# Color rendering index

Color rendering index of some light sources
lamp execution Index R a
sunlight 100
Lightbulb until 100
White LED earlier
current
65… 97
75… 98
OLED White 80 ... 90
Fluorescent lamp white de luxe 85… 98
White 70 ... 84
default 50 ... 90
Compact fluorescent lamps, energy saving lamps 80 ... 90
Fluorescent lamps, three-band lamps > 80
Fluorescent lamps, five-band lamps 70 ... 85
Metal halide lamp 60… 95
Metal halide lamp with ceramic burner > 90
High pressure sodium lamp warm white 80 ... 85
color enhanced 60
default 18… 30
High pressure mercury vapor lamp 40… 59
Low pressure sodium vapor lamp 20 ... 44

Color rendering is a quality feature of artificial light compared to natural light. A light source whose light contains all the spectral colors as in sunlight makes the colors of the illuminated objects look natural - the color rendering is optimal.

The color rendering index R a ( R eferenzindex a enerally , English color rendering index , CRI ) is a measure of a photometric quantity with which the quality of color reproduction of light sources of the same correlated color temperature is described (CCT). The index a stands for a ENERAL color rendering index, only the values of the first eight test colors according to DIN involves.

## history

With the increasing spread of energy-saving fluorescent lamps in the 1960s, large numbers of inexpensive light sources that were not thermal radiators were available for the first time . In contrast to incandescent lamps , which emit a continuous spectrum , fluorescent lamps, like all discharge lamps , have a discrete spectrum . The color rendering in the light from fluorescent lamps therefore differs more from that in the light from incandescent lamps, the narrower the wavelength ranges that the fluorescent lamps emit. For this reason, a yardstick was needed to be able to compare the new light sources with classic incandescent lamps or daylight . The aim was to use the color rendering index to calculate a reproducible value that indicates for each light source how far the color rendering of a lamp to be tested deviates from the ideal of incandescent and daylight.

With the spread of LED light sources , difficulties arose in evaluating the subjective color impression , which should initially be remedied by changes to the existing standard. From NIST finally became color quality scale ( English Color quality scale , CQS) developed to some shortcomings to fix the color rendering index. In particular, with the CQS the individual values ​​are combined as a square mean , while with the CRI this is done as the arithmetic mean .

In 2012, the European Broadcasting Union (EBU) introduced a further parameter for film lighting , the Television Lighting Consistency Index based on 24 reference colors .

## Calculation method

The color rendering index is designed in such a way that the test light source is measured spectrally and all further steps are carried out using purely numerical methods. According to CIE  13.2, the standard color values ​​of the test colors are determined for the test and reference light source. These are then transformed into UCS coordinates.

Since the standard only specifies discrete reference light sources with defined color temperatures, a color conversion must be carried out to compensate for the error between the color temperatures of the two light sources. The individual values ​​are combined as an arithmetic mean to form an index.

## description

### General

Under light sources whose light contains an uneven distribution of the spectral colors, the colors of illuminated objects will also appear unnatural - the color rendering is worse here. If the color of an illuminated area is missing in the color spectrum of the light source, you will only be able to see a dark gray area.

Up to a color temperature of 5000 K, the light emitted by a black body of the corresponding color temperature serves as a reference for assessing the reproduction quality . Above 5000 K it is compared with a standardized spectrum similar to daylight. For example, the spectrum of a black body with a temperature of 2700 K is used as a reference for calculating the color rendering of a household  incandescent lamp (which is itself a black body in a good approximation), for a fluorescent lamp with the light color 865 (8xx stands for a color rendering index of 80 up to 89.x65 for a color temperature of 6500 K), on the other hand, the daylight spectrum of the standard  illuminant D65.

According to its definition, the color rendering index is a special metamerism index .

The color rendering index does not depend on a specific color temperature. Every light source that perfectly reproduces the spectrum of a reference light source of the same correlated color temperature in the range of visible wavelengths achieves a color rendering index of 100. (Spectral components outside the visible range are ignored because they are not visually perceptible.) In theory, it is possible to add a light source synthesize (for example, to assemble from five different colored LEDs ), the spectral course of which deviates completely from that of a black body, but still reaches a color rendering index of 100.

According to the EU, only light sources with a color rendering index of more than 80 are permitted indoors (European Regulation 1194/2012). An R a of 65 is sufficient outdoors and for industrial use .

### calculation

The 14 test colors according to DIN 6169

To calculate the color rendering index, 14 test colors with a standardized reflectance curve were defined in DIN 6169 . The deviation of the secondary spectra between the reference and test spectrum serves as a measure for the 14 special color rendering indices  R i for color i. To calculate the general color rendering index R a , the arithmetic mean is only formed from the first eight values:

${\ displaystyle R_ {a} = {\ frac {R_ {1} + R_ {2} + \ dots + R_ {7} + R_ {8}} {8}}}$

An incandescent lamp with a colorless glass bulb has an R a of almost 100, while fluorescent lamps have a value of 70 to 90. Light sources that only emit light in a single wavelength , such as low-pressure sodium lamps , do not allow colors to be differentiated at all and therefore have a very low or even negative R a value (see table). Such negative color rendering indices are possible because when the color rendering index was determined in the 1930s, the reference light sources were set at 100, but the fluorescent lamps common at the time were (somewhat arbitrarily) set at 50 and the color rendering index is by no means a percentage value.

