If you buy clothes in a store and find that the color looks different in natural light, it may be due to poor color rendering of the store’s lighting. Specifically, color rendering is the ability of a light source to accurately display the colors of objects. The most commonly used measure of color rendering is the color rendering index (CRI).
Sunlight reveals the true colors of objects, so it has a perfect CRI of 100. This serves as the natural light standard against which all other light sources are compared when calculating CRI. The colors of clothes in stores, food on kitchen countertops, and even your skin can all look different depending on the light that illuminates them.
The Difference Between CCT and CRI
CRI should not be confused with CCT, which is commonly referred to as color temperature. While CCT can vary even in natural sunlight throughout the day and under different weather conditions, CRI remains constant.
The CCT of sunlight, for example, varies throughout the day and under different weather conditions. You can also compare the CCT of different light sources.
Understanding CRI in Commercial Applications
- A CRI of 90 means that an artificial light source is replicating about 90% of the visible spectrum, as the sun would produce that same visible spectrum in that particular color temperature.
- Commercial applications that may require lighting with a CRI of 90+ include art galleries, museums, and medical facilities.
- In the home, lighting with a high color rendering index can be used in areas where people apply makeup or for hobbies where color is important, such as fly-tying and needlepoint. Artwork and food also look better under high CRI lighting.
Compare the color under different types of light bulbs.
Light sources with a CRI below 80 are usually only used in areas such as logistics corridors or storage rooms where people are passing through or performing short-term tasks.
How is CRI measured?
- The traditional CRI metric is based on a set of eight reference colors. The formal measurement of CRI is typically done in lighting laboratories, often as part of a larger process of product testing and certification.
- In essence, the light source being tested is aimed at the reference colors and its color rendering performance is measured using a spectrophotometer. Then, for each of the eight colors, the performance difference between the reference and test light sources is calculated and given as a percentage. These eight values are then averaged to give an overall CRI number.
- In situations where CRI is unknown, the simplest way to assess a light source’s color rendering performance is to use a set of reference colors or a rainbow color sample printed with good color accuracy. Check the sample colors under the light source being evaluated. Do any colors look dull or pale? If so, the color rendering performance of those wavelengths may be lower. If all colors appear bright, clear, and true, the light source may have reasonable CRI.
Light Source and CRI
- Incandescent and halogen light sources, while inefficient in terms of energy usage, both exhibit colors that are very close to true. Light from fluorescent and other discharge lamps consists of a limited range of wavelengths, so these light sources typically have a low CRI.
- Early LED lighting exhibited poor color rendering in certain hues, particularly red tones that may appear gray or muddy. Red tones are not well-represented in the standard CRI color test, so light sources with poor red rendering may still have a high CRI; and since CRI is an average of eight values, overall CRI may have some high and some low values.
- Current LED lighting, particularly commercial-grade LED lighting, typically has high CRI.
What CRI Can’t Do
- CRI cannot tell us the brightness of the light source.
- CRI cannot tell us color temperature. For example, two light sources with the same color temperature of 5000K may make the subject look accurate in color rendering, while the other may make the subject look dull and faded.
- CRI cannot tell us the spectral distribution of the light source, i.e., which wavelengths are mixed together to create the color of light we see. Due to the limited number of reference colors used in CRI measurements, it can only give us a concept, but not a complete picture.
It is well known that the eight-color CRI index is limited. Therefore, in recent years, an expanded CRI with fifteen reference colors has been used.
However, the more modern TM-30-15 standard is replacing CRI as the preferred method of evaluating color performance of light sources. TM-30-15 has a complete set of 99 reference colors. The new standard also includes the use of the gamut index to quantify color saturation, which is a useful index that is used more frequently when describing light color quality.
The practical application of CRI
- The CRI of a light source has a profound impact on the appearance of paint, furniture, and people’s appearance and perception in any given space.
- Choosing lighting with a high CRI is beneficial in various ways. For example, selecting high CRI lamps in retail stores can make your products more eye-catching and increase customer desire to purchase them.
- Choosing high CRI lamps in clothing stores can best reproduce the fabric quality of the clothes. Selecting high CRI lamps in restaurants can make the food look more appetizing.
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