An authoritative guide to light source color rendering index (CRI)

2023 04 03 15 51 15


When we purchase lighting fixtures, our first consideration may be the power and luminous flux, and another parameter that cannot be ignored is the color rendering index (CRI).

Many lighting fixture manufacturers and designers will tell you that lighting fixtures with a CRI greater than 90Ra have excellent color rendering performance.

Using lighting fixtures with a CRI greater than 90Ra can best restore the color of the illuminated objects, such as making food look more delicious, vegetables fresher, and clothes more textured.

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But is this really the truth? Does a CRI greater than 90Ra, or close to 100, guarantee the best color rendering? In this article, I will share how to achieve the best color rendering.

What is the color rendering index?

Let’s first understand what the color rendering index is. The International Commission on Illumination (CIE) defines color rendering as the effect of a light source on the color appearance of objects, compared to a reference source.

The color rendering index is the value obtained by comparing the color appearance of a light source on a set of 15 color samples specified by CIE to that of the standard reference source.

The color rendering index does not refer to the color emitted by the light source, but rather evaluates the color rendering performance of the illuminated object.

The color rendering index is represented by a value between 0 and 100. The closer the value is to 100, the closer the light source is to sunlight, indicating better quality.

1、How is the color rendering index measured?

The color rendering index is measured by comparing the colors produced by a light source on 15 specified color samples to those produced by a standard reference source on the same samples.

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The first 14 color samples are selected from the Munsell color coordinate system, and the last color sample is a representation of the skin color of Asian women.

The Munsell color coordinates can be expressed by a mathematical model, so in actual measurements, real color samples are not used to determine the CRI value of the tested light source.

The standard reference source can also be calculated using a mathematical model, so in actual measurements, the spectral distribution of the tested light source is measured using an instrument, and the CRI value is calculated using mathematical analysis.

2、General color rendering index

The general color rendering index is the average of the sum of R1-R8 values, calculated as (R1+R2+R3+R4+R5+R6+R7+R8)/8. It is a subset of 8 color samples proposed by Nickerson (1960), belonging to relatively low saturation colors and evenly distributed throughout the entire hue range.

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This standard is sufficient for lighting fixtures used in daily life, and lighting fixtures with a CRI above 80Ra are defined as having excellent color rendering performance, while those above 90Ra are defined as having particularly excellent color rendering performance.

However, although the color rendering index of a lighting fixture is higher than 90Ra, it only reflects the color rendering ability for the R1-R8 color samples, which are relatively soft colors.

For plain clothes and the construction industry, this evaluation standard is sufficient. But it cannot express whether the light can reproduce more saturated colors, so a method of measuring the extended color rendering index has been developed.

3、Extended color rendering index (Re)

According to CIE (1995) specifications, the extended color rendering index (Re) complements the general color rendering index (Ra) by adding R9-R15, seven additional evaluation color samples, to the original R1-R8.

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The original test color samples (TCS) are taken from early versions of the Munsell Atlas. The last seven samples provide supplementary information on the colorrendering characteristics of the light source.

The first four are highly saturated, and the last three are representatives of well-known objects. The reflectance spectra of these samples can be found in CIE (2004), and their approximate Munsell notations are listed next to them.

4、Special color rendering index

The special color rendering index usually refers to the situation where some of the R9-R15 color samples have high color rendering index values in addition to a high general color rendering index value (Ra).

Lighting fixtures with a special color rendering index are usually used in specific places in daily life. For example, when illuminating meat, it is necessary to better present the freshness of the meat.

To meet the high performance requirements of the general index Ra, the color rendering ability of the R9 index is strengthened.

When the illuminated objects are vegetables, fruits, and other green-yellow objects, the color rendering ability of R10 and R11 is strengthened to better present their colors and make the vegetables and fruits more green and fresher.

In lighting used for makeup or selfie, the color rendering ability of R9+R15 is usually strengthened to achieve better skin color rendering effects.

5、The indispensable color rendering index R9

R9 is the ability of the light source to reproduce the color red of objects. Red is one of the indispensable colors in our daily life, as it is one of the primary colors.

It is well known that all common colors in the external environment are created by the combination of the three primary colors of red, green, and blue. This also creates the irreplaceability of red in many application scenarios.

For example, in the film and video industry, without red light illumination, human skin will appear pale or greenish. In medical applications, without red light illumination, blood vessels and blood will appear close to white or light yellow.

In other industries, such as textile printing, image hard brushing, and artistic lighting, the demand for the rendering of red color is still high.

6、The abandoned R96a measurement method

At the 1991 quadrennial meeting of the CIE, the Technical Committee proposed the R96a method. The R96a testing method includes a new set of test color samples
six reference light sources: D65, D50, blackbody 4200 K, 3450 K, 2950 K, and 2700 K,
a new color adaptation transformation: CIECAT94, and color difference evaluation in CIELAB, making all colors adapt to D65 (since CIELAB has been well tested under D65).

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The committee was dissolved in 1999 and issued CIE (1999) without a definitive recommendation, partially because of disagreement between researchers and manufacturers.

