Q: Just as I understand the concept of color temperature (Kelvin), now CRI is being bandied about as LEDs come into the market. What is it exactly?
A: I am right there with you. From the beginning of my career, the terms color temperature and color rendering index (CRI) have always had an air of mystery.
I thought I just had to accept them, like daylight savings time and no-host bars.
Color temperature is a description of the warmth or coolness of a light source. When a piece of metal (often mysteriously referred to as a black body radiator) is heated, the color of light it emits will change. This color begins as red in appearance and then slowly turns to orange, yellow, white, and then blue-white to deeper colors of blue. The temperature of this metal is measured in degrees Kelvin. What’s confusing is that higher Kelvin temperatures are cool and lower temperatures are warm; directly opposite to the temperature in an oven. Color temperature is not an indicator of physical heat. Cool light is preferred for visual tasks because it produces higher contrast than warm light. Warm light is preferred for living spaces because it is more flattering to skin tones.
CRI is a measurement of a light source’s accuracy in rendering different colors when compared to a reference light source with the same correlated color temperature. The closer a light source is to a score of 100, the better its color rendering. The higher the CRI, the better the visual perception of colors. What’s confusing to me is that an incandescent lamp gets a score of 100, even though it shifts colors into the yellow range. When it comes to alternative light sources, the basic premise is that an LED or CFL alternative with a high CRI is close in its rendering of colors when compared to the lamp it is replacing. Now my head hurts.
Related Question
Q: How important is color temperature in ambient lighting? Should this be a concern if you are using fluorescents or LEDs for your ambient lighting?
A: Frankly, I think color temperature is important in all aspects of lighting. As the general public is inching its way toward the use of dimmable fluorescents and LEDs, we do want to look at the color temperature and the CRI (color rendering index). When houses were done with all incandescent light, it was very simple. All of the illumination was the same color and when dimmed, they all became warmer in color. If the house is done in all fluorescents or LEDs, then the coloration and dimming will be constant. It's when you start to mix light sources that you have more inconsistencies … like when you mix cocktails.
When providing indirect illumination, the color temperature of the fluorescents or LEDs should be close in color to those of the other light sources in the space. I personally use dimmable LEDs or fluorescents that are the color of dimmed incandescent (2200K to 2400K) so that when I dim the incandescent lights, they will all feel closer in color temperature at the lower light levels. Now I'm changing out my traditional halogen MR16 lamps with LED versions, but I still add a “warming filter” in front of the lamp to add an amber hue. For closets, garages and laundry rooms, though, I do like my lamps in the 4000K to 5000K range for color matching.
