Graph illustration of typical CRI values

CRI: understanding the Color Rendering Index

Occasionally we cover topics incidental to lighting, but not directly related to light sensitivity.  This is one of those topics.

If you’ve shopped for LED bulbs you might have seen the Color Rendering Index (“CRI”) scores advertised prominently on retail packaging.  CRI is a measurement relating to how well a light source accurately renders nearby colored objects.  A perfect score of 100 is generally only achieved by old-school incandescent bulbs, halogen light bulbs and sunlight.  These old-school light sources receive a perfect score because the light they emit is actually a properly balanced mixture of all colors, even though the combination of colors makes it appear a neutral white to your eyes (fun fact: a rainbow is actually all those colors separated).

LEDs are tremendously efficient but they don’t emit a balanced mixture of all colors.  Instead, they usually emit high levels of blue light and an absence of all others colors.  To overcome this limitation, manufacturers coat LEDs with special phosphors to absorb some of the blue light and convert it to green and red through fluorescence.  These coatings help balance out the light, but they don’t work perfectly, resulting in insufficient strength of some colors.  If a bulb fails to emit sufficient strength of a specific color, then a nearby object of that same color will appear discolored.  For example, if a bulb emits low levels of red, then nearby objects containing any shades of red will appear discolored.

How the CRI is calculated

Despite its shortcomings the standard CRI is Ra (we won’t confuse you with other upcoming standards, yet).  The RCRI is made up of 15 colors (R1-R15).  The CRI score itself is calculated by averaging the scores of only the first eight colors (R1-R8), which are a variety of unsaturated, pastel colors.  Colors R9-R15 are not calculated into the CRI score, but are still very important for color performance and may be measured individually.

The full list of CRI Rcolors are as follows.

R1 – Light greyish red
R2 – Dark greyish yellow
R3 – Strong yellow green
R4 – Moderate yellowish green
R5 – Light bluish green
R6 – Light blue
R7 – Light violet
R8 – Light reddish purple
R9 – Strong red
R10 – Strong yellow
R11 – Strong green
R12 – Strong blue
R13 – Light yellowish pink
R14 – Moderate olive green
R15 – Asian skin color

The R9-R15 scores play an important role in how a bulb will perform.  Specifically, the strong red (R9) often scores poorly in modern LED bulbs.  A poor strong red (R9) score will have a profound impact on the appearance of nearby objects as so many include subtle shades of red.

Smart shopping using CRI

Trying to shop for good quality LED bulbs is a bit of an effort using a patchwork of information.  As such, there are a few things to keep in mind.

A very good CRI score of 90 or higher means that not only are R1-R8 scores good, but usually the R9-R15 will also be quite good, too.  Importantly, with CRI scores of 90 or higher the more troublesome R9 red usually reaches a respectable 60 or higher.

For lower CRI scores in the range of 80-85, the R9-R15 usually don’t correlate as closely and may perform quite poorly.  Furthermore, the R9 red scores can dip to less than 20, leaving objects nearby the light bulb looking obviously discolored.

Be extremely wary of purchasing LED light bulbs without any indication as to how it scores on the CRI.  Marketing departments will certainly use a high CRI score to promote their products.  An absence of CRI scores will almost invariably mean the bulb has poor or inconsistent color performance.

Occasionally you’ll come across bulbs with an Energy Star logo on the packaging, but no CRI score.  Good news.  That Energy Star logo means much more than just energy efficiency.  To be certified Energy Star, light bulbs also need to adhere to a set of minimum performance requirements, including CRI.  The specification (as of January 2017) requires that all Energy Star bulbs have a CRI score of 80 or higher and an R9 red score of higher than zero.  Full stop.  That’s the minimum.  Unfortunately, the minimum won’t impress anyone.

Where things get interesting is that less efficient bulbs can still gain Energy Star certification if they achieve a higher CRI score.  For example, an omnidirection bulb (emits light in all directions) that has energy efficiency between 70 and 79 lumens per watt (“lm/W”) must have a CRI equal to or greater than 90.  A directional bulb that has an energy efficiency of 61-69 lm/W must also have a CRI equal to or greater than 90.  Both lumens and actual watts consumed will be on the packaging, so do some quick math to figure out whether the bulb falls into this range.  Sadly, the required minimum R9 red score is not elevated on less efficient bulbs, though as mentioned above R9 performance is usually relatively good on bulbs with CRI of 90 or higher.

Be mindful of marketing departments interfering with the usefulness of the CRI.  A CRI score ‘up to’ provides no helpful information whatsoever.  A CRI score of equal to or ‘greater than’ is helpful as it assures purchasers a minimum level of color accuracy.

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