In 2002, Hung proposed that the color blocks in the CIE (1995) specification that had already been established had better correlation with any color difference than the ColorChecker chart used in the R96a test method, and the samples in the R96a test method were unevenly distributed in a unified color space.

What are the benefits of high color rendering index?

It is widely known that high color rendering index (CRI) lighting has better color performance, but do you know the specific benefits of it? Keep reading to find out.

  • Better color reproduction:
    This is the fundamental benefit of high CRI lighting. Just as sunlight is known for its ability to bring out the true colors of objects, high CRI lighting can make food look more delicious, products more attractive, and clothing more luxurious. In other words, high CRI lighting can accurately reproduce the original colors of objects.
  • Better for eye health:
    CRI also has an impact on our health, specifically our eyes. Poor color rendering can lower the sensitivity of our cone cells, leading to a decline in color discrimination ability, color blindness, or color weakness. Furthermore, it can cause eye fatigue, difficulty concentrating, and other vision problems, including myopia and other eye diseases.

Various lamps’ color rendering index

Despite LED being the most widely used and popular lighting source in the world today, there are still many other lighting products and applications for different light sources, each with their own color rendering index (CRI) and characteristics.

Incandescent bulbs are thermal radiation sources that have good color rendering due to their continuous spectrum energy (power) distribution, typically between 99-100.

Halogen bulbs are an improved version of incandescent bulbs and also have good color rendering performance, usually between 95-100.

Short-arc xenon lamps are high-intensity point light sources with a light color close to sunlight, with a color rendering index usually between 94-98.

Fluorescent lamps, also known as neon lamps, use low-pressure mercury vapor to release ultraviolet light when powered, causing the fluorescent powder to emit visible light. There are two types, with a general fluorescent lamp having a CRI between 75-85, while a three-primary-color fluorescent lamp has a CRI between 85-95.

High-pressure sodium lamps use xenon gas as a starting gas, with high efficiency and long life, but poor color rendering performance, usually between 15-30.

Dysprosium lamps are high-intensity gas discharge lamps with high efficiency, good color rendering, and high brightness. They have excellent applications in sports arenas, film shooting, and color television broadcasting, with a CRI usually above 85.

LED lamps, due to their energy-saving, environmentally friendly, and long-lasting performance, are widely used in various scenarios and are the development trend of future lighting, with a CRI usually between 80-99.

New light source quality evaluation standard TM-30-18

With the rapid development of LED lighting technology, the shortcomings of the CRI color rendering index evaluation standard have gradually emerged. The first is that it cannot accurately represent color fidelity, and the second is the lack of relevant evaluation of color quality, resulting in significant errors between the Ra value and the actual perceived color by the human eye.

1、What is TM-30-18?

Against this background, the new generation TM-30-15 light source quality evaluation standard was released by the Illuminating Engineering Society (IES) in North America in 2015. It was updated and released again in 2018 as TM-30-18, which adjusts some functions, measurement intervals, and calculation proportion factors on the basis of TM-30-15.

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2、The TM-30-18 evaluation standard consists of three parts:

Rf (color fidelity or hue): The Rf value indicates the degree to which the color of the light source matches or differs from the reference color. For example, whether it is deep blue or light blue, the index is represented by 0-100, with higher values indicating better matching. The calculation compares the chromaticity and saturation of the test light source and the reference light source and then determines the arithmetic mean of these color differences.

Rg color gamut or saturation: Rg describes the quantity of colors. When the Rg value exceeds 100, it means that the color is oversaturated, while when it is less than 100, it means that the saturation is lower than the reference value.

Color vector graphic: TM-30 creates a graphic report that plots the color average of 16 hue boxes and the Rf and Rg scores of 99 color samples.

For more detailed information about the TM-30 method, please visit the U.S. Department of Energy website.

3、Difference between TM-30-18 and CRI:

Difference one: CRI can only present the color of the object singly, providing an effect similar to that of irradiation under sunlight, and cannot provide any information about saturation. TM-30-18 adds the use of gamut index (Rg) to describe saturation information compared to CRI.

Difference two: The general color rendering index Ra only uses eight color samples to determine color fidelity, and even the revised CRI index only increases color fidelity to 15 color samples.

TM-30-18 has 99 color samples for evaluation, which is more abundant than the color evaluation samples of the CRI evaluation standard. Therefore, TM-30-18 is a more authoritative evaluation method for color rendering index with higher accuracy.

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TLCI (Television Lighting Consistency Index) stands for the Television Lighting Consistency Index. It was officially released by the European Broadcasting Union (EBU) in 2011.

Due to differences in color perception between the human eye and cameras, TLCI is a lighting quality evaluation standard specifically designed for the film and television industry.

The use of lighting fixtures that comply with the TLCI standard can enable cameras to record more accurate colors and facilitate post-production processing, reducing the need for color error correction work in post-production. Measurement method of TLCI evaluation standard

The measurement method of TLCI is similar to that of CRI. The color presented by the light source when illuminated on the 24 designated color samples of EBU is compared to the color presented by the reference light source (a standard light source with chromaticity and color temperature falling on the Planck curve) when illuminated on the same 24 designated color samples of EBU, based on the color performance output by the television camera. This color performance refers to the color performance that can be seen on a standard high-definition television monitor.

