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The best way to describe the CRI rating of a lamp, is its ability to light objects in a way that is as close to natural daylight as possible in order to create a true representation of colour. Listed below are the three defining features of good CRI;

  • Produces a great variety of colours
  • Creates contrasting shades of colour
  • The objects look naturally lit 

 

The CRI of a lamp does not always coincide with an object looking the same as how it looks in daylight. To create this you have to balance both the CRI and the correct colour temperature. For example if we look at a light source such as a candle, it has a CRI of 100 meaning it will create the differences in the chromaticity’s of 8 CIE standard colour samples (or in simple terms the eight distinctive colours in the light spectrum).

Even though a candle flame has the ability to show the differences, its colour temperature is very low (1700ºK). This means it struggles to show the blue colours in objects giving a less realistic light. A colour temperature that is going to match natural daylight best is between 5000ºK-6000ºK with a CRI of over 95.

Even though CRI is usually measured between 1-100 there are some instances where the CRI comes out as a negative figure, for example low-pressure Sodium lighting. If we are looking at lamps that are more likely to be used around the house, a typical residential LED fitting or bulb usually has a CRI of 80+. When testing for the CRI, the lamps are split into various groups called Correlated colour Temperatures (CCT) to be tested at different temperatures. The groups are split into three groups depended on the temperature range up to 2700ºK, 2700ºK – 6000ºk and over 6000ºK. If this is under 2700ºK a Blackbody radiator is used in place of natural daylight as the control.

 

Hopefully this post has increased you knowledge a bit more in lighting terminology, more helpful posts to follow…