Knowledge Color Rendering Index (CRI): What Do Ra and R9 Mean?

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The color rendering index (CRI) is a quantitative measurement of a white light source’s ability to reproduce the colors of various objects faithfully in comparison to a reference light source of the same correlated color temperature (CCT). The reference light source is a Planckian radiator (for color temperatures below 5000 K) or a mathematical model of daylight (for color temperatures at or above 5000 K. In this method, the chromaticity of 14 Munsell color samples illuminated by the test source is compared with the chromaticity of the same color samples illuminated by a reference light source. The difference is scaled and subtracted from 100. The score for each test color is defined as the Special Color Rendering Index Ri. 100 is the maximum Ri score that indicates a perfect match.

The principal metric of the CRI system is the General Color Rendering Index, which is referred to as Ra. In common practice, "CRI" is used to refer the General Color Rendering Index value. So whenever "CRI" appears in lighting literatures, one can be certain that it means Ra. Ra averages the Ri scores for the eight standard test colors. The colors that make up the Ra are the first eight color samples—TCS1 through TCS8. An Ra score of 100 indicates that the source renders colors in a manner identical to the reference. Daylight has the highest CRI of approximately 100. Thermal radiators i.e. incandescent and halogen bulbs have a relatively close CRI of greater than 95. Fluorescent lighting achieves a significant improvement in luminous efficacy but, to a large extent, at the cost of compromised color rendering performance. The typical Ra of cool white fluorescent lamps is in the 70-80 range.

Administered by the International Commission on Illumination (CIE), the CRI system is currently the only internationally-accepted metric for assessing the color fidelity of light sources. However, the CRI score is not comprehensive. It only contains measurements for R1-R8 values which are for colors with primarily pastel shades. The special color rendering indices for additional six saturated samples, commonly referred to as R9 through R14, are not included in the CRI calculations.

The color rendering ability of a light source depends on the interaction between the light source's spectral power distribution (SPD) and the spectral reflectance function of objects. White light is made from a combination of various wavelengths between about 380 nm and about 770 nm. The distribution of radiant power across the visible spectrum has a significant impact on a light source's color rendering ability. LEDs and fluorescent lamps produce white light through phosphor conversion, which means the SPD of these light sources can be manipulated for various purposes. As there is a fundamental trade-off between the color rendering and luminous efficacy for phosphor converted light sources, color quality of is often outweighed by luminous efficacy. However, this trade-off may not be evident in Ra values. Phosphor converted light sources that are over-saturated in the blue and green spectral bands may still have a good CRI, even though they are deficient in key wavelengths important for rendering saturated colors (R9-R14).

Among the Ri values for six saturated test colors, R9 is particularly important as it shows the response with deep reds. Without the vibrancy that comes from this critical color, objects with strong color tones will appear dull, unnatural, or unflattering. Therefore R9 is listed individually as a supplement to the general index Ra when faithful color reproduction with artificial lighting is of paramount importance. R9 values above 75 are often required in museum, photography, retail display lighting applications. An R9 value of 25 is considered as good for residential applications.

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