Instinctive Comprehension of CRI

The color rendering index (CRI) is a standard used to measure the ability of a light source to accurately reproduce colors. Put simply, it determines whether or not the colors of objects appear distorted when illuminated by a particular light source, and to what extent.

For instance, the image below illustrates the colors of the same oranges illuminated by light sources with the same color temperature but different CRI values.

An intuitive observation is that the color of the oranges appears closer to its true color when the light source has a higher color rendering index.

Scientific definition of CRI

The color rendering index (CRI) is a measurement of how well a light source can reproduce colors. It is a qualitative evaluation that determines the degree to which the colors of an object are accurately presented. Light sources with high color rendering have better color reproduction and are closer to natural primary colors seen under sunlight. Conversely, light sources with low color rendering have poor color reproduction, leading to significant color deviation.

Currently, the CRI is a common method for defining the color rendering of a light source. It quantitatively evaluates the color rendering of the light source by comparing the color of an object under the measured light source to the color of the same object under a reference light source. The International Commission on Illumination (CIE) sets sunlight as the standard reference light source with a color rendering index of 100. Incandescent lamps are also considered as an ideal reference light source due to their color rendering similarity to daylight.

The CIE specifies 15 test/standard colour samples, which are divided into two categories:

• The numbers 1-8 represent 8 natural colours, selected from the Menzel colour scale, including a variety of representative colours, of medium shade and lightness;

• The numbers 9-15 represent 7 test colours, including red, yellow, green, blue of higher chromaticity, European and American young female skin tones, Chinese young female skin tones and leaf green.

To quantify the colour rendering ability of the light source for these colours, R1-R15 are used to represent the colour rendering index of the light source for these 15 colours. Where R1-R8 represent the colour rendering index of the light source for natural colours and R9-R15 represent the colour rendering index of the light source for test colours. The higher the colour rendering index of these colours, the better the colour rendering ability of the light source. The 15 colours tested are shown in the chart below

The color rendering index standard includes two different measurement methods:

The special color rendering index (Ri) is the color rendering index of the light source for a specific test color sample. It can be calculated using any color sample from 1 to 15, or a self-selected color sample with accurately determined spectral amplitude and brightness factor.

The general color rendering index (Ra) is the average value of the special color rendering index for the 8 colors in the number 1-8 of the light source, represented by the average value of R1-R8. Typically, Ra is used to characterize the color rendering of the light source, where a higher Ra value indicates better color rendering.

In our daily life, the color rendering index of common light sources is roughly as follows:

Notice:

While the color rendering index of incandescent lamps is theoretically 100, the actual Ra values may vary among different types and uses of the lamps. However, they are generally considered to have the best color rendering. Nonetheless, due to poor energy efficiency and environmental concerns, they are gradually being phased out. Even though LED lamps may not have the same color rendering as incandescent bulbs, they are a popular lighting option due to their energy efficiency and environmental benefits.

When purchasing LED lights, how should the color rendering index be chosen?

When selecting LED lights, there are generally two principles that can be followed in terms of color rendering: the principle of accurate color rendering and the principle of artistic color rendering.

The principle of accurate color rendering

The principle of accurate color rendering emphasizes the importance of using a light source with a high color rendering index to accurately reproduce the original colors of an object. When selecting a light source based on this principle, the Ra rating can be used as a guide. The higher the Ra value, the more accurately the colors of the objects will be rendered.

To facilitate the selection of appropriate lighting for different applications, the International Commission on Illumination (CIE) has divided the color rendering index into five categories:

Principle of effect color rendering

The principle of effect color rendering involves choosing a specific color rendering index to enhance certain colors in special scenarios, such as in meat product displays. In such cases, while ensuring that the Ra requirement is met, it is necessary to select a color that improves the corresponding special color rendering index.

• In meat product displays, a high R9 color rendering index is important as it makes the meat appear fresher and more appealing.

• In studios, a low R15 index of the lighting source must be avoided to ensure accurate reproduction of skin color.

What should be paid attention to in terms of CRI for indoor and outdoor led lights?

When it comes to CRI for indoor and outdoor LED lights, there are a few things to keep in mind:

Indoor lighting: For indoor lighting, it is important to choose LED lights with a CRI of at least 80 to ensure that the colors of the objects in the room are accurately represented. A higher CRI may be necessary for specific applications where color accuracy is critical, such as art galleries, museums, and high-end retail spaces.

Outdoor lighting: For outdoor lighting, CRI is generally not as important as it is for indoor lighting. A CRI of 70 or above is typically sufficient for outdoor lighting applications. However, it is important to consider other factors such as color temperature and brightness when selecting LED lights for outdoor use.

Color temperature: In both indoor and outdoor settings, it is important to consider the color temperature of the LED lights in addition to CRI. Color temperature is measured in kelvin (K) and refers to the perceived warmth or coolness of the light. A color temperature of 2700-3000K is considered warm and is suitable for creating a cozy and relaxing atmosphere, while a color temperature of 4000-5000K is considered cool and is suitable for work environments.

Brightness: It is important to consider the brightness of the LED lights when selecting them for indoor and outdoor use. Brightness is measured in lumens (lm) and refers to the amount of light emitted by the source. For indoor lighting, a brightness of 400-800 lumens is suitable for ambient lighting, while a brightness of 1000-1500 lumens is suitable for task lighting. For outdoor lighting, brightness requirements vary depending on the application.

There is a slight visual difference between high and low CRI in both indoor and outdoor LED lighting, but it may not be significant for all applications. In outdoor lighting, such as for architectural or landscape purposes, where the illuminated objects are relatively fixed and the scene is open, the requirements for light efficiency are often higher. This is because the viewing distance is far and the environment is complex, leading to fewer color changes.

Considering the budget and the level of scene accuracy required, it may be more cost-effective to use CRI>70 for landscape lighting fixtures, as high CRI LED lights are generally more expensive than ordinary CRI lights.