CRI vs. CQS – Understanding and Selecting the Standard

I.  What Are CRI and CQS?

Light is all around us – except when it isn’t. That is when we notice it most, of course, realizing that it was only because of light that we could perceive any of the colors of our environment. But did you know that there are specific ways to measure the ability of light to allow you to see color? For the last century, researchers, developers, and scientists have tried to come to terms with the best way or ways to do this. It may seem at times that a consensus is still far in the distance, but there are facts that we can examine to form an opinion for ourselves. Let’s take a look at the progress they have made so far.

Color Rendering Index (CRI)

Before we get into what the Color Rendering Index is, we should consider what color rendering is. Essentially, it refers to the way that light affects how the human eye perceives color. Think about your own hand: when you are in a dark room, it looks different from what it looks like when you are sitting outside on a clear day. We think of “perfect” color rendering as what occurs in daylight, and the scales that we use to measure color rendering try to quantify this in order to make it easy for us to communicate how accurately or how clearly a light source is illuminating its environment.

A CRI runs up to 100, so if you hit 100, you are talking about standardized daylight or something called a “black body,” which is a light source that effectively absorbs all the other light around it. A CRI value can be negative too – a point that we will touch on in more detail later. If you have a light source that measures a CRI of 64 and another light source that measures a CRI of 80, the light source that measures a CRI of 80 should be superior – which is to say, it should come closer to duplicating the color rendering of daylight. The way that its creators intended it, CRI should get better as it gets higher.

Color Quality Scale (CQS)

The intention of the Color Quality Scale (CQS) is to measure color rendering as well, and in the time since CRI entered the lexicon, the term “color rendering index” has become a term for any measurement of color quality or color rendering. This means that CQS is a color rendering index, as is CRI. Don’t let this bog you down: what matters is their ability to do what they purport to do.

There is no such thing as a negative CQS, and its maximum figure is also 100. This means that a CQS can run anywhere from 0 to 100, and in addition to measuring color fidelity (the primary concern of the researchers who developed CRI and the one attribute that CRI measures), it can measure things like chromatic discrimination and observe preferences – which both factor into the way that people perceive color quality and thus factor into the ability of a light source to illuminate an environment.

CQS is intended to correct several things that other color rendering indexes had overlooked or assumed incorrectly. First, it is meant to be simpler for people to handle and talk about. Second, it is meant to apply more equally to lights, especially new types of lights, such as LEDs.

This begs the question – has CQS accomplished its goals? Is it performing as it is supposed to perform, and are other color rendering indexes not performing as well?

II.  The History of CRI

Before we go any further into the technical details of color rendering indexes, let’s talk about the history of color rendering indexes – and the Color Rendering Index specifically. By getting a firmer grasp of how the current standards came to be and why they have remained as popular as they are, we will be able to glean some insights into our best path forward.

Our story starts in the 1930s and 1940s. At that time, researchers and scientists knew that light sources varied in quality – just as we all know that light sources vary in quality, seeing instinctively that daylight changes our perception of color relative to candlelight. What they wanted to do was find a way to talk about this reality meaningfully, discussing color rendering or color quality just as we would discuss weight, height, distance, or pressure.

The International Committee on Illumination (CIE for short because of its French name Commission internationale de l’éclairage), met in 1931 and decided on their first colorimetry system, coming to an agreement about human perception of colors and concluding that sight is trichromatic (a fancy way of saying that it uses three colors, red, blue, and green, which it mixes to form all the other colors).

In 1974, the CIE created CRI, just more than 25 years after they decided to use daylight as the standard against which they would measure all other light sources. While the mathematics that goes into calculating a CRI value is a little complex, what you need to know about it is that it is heavily dependent on temperature, which made perfect sense at the time because there were no light sources emitting light without emitting equivalent heat.

Enter LEDs.

III.  A Little Background about CQS

Within the last decade or so, researchers at the National Institute of Standards and Technology (NIST) examined the CIE’s Color Rendering Index. They reviewed the shortcomings, which we are going to go over next, and they set out to come up with something better. Backed by the full force of the United States government, they drew on a wide range of expertise, consulting with scientists and business leaders alike. They also looked to the CIE for their opinions about CRI and considered the work that the CIE had done in the mid-90s in an attempt to improve on their model from the 70s. One by one, they addressed the issues that consumers and professionals had reported in their day-to-day use of CRI.

NIST took a shrewd approach to the problem, and instead of starting over from scratch, they adopted the general calculation method of the CRI. However, they tried to tweak the data that was foundational to their calculations to arrive at values that more universally corresponded to people’s perception of color from one light source to the next, using additional color samples and accounting for the discrepancies that tend to arise in CRI for high-quality, low-heat light sources.

The result of the work that NIST did was CQS. Today, CRI (or more accurately, the version of CRI that the CIE re-released in the 90s) is still somewhat common, but more and more, corporations, academic institutions, and other lighting and coloring professionals have turned to CQS because of its capacity to reflect light sources in our modern world more consistently, its values applying more accurately between incandescent light bulbs and LED lights than more primitive color rendering indexes had.

IV.  Shortcomings of CRI

In the years since CRI came into common use, many experts have noted that for an industry standard, CRI creates far more headaches than it should. When researchers at CIE created CRI, of course, the world was a different place. Colorimetry was a new science, and it attempted to describe something some people thought of as subjective.

