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> Light sources / Characteristics of light sources / Colour rendering
For visual performance, comfort and well-being, it is important that the colours of the surroundings, objects and human skin are rendered naturally and realistically. This makes people appear attractive and healthy. Dependent on location and visual task, artificial light sources should ensure correct colour rendering comparable to natural daylight (see also figure and figure).
Figure 3.256: Good colour rendering
Figure 3.255: Insufficient colour rendering
Despite featuring identical light colour, light sources can have different colour rendering properties due to different spectral compositions of their radiation. In order to express colour rendering properties of a light source objectively, the general colour rendering index Rα was established. The colour rendering index is the measure of correspondence of the seen body colour and its appearance under the respective reference light source. To determine the value of Rα, colour variations from 8 standardised test colours occurring when the test colours are illuminated using the light source to be evaluated/the reference light source are identified. The smaller the variation, the better the colour rendering characteristic of the tested light source. A light source with Rα = 100 renders all colours ideally as under the reference light source. The lower the Rαvalue, the lower the quality of the colour rendering. A categorisation of market-based light sources for the colour rendering levels can be found in table 9.3. The reference light type for daylight white light sources is natural daylight with a colour temperature of 6,500 K, corresponding to an overcast sky without direct sunlight. For light sources with a colour temperature of < 5.000 K, the reference light type is the Planckian radiator.
Lamps with a colour rendering index below 80 should not be used indoors where persons work or linger for extended periods of time. Exceptions are permissible for particular lighting tasks requiring high luminous efficacy and therefore lower colour rendering for economic reasons, for example high-pressure sodium vapour lamps. This applies e.g. for the lighting in high halls. However, appropriate measures should be taken in these cases in order to ensure a higher colour rendering at stationary and steadily occupied workstations and at locations where safety colours must be identified correctly. As the use of LED luminaires becomes more widespread, this issue increasingly fades into the background. High illuminance values can also be realised with these light sources, featuring good colour rendering and at economically reasonable expenses. This has been already taken into account in the current efforts of photometric standardisation.
Manufacturers have committed themselves to a simple, generally comprehensive international labelling for light colour and colour rendering of lamps and LED luminaires in addition to manufacturer-specific labelling. It consists of three digits as depicted in table. Regarding LED light sources, colour rendering properties are expressed by the rated colour rendering index which refers to the colour rendering at initial commissioning.
Ranges for the general colour rendering index Ra
Colour labelling of lamps/luminaires with integrated light sources independent of manufacturers.
Light colour Lamp service life
Luminous efficacyLight colourColour renderingLamp service lifeService life of LED light sourcesLamp reference
Introduction Visual needs Need for orientation Non visual needs
Guidelines Overview Work places Visual task area Surrounding area Illuminance Luminance distribution Limitation of glare Lighting direction, modelling Aspects of colour Flicker and stroboscopic effects Daylight
Maintenance Factor Light and safety at work Light and non-visual effects Light and economy Light and environment Lighting design Lighting measurement Emergency lighting
Lighting of traffic zones and geneal areas in buildings Lighting of industrial activities and crafts Lighting of offices and VDU workstations Lighting of design and CAD offices Lighting of sports facilities Lighting of health care facilities Lighting of geriatric care facilities and nursing homes Lighting of public areas Lighting of salesrooms Lighting of educational facilities Lighting of traffic areas Lighting of car parks Ligthing of further indoor spaces
Luminaires The enhanced concept of quality Photometric classification Photometric properties Photo biology safety Mounting and electrical connection Luminaire labelling Luminaire operation LED luminaire operation Operation of FL luminaires Electrical safety Fire protection Electromagnetic safety Mechanical safety Chemical and miscellaneous impact Acoustic properties Ventilation properties
Firehazardous locations Room with bathtubs and showers Swimming baths Exterior installations Agricultural and gardening plant locations Food industry Clean rooms Requirements on mediacal rooms Emergency light installations
Luminaire selection chart
Light management application and function Presence detection Daylight-dependent regulation Interfaces Building management, KNX and other bus systems
Light sources types Characteristics of light sources
ABC, DF, G, ILM, NO, P, QRSUV, W
Subject index
StandardsGuidelines and instructionsEuropean directives and regulationsFurther publications
Lighting societies
Standardisation institutes
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