True Colors: What is Color Rendering Index?

If you’ve bought clothing in a store only to find out that the color looks different under natural light, the store’s lighting may be the reason. More specifically, the poor?color rendering?of the lighting.

Color rendering is the ability of a light source to accurately show the color of an object. The most common measure of color rendering is the Color Rendering Index (CRI).

Sunlight reveals the true colors of an object and, therefore, has a perfect CRI of 100. It’s the natural light standard against which all other light sources are compared when calculating CRI. The color of clothing in the store, food on the kitchen counter, even your skin, will look different depending on the light that’s illuminating it.

The Difference between CTT and CRI

CRI should not be confused with CTT, commonly known as color temperature. While color temperature can vary even for sunlight throughout the day, CRI doesn’t change.

The CTT of sunlight varies throughout the day and in different weather conditions. You can also see how various light sources compare.

A Deeper Look at CRI in Commercial Applications

A CRI of 90 means that the artificial light source is replicating roughly 90% of the visible color spectrum that the sun would produce on the same color.

Commercial applications that may require high 90+ CRI lighting include art galleries, museums, and medical facilities.?

In the home, lighting with a high Color Rendering Index might be used in areas where people apply makeup, or for hobbies where color is important like fly-tying and needlepoint. Artworks and food will also look better under high CRI lighting.

Light sources with a CRI of less than 80 are usually only used in areas like back-of-house corridors or storage, where people are either transiting or carrying out short-term tasks.

How is CRI Measured?

The traditional CRI metric is based on a set of eight reference colors. Formal measurement of CRI is done in a lighting laboratory, most often as part of the larger process of testing and certifying a product.

In simplified terms, the light source being tested is directed onto the reference colors and its color rendering performance is measured by a spectrophotometer. Each of the eight results is then compared with the benchmark (the reference colors as measured in sunlight).?

For each of the eight colors, the difference in performance between the benchmark and the tested light source is calculated and given as a percentage. The eight values are then averaged to give an overall CRI figure.

Testing color rendering index (CRI) with different lights

Where CRI is not known, the easiest way to evaluate the color rendering performance of a light source is with a set of reference colors or a rainbow swatch that has been printed with good color accuracy. Check the swatch color under the light source that you’re assessing.?

Do any of the colors on the swatch look dull or pale? If so, the color rendering performance may be low in those wavelengths. If all colors are bright, clear and true then the light source probably has a reasonable CRI.

Light Sources and CRI

Incandescent and halogen light sources, while inefficient in terms of energy use, both render colors very close to true. The light from fluorescent and other discharge lamps is made up of a limited range of wavelengths therefore these light sources don’t usually have a great CRI.

Early LED lighting was poor at rendering some colors, especially red-based tones which could appear grey or muddy. Red tones are not well-represented in the standard CRI color test so a light source that renders reds poorly may still have a high CRI; and because the CRI is an average of eight values, there can be some high and some low values in what is overall a good CRI.

Current LED lighting, particularly commercial-grade LED lighting, is typically high CRI.

What CRI Can’t Do

• CRI can’t tell us the brightness of a light source.
• CRI can’t tell us the color temperature. Of two light sources with the same color temperature of 5000K, for example, one may show colors accurately while the other makes subjects look dull and washed-out.
• CRI can’t tell us the spectral distribution of a light source, i.e. which wavelengths have been mixed together to make up the color of the light that we see. Because of the limited number of reference colors in the CRI measurement it can give us an idea but not the full picture.

It is widely acknowledged that the eight-color CRI metric is limited. So, in recent years, an extended CRI has been used that has fifteen reference colors.

However, a more modern standard, TM-30-15, is replacing CRI as the method of choice for assessing light source color performance. TM-30-15 has a comprehensive set of 99 reference colors. The new standard also covers quantifying color saturation using the?Gamut index, a helpful metric that is being used more often when describing light color quality.

Practical Application of CRI

In any given space, the CRI of the light source could have a profound effect on how the paint looks, how the furniture looks, and how the people look and feel.

Choosing lighting with a high CRI can be beneficial. Recent research shows both light and color can have an impact on how comfortable people feel in a space and this has implications for well-being, especially in places where people spend a lot of time under artificial lighting like hospitals, school rooms, prisons and offices.

In some ways, CRI has become less important now that LED products are available with pre-selected or tunable wavelengths of light. Retailers, for example, can specify LED lighting with the right mix of colors for displaying diamonds at their best. They can have tunable white lighting with sensors that will pick up the color of a handbag and tune the light to boost that color.

In an ideal world where all of our lighting had a high CRI, we would be confident we’d chosen the right shade of foundation at the makeup counter. At the store, red meat would truly be red and white fish would not look grey. At the gallery, the artist’s chosen colors would be vibrant and true to the artist’s vision, and at home our navy blue socks would never be paired with black.

Whether measured by CRI or by a more modern standard, color quality affects the way we see the world.