CRI or Colour Rendering Index, is probably the most misunderstood and misquoted term used by lighting engineers, lighting designers and manufacturers to specify the quality of light sources.
Traditionally, it is derived by comparing the reflected light using a standard CRI test chart of two dozen specially selected colours commonly found in every day scenarios.
CRI Test Chart
They include typical Caucasian Skin Tones, Japanese Skin Tones, Foliage, Jeans etc. Lighting engineers and lighting designers sometimes will use a specific CRI value to ensure their subject is accurately lit. For example, a high Caucasian Skin Tone CRI might be more important for photographers, while a high blue CRI might be more important to a retail shop selling Jeans. The overall CRI number is an average of all individual CRI measurements.
Specific CRI values for a low CRI light source
If a light source only has a single pure colour like blue (also known as a monochromatic light source) then it cannot accurately render other colours. Therefore, a light source with less colours will not accurately render a scene, and thus have a lower CRI.
Blue sky with clouds and sun does not have perfect CRI
It is often misunderstood that a CRI of 100 represents the how close the light source is to Sun light. However, sun light is made up by a combination of direct light from the sun and scattered light within the atmosphere. Depending, on the angle of the sun in the sky this will also change throughout the day. Moreover, the light will vary depending on the time of year and the latitude on the earth. Alas, the sun is not a reliable standard for comparing light sources.
Lower CRI lighting generally have a less saturated and unflattering scenes
Typically, a higher value on the scale is better. The CRI scale is out of 100, where 100 represents the a standard Incandescent or Halogen light source. These light sources represents perfect black body radiation as the light colour temperature (also known as CCT) is the actual temperature of the molten metal filaments inside those light sources.
The majority of light sources struggle to produce white light with a wide gamut of colours. Therefore, a lamp with 80 CRI will often be at a lower price point and have slightly higher brightness than a higher CRI lamp.
Lighting designers would often specify a light source with at least a CRI value of 80, and some times as high as 90. Innerscene Virtual Sun Model A7 has a CRI in excess of 90. These quotients would mean that the subject being lit will typically be colour accurate. However, there are some scenarios where another light source with lower CRI is preferred.
Sodium Street Light with very poor CRI but sometimes preferred over higher CRI LEDs
Take Sodium street light for example, they have a very high efficacy and has been used for decades across the world. Recently, many street lights have been retrofitted to LED with cool white rather than yellowish tone. However, there has been some push back from local residents who prefer the more traditional yellowish light sources with much lower CRI. Certain task lights are also monochromatic in nature such as the dimmed down red light used in submarines. These are critical in helping submariners adjust to the low lighting levels as is often used in other dark spaces such as planetariums, movie theatres, airport control towers etc. Recently, there has also been specialist LED light sources which accentuate certain colours above others. For example, a Vivid LED light source may have a higher red element, which accentuate skin tones and helps retail stores improve the look of customers while trying out new clothes. These Vivid LED light sources often have a lower CRI of 80 compared to typical higher end CRI of 90.
Author: Thanh Tran, Senior Engineer at Innerscene Ltd. Tan Tran | LinkedIn