Color rendering index

Under color rendering index (English Colour Rendering Index, CRI ) refers to a photometric variable with which can be described the same correlated color temperature, the quality of color rendering of light sources. The abbreviated notation for the color rendering index Ra. Here, the index a represents a general color rendering index, which includes only the values ​​of the first eight test colors, in accordance with DIN.

History

With the widespread use of energy-saving lamps in the 1960s for the first time were inexpensive light sources in large numbers available that were not thermal radiators. Fluorescent lamps, like all discharge lamps, give off no light with a continuous spectrum. Colors may appear differently than under incandescent light, especially when they only give light in narrow bands under this light. It was a measure necessary in order to compare the new light sources with the traditional incandescent or daylight can. The aim was to calculate a reproducible number that indicates for each light source, as much as the color rendering of a test lamp from the ideal of incandescent and daylight is different. The color rendering index was the amount which was prepared at this time.

Increasingly With the proliferation of LED light sources difficulties were encountered in the evaluation of the subjective color sensation that should be initially resolved by changes to the existing standard. From NIST finally the color quality scale was (English Color quality scale, CQS ) developed to correct certain imperfections in the color rendering index, especially in this case, the individual values ​​are passed as the geometric mean to an index.

Calculation method

The color rendering index is designed so that the test light source is spectrally measured and done all the other steps purely by numerical methods. According to CIE 13.2, the tristimulus values ​​of the test colors for the test and reference light source can be determined. These are then transformed in UCS coordinates. As defined by the standard reference-light source only discrete color temperatures are defined, a color conversion to be performed, which compensates for the error between the color temperatures of the light sources. The individual values ​​are summarized as arithmetic means to an index.

Description

As a reference to evaluate the reproduction quality is used up to a color temperature of 5000 K, the light emitted by a black body temperature of the corresponding color. Over 5000 K is referenced to a daylight-like spectral distribution. For example, for calculating the color rendering of a household incandescent lamp (which is itself a good approximation, a black body is ) is the spectrum of a black body at a temperature of 2700 K is used as a reference for a fluorescent lamp with the color of light 865 ( 8xx represents a color rendering index of 80 to 89 x65 for a color temperature of 6500 K ), however the daylight spectrum to illuminant D65. The color rendering index is by definition a special index of metamerism.

The color rendering index is not dependent on a specific color temperature. Each light source of the same ( correlated ) color temperature in the range of visible wavelengths simulates the spectrum of a reference light source perfectly achieves a color rendering index of 100 spectral components outside of the visible region be ignored because they are not visually perceptible. Theoretically, it is also possible to synthesize a light source ( for example, five different colored light emitting diodes ( LEDs) ), which completely differs from the spectral curve of a black body radiator, but nevertheless achieve a color rendering index of 100.

To calculate the color rendering index of 14 test colors defined with a standard reflectance curve. The deviation of the secondary spectra between reference and test spectrum is used as a measure of the 14 special color rendering indices. To calculate the general color rendering index Ra, however, only the first eight test colors are used. The 14 test colors are selected by DIN 6169. In this case, the color rendering index R for color can be found i. A theoretical value of the colors of # 1 to # 8 is denoted by Ra. Since when determining the color rendering index in the 1930s, the reference light sources 100, the then current fluorescent lamps (sort of random ) were set at 50 and the color rendering index is by no means a percentage value, and negative color rendering indices are possible.

A light bulb with clear glass piston has an Ra of almost 100, while fluorescent lamps reach a value of 70 to 90. Light sources that emit light of a single wavelength only, such as the sodium vapor low-pressure lamps, allow no differentiation of colors and accordingly have a very low or even negative Ra value.

As the color rendering index only describes the similarity with the reference color of light, a high color rendering index does not imply that all of the colors are also well reproduced. For example, the reference light the bulb contains ( temperature radiator at 2700 K), very little blue and violet light components, resulting in a poorer reproduction of these colors together. For good color reproduction is therefore necessary that not only the color rendering index is high, but also the reference light having a possible " complete" spectrum. This is the case of color temperatures from 4500 K to 6000 K well.

A lamp with a high Ra value alone is not sufficient to judge colors, such as in prepress, in a standardized environment. There also depends on the spectral distribution of the light source. In order to better assess the quality of a light source and special color rendering indices are in addition to the general color rendering index Ra often used, for example, the R9 ( saturated red ), as the general color rendering index is often not a sufficient criterion. Reason for this is that, in particular for calculating the general color rendering index only the first 8, more pastel tones are used ( with a soft reflectance curve ).