you could refer to luminous flux as “light output”. The rated luminous flux for light sources is indicated by the manufacturer. Luminous flux does not contain information concerning the spatial distribution of the emitted light.

Luminous intensity

Symbol: I
Unit of measure: Candela (cd)
The luminous intensity (usually “I” in formulas) describes the luminous flux emitted by a light proportionate to a solid angle source in a particular direction. The spatial distribution of the luminous intensity is graphically depicted in luminous intensity distribution curves.

Luminous intensity distribution curves

A luminaire’s constructional features and design determine the spatial distribution of the luminous intensity. The spatial distribution of the luminous flux is graphically depicted in a luminous intensity distribution curve, which is usually expressed in a polar coordinate diagram. For rotationally symmetrical light sources, the luminous intensity distribution can be shown by a single curve, whereas two or more curves are required for axially symmetrical light sources, e.g. the C o and C 180 planes parallel to the street axis or the C 90 and C 270 planes perpendicular to the street axis.

Modules or Light Engines

Many different LED concepts are available on the market:
  • LED retrofit modules for luminaires already in operation 
  • Luminaires with permanently installed LEDs
  • Luminaires with replaceable LEDs
  • Luminaires that combine LED technology and reflector-projector systems
  • Luminaires with a single optical system for all LEDs
  • Luminaires in which each LED is configured with its own individual optical system
Hess offers 4 modules, which together cover virtually every street lighting application and luminaire type. Hess light engines have been designed for optimum efficiency and top performance for years to come, meaning you can begin the future without worry.

Safety classifications

Luminaires are classified in three categories depending on the protection they provide against excessively high touch voltage:
Protection rating I
The luminaires in this class have basic electrical insulation and all of their potentially electrically conductive parts are connected to a protective conductor. The protective conductor terminal displays this marking.
Protection rating II
Live parts are fitted with additional insulation. Connection to a protective conductor is not permitted.
Protection rating III
Protection against electrical shock is based on use of safety extra low voltage (SELV).

Technical symbols

Minimum distance to the illuminated surface.
Nominal wire and cable temperature.
High pressure sodium lamp requires an external ignitor for operation.
High pressure sodium lamp requires an internal ignitor.

Thermal Management

In particular, the operating and ambient temperatures are of critical importance: the colder, the better. High temperatures decrease luminous flux and shorten the light source’s lifetime. For this reason, it is impossible to have an efficient LED system without an effective and reliably functioning thermal management system. The heat is not generated by the heating of an element that glows, but rather via the flow of current in the semiconductor.