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Similarly, the TLCI value is also between 0 and 100, with values closer to 100 indicating better results in the footage captured by the camera. TLCI index and its impact on post-production

  • TLCI: 85-100, minimal color error, excellent lighting quality, colorists or lighting engineers may consider not making any adjustments or making only slight adjustments.
  • TLCI: 75-85, some color error, good lighting quality, colorists can quickly complete lighting adjustments and color calibration.
  • TLCI: 50-75, more significant color error, colorists or lighting engineers may need a lot of time to adjust the camera or use color paper to correct the color defects of the lighting. Experienced professionals are needed to obtain an acceptable result.
  • TLCI: 25-50, severe color error, extremely poor lighting quality. Even with comprehensive adjustments by colorists or lighting engineers, basic viewing requirements may not be met.
  • TLCI: 0-25, such lighting fixtures are a terrible choice, and no matter how they are shot and adjusted in post-production, they cannot meet basic viewing requirements.

Other quality management methods for color rendering of light sources

  • Gamut Area Index (GAI):

In 2010, Rea and Freyssinier developed the Gamut Area Index (GAI) to improve the deficiencies found in the CRI evaluation standard. They claimed that GAI was better at predicting color discrimination in the standardized Farnsworth-Munsell 100 hue test than CRI and could predict color saturation.

Testing showed that using GAI in combination with CRI to evaluate color rendering was more favored by test subjects than just using high-value light sources. It also showed that using GAI and CRI in combination was a better method for evaluating color rendering for new LED technology.

  • Color Quality Scale (CQS):

Developed by NIST researchers, it also aimed to improve the use of unsaturated colors and the small number of test sample colors in the CRI evaluation method.

CQS uses higher saturation colors and proposes 15 new colors to evaluate light sources. It also uses a numerical value of 0-100 to represent light source color rendering, with higher values indicating higher quality.

Is a light source with Ra>90 always good for color rendering?

If you have carefully read the above analysis, you may already have the answer in your mind, or you may feel confused because of the abundance of shared knowledge. Now I can answer you with certainty: a light source with Ra>90 does not necessarily have good color rendering.

To summarize briefly, the different color rendering index evaluation standards have different applications, but they all use numerical values of 0-100 to represent color rendering.

A light source with Ra>90 is generally better than Ra>80, but if the extended color rendering index Re is included, a light source with Re>90 may have better color rendering than a light source with Ra>90.

However, if compared with the TM-30 index, a light source with Rf>90Rg>90 will have better color rendering than a light source with Re>90.

For everyday lighting, the quality ranking order with indices above 90 is: TM-30 > Re > Ra. For the film and television industry, the TLCI evaluation standard is more appropriate, and a light source with TLCI>90 is better than a light source with Ra>90.

In the following, we will share how to choose the appropriate lighting fixture by checking the color rendering index parameters.

How should I choose lighting fixtures?

  • Workplaces:

Workplaces usually include offices, conference rooms, and design studios. We focus on work for long periods, so for eye health reasons, we recommend lighting fixtures with a color rendering index above 90, sufficient to meet the Ra index.

Without considering costs, fixtures with extended color rendering indices Re and TM-30 indices greater than 90 are better choices.

  • Living spaces:

Living spaces typically include commercial shops, restaurants, and exhibition areas. For these areas, specific color rendering index requirements can be very high.

For example, to better restore product colors while meeting Ra>90, attention must be paid to specific color enhancements based on your products.

If the colors are rich, such as with various products in a supermarket or various colors of display items in an exhibition hall, we recommend choosing fixtures with TM-30 indices greater than 90 to better restore their color and texture.

  • Learning spaces:

Learning spaces typically include classrooms, libraries, and e-reading rooms. Like offices, the main users of these spaces are minors, and we need to pay special attention to eye health.

We recommend fixtures with extended color rendering indices Re>90. Of course, fixtures with TM-30 indices greater than 90 are even better choices.

  • Resting places:

Resting places generally refer to homes, apartments, hotels, and other places. If specific color requirements exist, we recommend Ra>90 to provide a better color experience, and fixtures with TM-30>90 can be selected for even better quality.

  • Requirements for the film and television industry:

With the development of society, the film and television industry is no longer a distant field.

With the development of the Internet, various photo and video websites influence everyone, giving us all the opportunity to showcase ourselves.

Of course, the important factor in improving photo or video quality is lighting. Therefore, we need to pay special attention to the TLCI index.

To improve photo and video quality, we not only need to pay attention to the lighting fixtures’ illumination but also the color rendering index.

If you’re just sharing personal photos or videos, Ra>90 is sufficient. But if you have more sophisticated color adjustment requirements, you must use lighting fixtures with a TLCI>90.

This will reduce the workload of color correction in post-production, improve efficiency, and provide the best photo or video quality.

When purchasing film and television lights, the TLCI index value is always marked in the parameters of the lighting fixture. If this value is not marked, it is not a professional film and television light.

The above is a knowledge sharing about color rendering index related to light sources. We hope it will be helpful to you, and we will continue to share knowledge about lighting and lighting fixtures.

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