Of course, when working on a lighting design, you take your time when it comes to choosing light sources. It’s a choice that is not always obvious. You should of course take into account parameters such as power usage (Watts), energy consumption (kWh), light output (Lumens) and lamp life performance. But do you also take into account the correlated color temperature (CCT) or the color rendering index (CRI)?

Color temperature, expressed in degrees Kelvin, indicates whether your light source will emit rather "hot" or "cold" light and is a choice that best depends on the functionality of the room. A space in which someone will primarily perform functional tasks rather demands a higher color temperature resembling daylight (4000-5000 K). If lighting in a room revolves all around creating a specific atmosphere, you could opt for a lower color temperature (2000-3000K) or 'warmer' light. LED lights come in a very wide range of colors.

The color rendering index of a lamp, CRI in short, is an often forgotten concept that is somewhat more complex, but at least as important.

What is CRI / Color Rendering Index?

The CRI or Ra value of a lamp is a metric between 0 and 100 which indicates how accurate this light source displays the colors of an object. Also referred to as 'color accuracy'. The higher the CRI of a lamp, the more natural a color will look in this light.

If you can’t see the difference between a black and a dark blue object under a particular lamp, then this lamp might have a lower CRI. Not all light sources show colors in the same way. In the lighting industry color rendering index is a universal benchmark to distinguish between the vibrancy, naturalness and saturation of colors in certain lighting.

Lamps with a CRI higher than 80 are considered "more than acceptable" for just about all applications. Lamps with a CRI higher than 90 are known as "high CRI lighting”.

CRI does not say everything: the shortcomings of CRI

The way we measure CRI has remained the same for 50 years and is still the standard among traditional lighting manufacturers. But the way in which CRI is being measured is far from perfect.

Each lighting product is subjected to a CRI test, assigning a score (Ra) between 1 and 100, with 100 being the light of the reference lamp, a light bulb. Both the test light and the reference light are aimed onto 8 color samples. With each color the test light is assigned a score according to the similarity to the color sample being illuminated by the reference light. The average of these 8 scores is considered the CRI of this lamp.

The limitations of this test method have led to 2 major misconceptions:

1. Light sources with the same CRI show the same result. 
   You do not know which color samples show the biggest deviation.
2. A light source with a higher CRI, by definition, produces a more beautiful result than a light source with a lower CRI.
   It is not exceptional that a lamp with a CRI of 70 creates a much more enjoyable effect than one with a CRI of 85.

LED lighting became a game changer

By 2015, scientists from the lighting industry revolutionized this testing method and TM-30-15 ('TM' stands for testing method) quickly became a popular way of measuring lighting quality. However, keep in mind that TM-30-15 is not yet the new standard, it’s only a first major step in the right direction.

The shortcomings of the old CRI system have only been emphasized in recent years, as LEDs have experienced great advances. LED technology quickly reached CRI scores of 80 or higher. But in recent years, after seeing big improvements of LED light quality, we did not see any increase in CRI. This only supports the assumption that the old testing method is obsolete.

The update to TM-30-15 was done through 2 major changes:

1. The number of color samples used in the test was increased from 8 to 99.
   (99 is already a big improvement, but in the future this may become even more accurate)
2. In addition to an improved “color fidelity" (Rf), the similarity of color under the test light versus the reference light, ‘color gamut’ (Rg) was introduced as a new testing criterion: an index for the saturation or intensity of the displayed colors.

The direct result of an improved test method like TM-30-15, which is not simplified to a single metric, is that it provides us with much more information and enables lighting designers to make better choices for the customer.