What is the Color Rendering Index (CRI)?
Put simply, the Color Rendering Index (CRI) measures the ability of a light source to accurately reproduce the colors of the object it illuminates.
This is a seemingly simple definition, but there is a lot going on, so we'll help break it down into three parts
Part 1: Color Rendering Index (CRI) is a score with a maximum of 100
What does it mean to measure the ability of something? Like test scores, CRI is measured on a scale where a higher number represents higher ability, with 100 being the highest.
CRI is a convenient metric because it is represented as a single, quantified number.
CRI values that are 90 and above are considered excellent, while scores below 80 are generally considered poor. (More on this below).
Part 2: Color Rendering Index (CRI) is used to measure artificial, white light sources
Light sources can be grouped into either artificial or natural light sources.
In most situations, we are concerned about the color quality of artificial forms of lighting, such as LED and fluorescent lamps.
This is compared to a daylight or sunlight - a natural light source.
Part 3: Color Rendering Index (CRI) measures and compares the reflected color of an object under artificial lighting
First, a quick refresher on how color works.
Natural light such as sunlight is a combination of all the colors of the visible spectrum. The color of sunlight itself is white, but the color of an object under the sun is determined by the colors that it reflects.
A red apple, for example, appears red because it absorbs all colors of the spectrum except red, which it reflects.
When we use an artificial light source such as an LED lamp, we are attempting to "reproduce" the colors of natural daylight such that objects appear the same as they do under natural daylight.
Sometimes, the reproduced color will appear quite similar, other times quite different. It is this similarity that CRI measures.
As you can see in our example above, our artificial light source (an LED lamp with 5000K CCT) does not reproduce the same redness in a red apple as natural daylight (also 5000K CCT).
But notice that the LED lamp and natural daylight have the same 5000K color. This means that the color of light is the same, but the objects still appear different. How could this be?
If you take a look at our graphic above, you will see that our LED lamp has a different spectral composition compared to natural daylight, even though it is the same 5000K white color.
In particular, our LED lamp is lacking in red. When this light bounces off of the red apple, there is no red light to reflect.
As a result, the red apple no longer has the same vibrant red appearance that it had under natural daylight.
CRI attempts characterize this phenomenon by measuring the general accuracy of a variety of objects' colors when illuminated under a light source.
CRI is invisible until you shine it on an object
As we mentioned above, the same light color can have a different spectral composition.
Therefore, you cannot judge a light source's CRI by simply looking at the color of the light.
It will only become evident when you shine the light onto a variety of objects that have different color.
How is CRI measured?
The method for calculating CRI is very similar to the visual assessment example given above, but is done via algorithmic calculations once the spectrum of the light source in question is measured.
The color temperature for the light source in question must first be determined. This can be calculated from spectral measurements.
The color temperature of the light source must be determed so that we can select the appropriate daylight spectrum to use for comparison.
Then, the light source in question will be virtually shone onto a series of virtual color swatches called test color samples (TCS) with the reflected color measured.
There are a total of 15 color swatches:
We will also have ready the series of virtual reflected color measurements for natural daylight of the same color temperature.
Finally, we compare the reflected colors and formulaically determine the "R" score for each color swatch.
The R value for a particular color indicates the ability of a light source to faithfully render that particular color.
Therefore, to characterize the overall color rendering capability of a light source across a variety of colors, the CRI formula takes an average of the R values.
Which and how many R values are averaged will depend on which definition of CRI you are using - general CRI (Ra) or extended CRI.