Don’t tell that you understand CRI of LED Light Source

Needless to say, the color is an important aspect for the evaluation of the quality of lighting.CRI (Color Rendering Index) is an important method to evaluate the color of the light source.And an important parameter to measure the color characteristics of the artificial light. It’s widely used in the evaluation of artificial lighting light source.

Generally, the higher the color rendering index indicating the color of the light source, the better for color reproduction of the object and also the stronger.  But that’s just generally speaking. Is it? Is evaluation with color rendering index of the light source color reproduction absolutely reliable?There will be exceptions?

In order to clarify these issues, we should understand CRI first and what is it meant. CIE provides a good method to evaluae the light source color in accordance with a reasonable model. It uses 14 kinds of test color samples,and get a series of spectral brightness numenrical value with the standard light source test, and the provisions of its color rendering index is 100. The CRI of The light source will score to compare to the standard light source . These 14 kinds of color samples are as follows:

LED Lighting CRI 1 LED Lighting CRI 2
Among No. 1-8 for the evaluation of the general CRI Ra, it’s selected in eight typical hues of medium saturation. Except the 8 standard color samples of general CRI, CIE added 6 kinds standard color samples of special CRI,for testing something special CRI to use, respectively are red, yellow, green,blue,Europeans’ skin color and leaf green (No. 9-14).The computing method of CRI in Asia even increases the Asian women’s skin color samples.

The question is here. Usually we are talking that CRI value (Ra) is obtained from the 8 kinds standard colors. 8 colors samples have moderate chroma and lightness,they are non-saturated colors,used to measure the spectrum continuous and wide band of color rendering with good results. And for the evaluation of the waveform steep and narrow-band light source will cause problems.

High CRI Ra,it means the rendering must be better?

As an example, have a look at the following two pictures. Each picture in the first row is the performance of the various standard colors samples. The second line is the LED light source’s performance.

LED Lighting CRI 3

LED Lighting CRI 4

Both CRI of LED light sources, the result is 67 and 80 according to the standard test method.Can you tell which LED is Ra = 67, which one is Ra =80 ?

The answer is, above one Ra = 80, below one Ra = 67. Unexpected, right? why? The reasons are already talked about as above.

So is there a new and better way to evaluate the rendering of light? NIST’s (National Institute of Standards and Technology)answer is CQS (Color Quality Scale). Similar to the CRI, CQS also uses color test method, but the CQS select 15 kinds of saturated colors. They are evenly distributed across the entire visible spectrum, as shown below:

LED Lighting CRI 5
What are the advantages of CQS, you can search on the web to learn about it? What I think is that any kind of methods maybe inapplicable for all. So,if for a strict space with color,what method should we determine applicative or not?My foolish method: just look at the spectrum.

Here are some typical spectral distributions of the light source, namely daylight (Ra100), Incandescent (Ra100), fluorescent lamp (Ra80), LED of some brand(Ra93), and metal halide lamp (Ra90).

LED Lighting CRI 6

LED Lighting CRI 7

LED Lighting CRI 8

LED Lighting CRI 9

LED Lighting CRI 10
From the analysis,the test resulting that CRI Ra is greater than 80,belong to master, But the real color reproduction ability is different.

So,when pay attention to the light source color rendering. First we should look at the Ra. Then, for most of the LED light source (above example considered unusual exception), it usually is not continuous, narrow band, depending on the spectral distribution. Overall, in the places with strict requirement and rich colors, such as museums, supermarkets, shops, restaurants,we should choose a relatively continuous and wide band spectrum of the light source. So as to restore the goods to be real, beautiful and comfortable color, it helps to display and sell goods, and create ambience.
The effect of different Ra for flowers as the picture below:

LED Lighting CRI 11

LED Lighting CRI 12

LED Lighting CRI 13

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