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EFFECT OF LIGHTING ON COLOR TRANSMISSION ACCURACY

31.08.2021 23:10

[1. Information systems and technologies]

Author: Pastushenko M.М.


The same color in different lighting is perceived differently. The color rendering factor is the value at which the connection between light and the visible color is considered. CRI (Color Rendering Index) – color rendering index.

The CRI color rendering index measures the ability of a light source to accurately reproduce the colors of the object it illuminates.

CRI is measured on a scale where a higher number represents the highest color rendering capacity and 100 is the highest. CRI values of 90 and above are considered excellent.

The color rendering index shows how natural the objects are when illuminated. The standard is considered to be sunlight, the CRI of which is equal to 100.

Natural light, such as sunlight, is a combination of all colors of the visible spectrum. The color of the sunlight itself is white, but the color of the object under the sun is determined by the color it reflects. When we use an artificial light source, such as an LED lamp, we try to "reproduce" the color of natural daylight so that the objects look the same as in natural daylight.

Not only the temperature of light affects the color reproduction of objects that it illuminates. For example, an artificial light source (5000K LED lamp) will not transmit red in the same way as natural daylight (also 5000K). The LED lamp has a different spectral composition compared to natural daylight, although it has the same 5000K white color.

The CRI color rendering index attempts to characterize this phenomenon by measuring the overall accuracy of different colors of objects when illuminated under a light source.

For most rooms, 80 CRI (Ra) lighting is sufficient for acceptable color. For textile factories, printing houses, or dyeing shops, where a color transfer is most important, 90 CRI (Ra) is used.

The CRI calculation method is performed using algorithmic calculations after measuring the spectrum of the considered light source. First, you need to determine the color temperature for the light source. This can be calculated from spectral measurements. The color temperature of the light source should be determined so that we can select the appropriate daylight spectrum to use for comparison.

Then the light source in question will actually be illuminated by a series of virtual color samples (TCS), with the measured reflected color. 15 color samples are shown in Table 1.




To determine the CRI, a tested light bulb or lamp is taken, and the light from them is sent to each pattern in turn. Next, special devices measure the color that the pattern has acquired. After that, the same samples are illuminated with reference light and the measurements are performed again. All that's left is to compare the color difference.

When all the measurements are made, calculate the arithmetic mean between the eight basic patterns. The standard CRI values for different rooms are given in Table 2.




It is often necessary to add a samples №9 – deep red to standard measurements. CRI R9 is a useful indicator for assessing the color rendering ability of a light source, especially for objects whose display spectra contain red waves. Due to the averaging of the values in the CRI calculation, the red transfer rates may be poor, with good overall readings (see Figure 1). 




Achieve R9> 95 Quite difficult, which is due to the color space CIE 1960 UV. When calculating its value is distorted, which gives a very high sensitivity in the red spectrum.

Until 1974, the International Commission on Lighting (IEC) methodology determined 8 reference colors used in the measurements.  

The more modern CQS (Advanced CRI) methodology, developed by the US National Institute of Standards and Technology (NIST) in 2010, uses 15 saturated colors that are prone to color changes [1].

Formulas for calculating CRI and CQS are given in formulas 1.1 and 1.2.




Due to this, the shift even by one color will already significantly affect the final value of the color rendering quality index, and there will be no visual error as with CRI.

The general difference between CQS and CRI is the small dependence of the new coefficient on three parameters: brightness, tonality, saturation.

In 2015, America introduced a new method of measuring light quality IES TM-30-15, where the standard is an ideal light source and includes 2 indices: FA (fidelity – accuracy) – the correspondence of 99 colors from the tested light source. It is measured in the range from 0 to 100 and GS (gamut – saturation) shows the saturation of these colors. Measured from 60 to 140 [2]. Here, in addition to the old artificial multi-colored plates, for comparison, and used "living" objects found in nature.

The world's leading manufacturers and companies provide data on all fixtures based on all three ratios.

References:

1. Anastasia Kolesnik (November 8, 2019). Color Transfer Index: RA, CRI AND CQS. LED expertise. URL: https://ledtest.vestum.ua/study/6903/

2. Light Ru (2020). Measuring color temperature. URL: https://lightru.pro/izmerenie-tsvetovoj-temperatury/

3. Light inspection (2018). The CRI is no longer the same - how to evaluate the quality of light using coefficients. URL: https://svetosmotr.ru/indeks-tsvetoperedachi-cri/



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