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LEDs and CRI
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Correlated Color Temperature Chart
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Color Rendering Index (CRI) indicates how well a test source renders eight standard colors of intermediate saturation, when compared to a reference lamp of the same color temperature. Lab measured CRI is a comparison against a spectrally continuous red-weighted reference. Field conducted CRI tests are subjective with Human observers when luminance levels are below 3 cd/m2.
The industry is discovering that CRI is not the best metric for comparing LED light sources, especially at Mesopic levels. Originally developed in 1964, this index is based on outdated color models and assumes illumination sources with broad spectral distributions, whereas LEDs are narrow-band sources. And, nighttime lighting requirements fall primarily in the Mesopic range where our color sensitivity shifts with luminance, and there is no defined index. Several standards bodies are addressing this deficiency, and in the interim, Color Temperature may be the most suitable tool for comparison because it is independent of observer subjectivity.
Correlated Color temperature (CCT) defines a color as the temperature in degrees Kelvin that a "black body" source must reach in order to produce that same color. CCT describes the dominant color without regard to Human visual response or the source technology and is more appropriate for comparison of visual effectiveness at lower light levels and among different technologies. |
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