Encyclopedia

Often overemphasized in the context of horticultural lighting, CRI stands for color rendering index. This measurement quantifies a light’s ability to illuminate plants or any other object reliably without color distortions compared to a natural and/or visually-ideal light source. Typically this is a scale from 1-100, with 100 being closest to “standardized daylight.” A light source with a CRI of 30 would be considered a heavily distorted illumination. For example, ceramic metal halide lights traditionally have very high CRI (typically 90+). A classic Tri-Band LED that favors heavy reds and blues (purple lights) has very poor CRI ratings that are often below 50. Color Rendering Index does not really affect your plant growth. Still, you should be aware that you may find identifying plant health issues like toxicities and deficiencies under the LED difficult because you won’t be able to see the ‘true’ colors of the symptoms. HPS lamps are another popular light source that traditionally has a poor CRI. Some simple solutions for working with lights that have poor CRI would be to use color-correcting grow room glasses or add a standard light source to use when you’re inspecting the plants for problems.

Many lamps generate light by means other than a filament, which creates light with slightly different. In this case, the actual metric used is CCT or correlated color temperature. A common example of CCT is seen when describing fluorescent T5 bulbs, but this has been used to describe the visual spectrum of high pressure sodium lights and LED’s as well. Lamps marketed as 4000K and lower will emit more red light, lamps which will be good for flowering plants and lights rated 7000K and above emit more of a blue spectrum which will be good for seedlings and vegetative growth. Lamps with a Kelvin between 5500-6500 will emit the closest spectrum to natural daylight. It’s important to note that color temperature, CCT and Kelvin only refer to the visual spectrum emitted from a lamp.