### Quality of the reference light

Since the color rendering index only describes the similarity to the reference light color, a high color rendering index does not automatically mean that all colors can be reproduced well and assessed accordingly, for example in prepress . For example, the reference light of the incandescent lamp (temperature radiator with 2700 K) contains only very few blue and violet light components, which results in a poorer rendering of these colors. For good color rendering, not only is the color rendering index high, but the reference light must have as full a spectrum as possible. This is good at color temperatures of 4500 K to 6000 K. In order to better assess the quality of a light source, in addition to the general color rendering index R a , special color rendering indices are often used, for example R 9 (saturated red).

Recordings with a compact fluorescent lamp (A), a high-voltage halogen lamp (B) and a flash (C).
The different color rendering indices cause differences in color rendering.

## Use of the term for light sources

The color rendering index (R a ) was introduced in order to be able to specify the color rendering quality of a lamp precisely . The best value with the most natural color rendering is R a = 100.

The color rendering index is determined by evaluating the color shift to a reference light source on eight different color samples of frequently occurring test colors. The value of each color sample is added up with ⅛ to the color rendering index.

Depending on the visual task, the suitable light source should be selected based on the color rendering. In graphic companies, the highest demands are placed on color reproduction. Only lamps with a color rendering index that is significantly higher than 90 can be used here. In order to ensure good and relaxed vision, however, the color rendering index should not fall below 80 in offices and workshops either. In living rooms and bedrooms, where color vision is not in the foreground, one can confidently neglect the color rendering and choose the light source according to the mood to be created.

The color rendering is also deliberately worsened in the trade in order to make it appear appetizingly red and not unpleasantly green, like meat products. However, poor color rendering can unintentionally mean that products such as textiles look different in shops than they do in daylight.

## Further light quality indices

In addition to the aforementioned CRI or Ra with eight reference colors, the number of reference colors was expanded with the introduction of LED technology. One speaks of a Re with 14 or 15 reference colors as shown above. This higher number of color samples describes the light quality more clearly. In addition, the European Broadcasting Union (EBU) has introduced an index consisting of 24 reference colors under the name TLCI-2012 for their requirements and is increasingly used.

All of these only take samples from the large color spectrum. The GAI (Gamut Area Index) takes a different, two-dimensional approach. The standard color table according to CIE1931 shows all colors that the human eye can perceive. However, depending on the light source, only part of the entire color space can usually be mapped. Using the color locations of the eight reference colors, the area of ​​an octahedron is mapped in the color space as a reference color space (gamut). Using a spectrometer, the imaged area of ​​a light source is compared with the reference and the GAI value is calculated. According to the recommendation, this should ideally be between 80 and 100. The GAI value provides information about the naturalness of the light and allows conclusions to be drawn about the saturation of illuminated objects and the "vividness" of the colors. The GAI is mainly used in color-critical applications, for example in museums or in medical technology.

## Other usage of the term

The term is used in printing ; what is meant is the color tolerance .

## literature

• Robert GW Hunt: The Reproduction of Color. 6th edition. Wiley, 2004, ISBN 0-470-02425-9 .
• Hans Rudolf Ris: Lighting technology for practitioners. 4th edition, VDE / AZ, Aarau 2008, ISBN 978-3-8007-3013-1 (VDE) / ISBN 978-3-905214-58-1 (AZ).
• Wilhelm Gerster: Modern lighting systems for indoors and outdoors. Compact Verlag, Munich 1997, ISBN 3-8174-2395-0 .
• DIN 6169-1: 1976-01 - Color rendering; Common Terms
• Horst Stöcker: Pocket book of physics. 4th edition. Verlag Harry Deutsch, Frankfurt am Main, 2000, ISBN 3-8171-1628-4

## Individual evidence

1. Hans-Georg Buschendorf (Hrsg.): Lexicon light and lighting technology . Verlag Technik, Berlin 1989, ISBN 3-341-00724-5 , p. 64.
2. Source: Megaman R9 technology ( Memento from February 12, 2015 in the Internet Archive ) (PDF)
3. Source: Osram
4. Source: Tridonic ( Memento from July 27, 2014 in the Internet Archive )
5. ^ The Sodium Lamp - How it works and history. (No longer available online.) In: edisontechcenter.org. Archived from the original on September 18, 2014 ; accessed on January 5, 2017 .
6. Definition of color rendering index R a (CRI)