There is a more scientific way to describe the colour rendering index but for the purpose of this post I shall keep this as simple as possible to understand so that we can concentrate on the points that really matter to our wider design community.
In general terms, CRI is a measure of a light source’s ability to show object colours “realistically” or “naturally” compared to a familiar reference source, either incandescent light or daylight. CRI is the world’s most widely accepted metric to describe a light source’s ability to accurately reproduce colour.
Why is CRI Important?
The type of light source you use in your room could have a profound effect on the way a room and its furnishings look. In essence a higher quality CRI light source will make your room look more colourful as the fabrics and paint surfaces intended, whereas a lower CRI light source will create a duller appearance. To explain this we first need a refresher on how we see light:
Natural light such as sunlight is a combination of all the colours of the visible spectrum. The colour of sunlight itself is white, but the colour of an object under the sun is determined by the colours that it reflects.
A red apple, for example, appears red because it absorbs all the colours 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 colours of natural daylight such that objects appear the same as they do under natural daylight.
Sometimes, the reproduced colour 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 colour. This means that the colour 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 colour.
In particular, our LED lamp is lacking in red. When this light bounces off 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’ colours when illuminated under a light source.
This consideration is really important to understand when choosing your lights, especially architectural lights because in most cases colour helps tell us if something is inviting or not. For example, a supermarket will choose the highest CRI lights it can afford as it will help sell their fruit. A bright red apple is mush easier to sell than a dull looking apple.
CRI is invisible until you shine it on an object
As I mentioned above, the same light colour can have a different spectral composition. Therefore, you cannot judge a light source’s CRI by simply looking at the colour of the light. It will only become evident when you shine the light onto a variety of objects that have different colours.
What are common CRI values and what is acceptable?
For most indoor and commercial lighting applications, 80 CRI (Ra) is the general baseline for acceptable colour rendering, but still not great.
Please note that most lighting manufacturers quote their products using an averaged general R1 to R8 test colour samples known as Ra, where ‘a’ is the abbreviation for average.
For applications where colour appearance is important for the work being done inside or can contribute to improved aesthetics and feel of your space, 90 CRI (Ra) and above is a good starting point. 90 CRI (Ra) to 98 CRI (Ra) lights in this CRI range are generally considered high CRI lights.
Types of applications where 90+ CRI (Ra) might be needed for professional reasons include hospitals, Museums, Galleries, textile factories, printing facilities or paint shops.
Areas where improved aesthetics are important include luxury hotels and retail stores, residences and photography studios.
When comparing lighting products with CRI values above 90, it can be very helpful to compare the individual R values that make up the CRI score, particularly CRI R9.
So why don’t we just specify high CRI lights all the time?
The challenge is the higher CRI indexed lights use higher quality chips and consequently higher cost. This squeezes budgets especially when lighting is an afterthought in the design process and budgets have already been squeezed. In these cases lower quality mass-produced lamps are purchased instead resulting in washed-out spaces and inconsistent light colouring across your ceiling. Many mass-produced manufacturers have been known to exaggerate their CRI index performance because it is their key selling point. Unfortunately, in some projects you can actually see the same lights with different light temperatures on the same plane which is quite unsightly.
A Deeper look at R Values
As we read earlier CRI is the world’s most widely accepted metric to describe a light source’s ability to accurately reproduce colour.
The general concept involves using a set of 15 predefined colours called test colour samples (TCS) and determining how accurate a light source would make each of these colours appear.
Below are the 15 test colour samples reproduced:
Accurate” as a simplification is defined as its similarity to natural daylight or an incandescent bulb, depending on its colour temperature. Each of these TCS scores is called Ri, where R stands for Rendering Score, and i is the TCS index number. For example, the score for TCS4 (“Moderate yellowish green”) would be calculated and labelled as R4. Once each of the R values is calculated, two types of CRI, called General CRI and Extended CRI, can be calculated.
General CRI is calculated as the average value of R1 through R8. Formulaically, this is often referred to as Ra, where a is an abbreviation for “average.”
Please note that only R1 through R8 are used, and R9-R15 are NOT used in the calculation of Ra.
Extended CRI is calculated as the average value of R1 through R14. Sometimes the symbol “Re” is used, where the letter “e” represents “extended.”
Notably, extended CRI captures the influence of saturated colours such as deep red (R9) and strong blue (R12) that general CRI does not.
This is one of the criticisms of the general CRI, and it is therefore always a good idea to look at extended CRI and the specific R values when working on a project where colour quality matters.
Why is CRI R9 important?
CRI R9 is a very important metric because many light sources will be lacking in red content, but this fact will be hidden due to the averaging out of CRI calculations which do not include R9.
As the chart below shows, a light source can actually perform quite well with the first 8 test colour samples, scoring quite well for R1-R8. For the general CRI Ra metric, this means that an LED with poor red colour rendering can still get by with an 80 CRI (Ra) rating.
A closer look at the R9 value, however, reveals that the light will perform very poorly for red colours in particular.
Although the maximum possible value of R9 is also 100, unlike average CRI numbers, R9 should be judged a bit differently.
Mathematically, R9 is far more difficult to achieve a high score compared to the other R values that comprise the CRI calculations and is far more sensitive to spectral variations. Therefore, an R9 score of 50 or above would be considered “good” while an R9 score of 90 or above would be considered “excellent.”
You will therefore find that most lighting products available in the market will rarely specify the R9 value, and when they do, rarely will they guarantee anything higher than 50.
Why is red such an important colour?
Red is an important colour for many applications including photography, textiles and reproduction of human skin tones.
Many objects that do not appear red actually are a combination of colors, including red. Skin tones, for example, are very much influenced by the redness of the blood that flows right beneath our skin.
Therefore, a light that lacks red will make a person look pale, or even green. This can be problematic for medical applications where colour appearance is critical for accurate diagnoses. In other applications such as photography, aesthetic appearance is crucial and many times cannot be corrected for even in post-production and digital editing.
When searching for a high colour quality LED, be sure to inquire about the CRI as well as its R9 value.
Finally, ask the right Questions.
In the United States, the term CRI is used to refer to general CRI (R1-R8), while this is not necessarily the case in other regions of the world. In Europe, for example, CRI is typically used to describe extended CRI (R1-R14).
Depending on who you are speaking to, CRI can take on a very different meaning.
Our recommendation is to be explicit when discussing these metrics with manufacturers and customers. When discussing general CRI, it is best to use the term “CRI (Ra)” or general CRI (R1-R8). When discussing extended CRI, use the term “CRI (e)”, “Re” or extended CRI (R1-R14).
Typically, extended CRI is used less frequently than general CRI, but when in doubt, it is always best to clarif