Colour rendering is the ability of a light source to accurately display the colour of an object. The most commonly used measure of colour rendering is the Colour Rendering Index (CRI).
Sunlight reveals the true colour of an object and therefore has a perfect CRI of 100. This is the natural light standard against which all other light sources are compared when calculating CRI. The colour of the clothes in the shop, the food on the kitchen counter and even your skin will all look different depending on the light that illuminates it.
The difference between CCT and CRI
CRI should not be confused with CCT, which is often referred to as colour temperature. While the colour temperature can change even with sunlight throughout the day, the CRI does not.
The CCT of sunlight can change throughout the day and in different weather conditions. You can also see how the various light sources compare.
An insight into CRI in commercial applications
A CRI of 90 means that an artificial light source is reproducing approximately 90% of the visible colour spectrum that the sun produces in the same colour.
Commercial applications that may require 90+ CRI lighting include art galleries, museums and medical facilities.
In the home, lighting with a high CRI can be used in areas where people wear make-up or for hobbies where colour is important, such as fly knots and needlepoint. Artwork and food will also look better with high CRI lighting.
Compare the colours under different types of bulbs.
Light sources with a CRI below 80 are usually only used in areas where people are in transit or on short term assignments, such as logistic corridors or storage rooms.
How is CRI measured?
The traditional CRI indicator is based on a set of eight reference colours.The formal measurement of CRI is done in a lighting laboratory, usually as part of a larger process of product testing and certification.
In simple terms, the light source under test is aimed at the reference colours and its colour rendering properties are measured using a spectrophotometer. Each of the eight results is then compared to a reference (the reference colour measured in sunlight).
For each of the eight colours, the difference in performance between the reference and test light sources is calculated and given as a percentage. These eight values are then averaged to give an overall CRI figure.
Where the CRI is not known, the easiest way to assess the colour rendering performance of a light source is to use a set of reference colours or a rainbow of colour samples printed with good colour accuracy. Check the colour of the sample under the light source you are evaluating.
Do any of the colours on the sample look dull or pale? If so, the colour rendering performance at these wavelengths may be low. If all the colours are bright, clear and true, the light source is likely to have a reasonable CRI.
Light sources and CRI
Incandescent and halogen light sources, although inefficient in terms of energy use, both exhibit very close to true colours. Light from fluorescent and other discharge lamps consists of a limited range of wavelengths, so these sources usually do not have a large CRI.
Early LED lighting did not perform well in rendering certain colours, particularly red tones which may appear grey or muddy. Red tones are not well represented in standard CRI colour tests, so sources with poor red rendering may still have a high CRI; and as the CRI is an average of eight values, there may be some high and some low CRI values overall.
Current LED lighting, particularly commercial grade LED lighting, typically has a high CRI.
What CRI can't do
- CRI cannot tell us the brightness of a light source.
- CRI cannot tell us the colour temperature. For example, in two light sources with the same colour temperature of 5000K, one will show the colour accurately, while the other will make the subject look dull and faded.
- The CRI cannot tell us the spectral distribution of the light source, i.e. which wavelengths are mixed together to make up the colour of the light we see. Because of the limited number of reference colours in the CRI measurement, it can only give us an idea, but not a complete picture.
It is well known that the eight-colour CRI indicator is limited. Therefore, in recent years, an extended CRI with fifteen reference colours has been used.
However, the more modern standard TM-30-15 is replacing the CRI as the preferred method for assessing the colour performance of light sources.TM-30-15 has a complete set of 99 reference colours. The new standard also includes the use of the gamut index to quantify colour saturation, a useful metric that is used more frequently when describing the quality of light colour.
The standard CRI scale measures the colour accuracy of only 8 colours, while the extended scale measures 15 colours. The new TM-30-15 measures over 99 colours.
Practical applications for CRI
In any given space, the CRI of a light source can have a profound effect on the look of paint, the look of furniture and the look and feel of people.
Choosing lighting with a high CRI can be very beneficial, for example in a retail shop choosing high CRI luminaires can brighten up your merchandise and make customers more likely to buy.
In clothing shops, choosing high CRI luminaires will give the best reproduction of the fabric quality of the garment.
Choose high CRI luminaires in a restaurant to make the food more tasty.
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