Colour Rendering Index (often referred to as CRI) is a commonly used but rarely understood concept that is basically a way of measuring the quality of an artificial light source in comparison to a natural one. We say basically, it does get a little bit more complicated than that. Though it isn’t that widely used in the hobby aquarium industry, its popularity is growing due to the rise of LED aquarium lighting. As an aquarium keeper, it’s important to understand the basis of CRI, how it is measured, and what it means when it comes to aquarium lighting and your tank in general.

Achieving the Right Colour Temperature
Assume, if you will, that in an ideal world, coral systems and plant life look best under their natural light source, as it is in these conditions that they develop and grow. It’s also generally understood that most tank keepers want their aquariums to resemble the authentic, natural environment as closely as possible Obviously, with the dreary state of British weather, they’re never going to be getting this type of light naturally, so you need to make sure your aquarium lighting is doing the right job. Long story short, the light that corals receive in natural ecosystems is pretty much the exact right colour temperature for a healthy and visually appealing reef; a product’s Colour Referencing Index indicates just how accurately it can imitate this colour temperature.
A Numbers Game
Now widely used both within and outside of aquarium keeping, a light’s Colour Referencing Index is measured on a scale of 1-100, with light sources that emit blackbody radiation coming in at around 100. Although the Commission on Illumination (CIE) who devised this test stress that it’s largely relative, and that each artificial light source must be measured against its desired natural light, specialists tend to opt for a “higher the better” rule when it comes to CRI. For the purposes of tank keeping, you need to make sure that your aquarium lighting systems have a CRI of approximately 90 or above. This then indicates that that artificial light source will render the colours of your corals properly and vividly.
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