One property associated with lighting that everyone is aware of, but probably not familiar with, is Corelated Color Temperature, or CCT. CCT is simply the color of the light that is emitted from a source. A low CCT incorporates more warm colors; reds, oranges, and yellow, while a higher CCT will be more white or blue. While there are variations in all light sources, some common examples are:
High Pressure Sodium 2,200K
Standard incandescent 2,700K
Standard Metal Halide 4,000K
Cool White fluorescent 4,200K
Daylight Metal Halide 5,500K
Interestingly, the CCT of daylight varies throughout the day. At sunset and sunrise, the CCT is very warm, about 1800K. At high noon on a clear day, the CCT can be 10,000K+.
In the early years of LED, manufacturers sought to maximize the efficacy of the LEDs. In order to accomplish that, they typically used a 5,000 degree kelvin color temperature LED, which is a very white/blue color. As the technology developed, manufacturers were able to incorporate other color temperatures in their luminaries, while still maintaining a very high level of efficacy. Today most standard luminaries are available 3,000K, 3,500K, and 4,000K color temperatures, and can typically be special ordered for 2,700K and 5,000K.
Another important aspect of the quality of light is CRI, or Color Rendering Index. This is a measure of how accurately a light source can reveal the color of various objects as compared to natural light. Sunlight is considered the perfect light source, and thus is assigned a CRI of 100. The CRI of some typical light sources:
High Pressure Sodium 24
Cool White Fluorescent 64
Standard Metal Halide 85
Standard Incandescent 100
As we can see, there is a large amount of variation in CRI of common light sources. What this means is that objects lit with High Pressure Sodium light source will look bland and dull, with no discernable difference between browns and reds, or greens and yellows. Cool White Fluorescent will reveal much more variation in colors as compared to High Pressure Sodium, and Standard Metal Halide more than Cool White Fluorescent. Generally, an acceptable CRI for most environments is 85. The current standard in most commercial and residential LED luminaires and lamps is 80-85 CRI. However, some situations, like an art gallery, or medical application, may demand a higher CRI.
In the past, a requirement for a certain color temperature or CRI value may have meant changing light sources. But thanks to the wide variety of LED offering, today all of those options are available to us with a single light source; LED.