Light Thoughts

Equally important as the correlated color temperature for fluorescent lamps, the color rendering index (CRI) is used to determine how things are going to look under fluorescent lighting. The CRI is a made-up scale ranging from 0 to 100, where 100 is the expected color rendition of incandescent/halogen sources

The colors of objects that you see are actually those colors being reflected back at you. If you start with white light shining on “red” object, the blue and green parts of the white light are absorbed by the object and the red is reflected back for you to see. If instead of a “white” light that has a good balance of red, blue, and green, you have a “white” light that is weak in the red part, the blue and green still get absorbed by the object, but there is less red to be reflected back at your eye. The same object is going to look less “red” under the second “white” light.

Since an incandescent source is like a black-body radiator, the color spectrum of of the light generated by the filament has a decent mix of all the visible colors. However, since the colors present in the “white” light of a fluorescent is made up by phosphors which each do just a part, there can be a poor mix of colors in the light. The better the phosphors do at creating that mix of colors the better the CRI rating.

The more important color is, the higher the CRI of your lamp should be. Manufacturers will make similar lamps (in terms of size, power, etc.) with different CRI ratings since the lower CRI lamps can be made more cheaply. People will either select the lower CRI lamps because color doesn’t matter as much as cost or because they don’t know that there is a difference. For example, for linear tube fluorescents, like you find in offices, the cheaper lamps will have CRI ratings in the low to mid 70s and the higher quality lamps will have CRI rating in the low to mid 80s.

For your home or office I recommend getting the lamp with the highest CRI you can find. For standard fluorescents I only specify lamps with a minium CRI of 85. For compact fluorescents I have to drop my minimum to a CRI of 82, since higher CRI rated lamps aren’t readily available.

Note that both the CRI and the CCT of a fluorescent lamp will be printed on the lamp itself, so if you need to find out what something is you can just look at it. For a linear tube style, the information will be printed on the glass at one of the ends. For a compact fluorescent lamp the information will be printed on the base. Most of the time the information is just printed as “82 CRI” and “3500K.” Sometimes only the manufacturer’s coding will be printed, but typically it will show the CRI by either a “7″ or “8″ somewhere in the lamp code (for low to mid 70’s or low to mid 80’s as appropriate) and the CCT as the first two digits of the CCT (such as 27 for 2700K or 35 for 3500K).

To see an example of what happens when you use a lamp with a low CRI rating most people can probably just find a nearby streetlight. Many streetlights are still high-pressure sodium because HPS lamps are very energy efficient. However, they also have a CRI around 22 and a CCT of around 1800K. That’s why the “orange glow” of a streetlight makes everything look orange or gray.

If you have streetlight around you that look more “white” than orange they are either very old mercury lamps (which are now banned from new installation but utility companies who stockpiled them can use up their inventory until they replace the fixture) or metal halide lamps. They probably have a CCT of around 4000K and a CRI ranging from about 50 (mercury vapor) to 65 to 75 (metal halide). There are also new ceramic metal halide lamps that can have a CRI as high as the low to mid 90’s.

Using streetlights doesn’t provide a perfect example since the darkness of night also affects how we perceive colors, but it’s a good shorthand.