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Why the LED R9 value isn’t important

Why the LED R9 value isn’t important

Whenever LED light quality is discussed—especially in terms of color rendering—it’s just a matter of time before the R9 value is mentioned. So what exactly is this point of dissention in LED lighting circles? Is the R9 value really that important?

The R9 value is one of the 14 pigment colors scientists have established to measure color rendition. However, Color Rendition Index (CRI), which is the baseline measurement used to determine how well colors from a light source compare against those from a “natural” light source such as sunlight, does not account for the R9 value. In fact, CRI only measures the light source against the first 8 pigment color samples:

  • R1-R8 (pastels)
  • R9-12 (saturated solids) <– not measured in CRI
  • R13-R14 (earth tones) <– not measured in CRI

CRI R9

CRI pigment color samples

 

The R9 value produces strong, vibrant reds. Is it really important to include a strong red when measuring color rendering? No, it’s not important. It’s fundamental. Paramount. Vital. Essential. Critical. Take your pick of adjectives.

Why should we care?

People should care because strong reds are prevalent in skin tones and clothes. Food retailers should care because strong reds are prevalent in grocery store produce and meats. Gallery owners and artists should care because strong reds are prevalent in art work. Hospitals should care because strong reds are the most critical color for surgical procedures.

The R9 value is so important, that some utility companies are now offering rebates for positive R9 value lamps.

Can you get a reasonable colour rendering without a good R9? For most applications you can, but where red reproduction is of paramount importance you need to look further. Energy Star defines an acceptable CRI as having a value greater than 80 and an R9 value greater than zero and this produces very good light quality and colour reproduction. However many LED products fail to meet these standards.  Further, when it’s really critical to produce good reds of the quality produced by halogen or incandescent lamps (such as the vegetable or meat counter at your local grocery store) you need to look for a lamp with a CRI above 90 and an R9 above 60.  It’s worth noting that R9 does track with increasing CRI, but because the measure is in effect an average value, it’s only at very high CRI’s that R9 tracks closely.

Aside from using a different set of standards in which to judge LED light quality (see Attention CRI: You’re no longer relevant), what can you do to ensure the LED lamp you want to buy renders color accurately?

  • Don’t select an LED lamp based on the CRI number alone. Ask the manufacturer to provide their R9 values—reputable manufacturers will be able to produce lamp spectrums.
  • Inquire as to whether the manufacturer employs a design strategy to account for R9
  • Ask whether they routinely measure the colour performance of their lamps and whether they can demonstrate the changes from lamp to lamp, and of the colour consistency over the lifetime of the lamp – after all, there’s no point in putting in the investment in a high CRI lamp if it ages badly

All of Leapfrog’s specialised high CRI products have a CRI of at least 95 and an R9 over 80.

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2 Responses to Why the LED R9 value isn’t important

  1. We have recently been working with our campus art gallery to replace their halogen spots and floods with LED lamps. The artists had been very disappointed with the LEDs until we found some in warm white with high CRI from Sylvania. These were much better, but we don’t have the R9 stats on them. Until today I was web searching on “High CRI LEDs” but just tried “High R9 LEDs” and got a much better range of potentially useful lamps and excellent discussions such as yours.

  2. There seems to be emotion attached around this subject. My background, and bias, is science. So the problem is for building owners is to spec out the type of lighting where clients are, at best, inarticulate in conveying what need is to be satisfied.

    You have 3 concurrent problems to address, technically. Emission light source attribution, reflected light, and human eye sensitivity. The emotive effects- perhaps in the discipline of psychology, are for someone other than myself.

    Regarding light sources- with LEDs, their spectral width is much smaller (narrower) than incandescent or fluorescent lighting. That is neither good or bad from an illumination perspective. It is clearly more efficient commercial power line usage.

    Manufacturers are evolving wider spectrum source LED. However, at this time, to sell what they have on the truck… the “temperature” is your primary discriminator.

    The degrees Kelvin only gives the central tendency of the luminous flux emissions. With the human eye as the receptor (not a digital sensor), a difference of ~30 lumens is the most common boundary for response fall off. With the evolution from gas lamps with cotton mantles to cotton filaments bulbs, to metal (tungsten) bulbs- you move up from 1700 K to 2700 K. For household lighting. Find a TV or movie studio where color balance with that spectral response is possible to achieve any balance. That’s why Arc lamps and other lighting addressed unsatisfied needs with early low temperature light emissions- such as approximating daylight close to the sun’s temperatures (5000 to 27000 K).

    With respect to ‘caveat emptor’- the marginal price difference going from a spectral “yellow” LED to a spectral white, blue-white, and blue also increases. And the quality control is also less rigorous (specific to the PNP junction devices). To manufacturers manufacturing unit quantities in the tens to hundreds of thousands- selling (versus telling) you marketing spin is valuable….to them. You are still left to make a calculus to satisfy competing interests/needs of your audience.

    Regarding reflectivity…light emissions and pigments work differently. The reflective properties, and in some cases, the refractive properties- is what the human eye gets to see (sense). Add that ff the creators of the works were working in yellow light, the “white balance” was already shifted for their personal environment’s reference. Their buyers- may or may not have the reference lighting conditions they developed their “art” in, however. (Hopefully- your artists handle the returns and not you.)

    With respect to eye sensitivity- the human eye evolving in Cro-Magnon’s 20,000 years does not have an even distribution of cones (color receptors). The distribution is weighted heavily against the “yellow”. So, depth perception and precision distance measurement were preferred in evolution and under our earth’s Sun…yellow loses out.

    As a gallery owner, you may prefer to have the traveled portions of you presentation area where the liability underwriting favors one lighting temperature (near daylight). Perhaps at the individual work of art, you might need a lighting that matches what the artist developed under.

    —Note; yes, I have left out some of the physics regarding the Half-wave spectral width. That is defined as the relative spectral energy distribution curve, corresponding to the two-and-a-half maximum intensity at the wavelength difference, and marks the spectral purity. That said, it can also be used to measure the dispersion of the semiconductor material of the light-emitting contributing the energy state. Which does result in the half width of the emission spectrum of the LED shifting 30 to 100 nm. That is what a narrow spectral width of the die means- good color, bright color, light color, and clearly visibly perceived. Where would one find these specification references in the sales materials to be able to specify them??

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