CRI, or Color Rendering Index, is a complicated topic. The wikipedia page for CRI
contains an exhaustive description of what it is and how it is measured. This week’s blog post will attempt to answer, in simple terms, the question “What is CRI and why should I care?”
What is CRI?
In the simplest of terms, CRI is the qualitative measure of how evenly a light source illuminates all colors of the spectrum. Because white light is a mixture of all the colors of the spectrum, an ideal source of white light emits and therefore illuminates, all colors equally. A source of white light that emits, for example, the color red slightly more or slightly less than other colors is an imperfect white light, and would have a lower CRI than one that emits all colors equally.
CRI is measured by using eight reference colors, as defined by International Commission on Illumination, to measure how much light the tested light source reflects back along these ten spectra. This is compared to a perfect light source along these eight spectra, and the average difference is taken to represent how close this light source is to a ‘perfect’ light source, which is given a CRI of 100.
As an aside, plenty of ‘perfect’ light sources exist. Practically all incandescent light bulbs have a CRI of 100. This is because incandescent light bulbs, which work by heating a filament until it gets hot enough to glow, output light evenly across all spectra. LEDs, which work by emitting a certain wavelength of light (usually blue), absorbing it into a phosphor coating, and then emitting it again at something approximating a perfect white, naturally don’t emit all spectra equally, hence the rarity of very high CRI LEDs. 100 CRI LEDs have not yet been produced, even at research/development levels.
Why should I care?
Clearly, light sources with low CRIs reproduce colors less accurately than those with high CRIs. In certain applications, high CRI is critical. If you’re installing lighting in a location where color reproduction is very important - such as anywhere you’re displaying art, graphics, or images, then high CRI is necessary to reproduce colors accurately. If you’re installing lighting somewhere that color reproduction is less important, such as a patio or shed, then CRI is probably not so important - for this particular installation at least.
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