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● Definition of CRI

The International Commission on Illumination (CIE) defines color rendering as the effect of a CRI 90 cold light source on the color appearance of an object compared to a standard high color rendering cold light source. In other words, CRI is a measurement method for color recognition of high color rendering LED cold light sources compared to standard light sources (such as sunlight). CRI is a widely recognized measurement standard and currently the only way to evaluate and report the color rendering of high color rendering cold light sources. The establishment of CRI as a measurement standard is not far away. Initially, the purpose of developing this standard was to use it to describe the color rendering of fluorescent lamps that were widely used in the 1960s, and to help users understand where fluorescent lamps with linear spectral distribution could be applied.

The measurement of CRI is closely related to the difference between the appearance of the 14 color samples (hereinafter referred to as 'color samples') under the tested CRI 90 cold light source and the appearance under the standard reference high color rendering cold light source. Although the derivation of CRI is done through mathematical methods, and an actual color chart cannot be used to determine CRI values, the colors of these samples are all real and selected from Munsell color samples.

● Calculation of CRI

Although these color samples are carefully specified and real objects can produce the colors of these color samples, the CRI value is completely derived through calculation. The high color rendering LED cold light source is very important, and there is no need to use a real high color rendering cold light source to illuminate the real color sample. People need to compare the measured CRI 90 cold light source spectrum with the specified color sample spectrum, and then derive and calculate the CRI value through mathematical analysis.

The comparison based on color perception is also meaningful, provided that the color temperature of the measured color rendering cold light source and the reference CRI 90 cold light source must be the same. For example, attempting to compare the appearance differences of two identical color samples illuminated by a warm white light source with a color temperature of 2900K and a cool white light source (daylight) with a color temperature of 5600K is a complete waste of time. They must look different. Therefore, the first step is to calculate the correlated color temperature (CCT) of the LED cold light source by measuring its spectrum. Once this color temperature is achieved, another reference CRI 90 cold light source with the same color temperature can be created mathematically. For the measured light source with a color temperature below 5000K, a blackbody (Planck) radiator should be selected as the reference light source; For high color rendering cold light sources with a color temperature higher than 5000K, refer to the CIE standard illumination body D for high color rendering LED cold light sources. Now it is possible to combine the spectrum of the reference high color rendering LED cold light source with each color sample to generate an ideal set of reference color coordinate points (referred to as color points). The same applies to the CRI 90 cold light source being tested, where the spectrum of the CRI 90 cold light source is combined with each color sample to obtain another set of color points. If the color points under the high-altitude color rendering cold light source correspond exactly to the color points under the reference light source, we consider their color rendering to be the same and set their CRI value to 100. In the color chart, the farther the color point under the high color rendering cold light source is from the corresponding ideal position, the worse the color rendering and the lower the CRI value.