High-CRI LED lighting
CRI is a quantitative measure of a light's ability to reproduce the colors of objects faithfully in comparison with an ideal or natural light source. In general terms, CRI is a measure of a light source's ability to show object colors "realistically" or "naturally" compared to a familiar reference source, either incandescent light or sunlight.
Efficiently achieving an acceptable CRI has been the most difficult metric for light bulbs attempting to replace incandescent bulbs. It is therefore frequently ignored in marketing (the CRI value only occasionally appears on product packaging). Light bulbs with a high CRI can be acceptable replacements for incandescent bulbs. Most LED lights do not have a CRI above 90. For example, the top bulbs listed in the 2016 Consumer Review have a CRI of 80.
In 2008, the US Department of Energy created the L Prize to find an incandescent light bulb replacement that met efficiency metrics and had a CRI above 90. On August 3, 2011 Philips was declared as the first winner of the L Prize.
CRI is calculated from the differences in the chromaticities of eight CIE standard color samples (CIE 1995) when illuminated by a light source and by a reference illuminant of the same correlated color temperature (CCT), commonly measured in kelvins, indicating the light color produced by a radiating black body at a certain temperature; the smaller the average difference in chromaticities, the higher the CRI. A CRI of 100 represents the maximum possible value. Lower CRI values indicate that some colors appear unnatural. Incandescent lamps have a CRI above 95. Cool white fluorescent lamps have a CRI of 62, however fluorescent lamps containing rare-earth phosphors are available with CRI values of 80 and above.
For CCTs less than 5000 K, the reference illuminants used in the CRI calculation procedure are the SPDs (Spectral Power Distribution) of blackbody radiators; for CCTs above 5000 K, imaginary SPDs calculated from a mathematical model of sunlight are used. These reference sources were selected to approximate incandescent lamps and sunlight, respectively.
The CRI measure in use in 2017 was developed by the CIE in 1974 and slightly updated in 1995. The measure has two main flaws. Its color differences are measured in a nonuniform color space. Its color sample set has just 8 items, which is too few to test lights with complex spectra. A light manufacturer can tune its SPD to the sample set so as to achieve an artificially high CRI. In 2015 the IES (Illumination Engineering Society) produced a replacement to the CRI measure that uses a newer color space and 99 color samples. In 2017 the CIE published an almost identical measure, but it did not deprecate its 1995 CRI measure.
CRI has been challenged because fidelity to reference illuminants such as correlated color temperature (CCT) is not all that measures the quality of illumination. Various CCTs are preferred, and scoring 100 at one CCT does not imply equal illumination quality as scoring 100 at another CCT. The "warmer" light colors, such as a 2700K incandescent bulb or a 1700K candlelight are more easily reproduced than more neutral white lights, such as 4800K direct sunlight, and thus usually have higher CRI ratings in alternative light sources such as CFL and LED bulbs; "warmer" light (redder) naturally renders colors less accurately. Think of how the world looks at sunset (2000K) compared to high noon (5600K).
Problems have been encountered attempting to use LED lighting on film and video sets. The color spectra of LED lighting primary colors does not match the expected color wavelength bandpasses of film emulsions and digital sensors. As a result, color rendition can be unpredictable in optical prints, transfers to digital media from film and video camera recordings. This phenomenon with respect to motion picture film has been documented in an LED lighting evaluation series of tests produced by the Academy of Motion Picture Arts and Sciences scientific staff.
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- IES-TM-30-15: Method for Evaluating Light Source Colour Rendition
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