CRI stands for Colour Rendering Index and it measures how closely a light's ability to illuminate an object matches natural/ideal daylight. The higher the CRI rating of a light, the more accurately it reproduces the colour of an object in daylight.
As with many concepts, marketers are generally correct in claiming that higher CRI lamps generally produce nicer looking colours, at least where general consumer lighting is concerned. However, this does not mean that low CRI lamps produce unattractive lighting.
There is a big difference between colour accuracy and vibrancy. When it comes to CRI, most people have the misconception that a higher CRI means more accurate colour rendering.
What the CRI actually measures is how accurate a light's colour rendering is compared to a black body radiator of the light's stated K rating. The sun is rated at around 5780K; the stars are near perfect black body radiators, so they are a good comparison. If you have a 5780K rated fluorescent lamp, in order for it to score 100 on the CRI scale, it needs to illuminate an object 100% visually similar to our 5780K Sun.
However, if instead you have an incandescent lamp rated at, say, 2700K, then in order for it to score 100 on the CRI scale, it needs to match the illumination standards of a 2700K black body lamp. Using stars as an example, this means illuminating an object with a red dwarf star that burns at 2700K and then comparing how visually similar it is to the incandescent bulb. If the similarity is 100%, then the 2700K bulb will score 100 on the CRI scale.
A lamp that has a CRI of 100 and is rated at 5780K will illuminate objects very differently to a lamp that has a CRI of 100 and is rated at 2700K. To illustrate this, remember when incandescent light bulbs were still in use?
A standard incandescent bulb has a K rating of around 2700K. Incandescent bulbs are blackbody radiators, similar to stars. Because of this technical definition, all incandescent light bulbs have a default CRI of 100 - yet they render objects in a visually (unattractive) yellowish tone.
The CRI index is therefore not particularly useful for hobbyist aquarium applications. There are many lamps that give attractive colour renditions and are great for growing plants, but may have a low CRI rating. Similarly, a high CRI lamp may actually be visually unappealing; because of the way it is calculated, it has little value for aquarium lighting purposes.
Aquarists are generally more concerned with having vibrant colours with high colour saturation. Many lamps can have a low CRI but produce very attractive visual colouration; my own LED fixture below only has a CRI of 78 as the overall spectrum is red/blue heavy. However, this allows it to produce more vivid reds and purples. Many others, including myself, find the colour rendering attractive.
The Fluval Plant 4.0 is marketed as a "Full spectrum" fixture with 90 CRI. However, it renders colors poorly compared to other fixtures such as the Twinstar S V that has a much better spectrum profile. The same aquascape was photographed with the same camera settings to show the difference.
Choose a light that makes the tank look the way you want it to. To make the colours of fish and plants pop more, mixing in the red/blue spectrum makes a huge difference. To get a really accurate sense of colour rendition you need to see the tank/light in person, and best of all with side-by-side comparisons with other light systems.
Click here to learn more about aquarium lighting for planted tanks
Click here to learn more about PAR values.
Click here to learn more about spectrum curves.
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