As Wendy L. Davis and Yoshihiro Ohno put it in the abstract of their 2010 Optical Engineering paper “The Color Quality Scale”: “The Color Rendering Index (CRI) has been shown to have deficiencies when applied to white light-emitting diode (LED) based sources. Further, evidence suggests that the restricted scope of the CRI unnecessarily penalizes some light sources with desirable color qualities.” Let’s break these criticisms down individually.

One of the most immediate criticisms of CRI is that it is confusing. Because the scale can run into negative values, it tends to be difficult for people to interpret correctly. A specific low-pressure sodium lights, for example, may not offer the best color rendering of any light source on the market – but what does it mean when it scores negatively and yet still seems capable of rendering some light? This has perplexed people for decades, ever since CRI entered wide usage.

Another criticism of CRI is that it draws on too few color samples – eight, to be exact. All of the color samples used to calculate CRI are pastels and completely unsaturated, which means that high-CRI light sources may render bright or saturated colors poorly. To bring this concept into life, imagine that you have just finished painting your room neon orange. You go shopping for a new light for your newly-painted room, and you settle on something that seems to be of the highest caliber – because of its high CRI. When you plug in the light and turn it on, though, you find that you can see almost none of the wonderful, bright orange color of your walls. The standard has failed you.

In particular, CRI fails to accurately describe the color quality of LED lights. Rather than corresponding to high values as they ought to given the nature of the scale, low-CRI LED lights actually produce higher color quality scores according to CRI. This is because LED lights emit their high-quality light without emitting correspondingly high levels of heat, something that was impossible at the time that the CIE called color rendering indexes a settled matter and assumed that it did not need to account for light sources below 5000 kelvins (which LED lights are). The same applies to fluorescent lamps, which also score poorly according to CRI but which have become popular because people can see so clearly in the environments that they light.

It is also worth mentioning that CRI scores incandescent bulbs at 100, even though to the human eye, it is clear that they are not replicating the color quality of standardized daylight. In their paper proposing CQS as the new standard, Davis and Ohno cited several other technical issues with CRI, including its use of an obsolete color space scale, its imprecise chromatic scale, its imprecise error calculations, its equal weight to all errors (irrespective of the ways that people perceive and even prefer some color rendering errors), and its inability to account for non-white light.

V.  Picking One or the Other

CRI and CQS are different. Although NIST researchers derived CQS through the same calculation that other researchers have used to derive CRI for decades, they did so in a way that better accounted for the light sources that are on the market today, the way those light sources function, and the variables that actually affect their color rendering. By drawing on more chromatic samples, by making the scale more straightforward, and by accounting for low-heat, high-quality light sources like LEDs, CQS tends to produce scores that correspond more closely with the human point of view – so that a higher score means a higher color quality, no exceptions.

CRI has become progressively more popular since its introduction, of course. Professionals and lighting manufacturers all around the world have become so used to using CRI and speaking in terms of it that even in spite of its glaring flaws, it remains widespread. Think back to the bright orange room that you just painted for yourself: as frustrating as it is, this is a scenario that is not only plausible but likely. Many lighting fixture suppliers will talk about CRI almost exclusively, brushing over its shortcomings or ignoring them completely.

To those who understand the principles of CRI and its flawed premises, however, it is unreasonable to brush over its shortcomings. They are too numerous and too substantial, and their impact on our ability to talk about light sources is too great. When a world-class LED can score lower than a flickering, half-burnt-out oil lamp, we know that there is a problem. We know that we cannot stand idly by while this issue persists and prevents meaningful discussions of light sources for art galleries, surgery centers, and our homes.

The work that NIST has done to develop CQS is world-changing, and by using one less than double the number of colors, it represents a drastic improvement over CRI, which is perhaps a few years past its prime. While CRI has served its purpose up until now, serving as a stepping stone from the complete absence of any measurement scale, CQS has clearly surpassed it in terms of usefulness.

Although it is useful to understand CRI even if only to understand its legacy in this field, CQS represents the cutting edge. For color rendering indexes, CQS meets the needs of the modern world and measures color quality more accurately. While the difference may not have been perceptible forty years ago, it is perceptible today. Walk into any LED-lit office or any brightly-painted bedroom and you will see immediately what the color quality is – and if you were to look at the CQS of the light source in the office or the bedroom, it would match up one to one. The same can’t be said for CRI.

VI.  Coloring the Future

When you decide that you need to select a standard for measuring color quality, there are two considerations that should be at the forefront of your mind. Let’s put these considerations into two easy-to-remember questions.

  1. Is the standard easy to talk about?

For CRI, the answer would have to be no because of its negative values. Confusing people who are trying to understand color rendering, CRI’s negative values seem to communicate something that they do not – that color rendering is something that can be absent in some light sources and not merely something that varies between light sources. Even that sentence is confusing, a strong indicator of how CRI can muddle the conversation about light sources.

In CQS, on the other hand, there are no negative values. The lowest score possible is zero.

  1. Is the standard accurate across light sources?

Once again, the answer for CRI would have to be no. Penalizing LED light sources because of their low heat levels, CRI does not communicate true color quality for all types of lighting.

CQS? It does communicate true color quality for all types of lighting. It is accurate across light sources.

On both counts, CQS seems to be the superior standard, both making it easier for people to talk about color rendering and making it easier for them to think about it. In today’s world, where LED lights are more popular than ever, the standard for measuring color rendering ought to be able to measure their color rendering just as well as it can measure that of a candle wick or an incandescent light bulb – which is why, between CRI and CQS, the choice is clear.

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