The Spectrum

The Color Temperature gives a coarse description of the color of a light source by identifying the temperature of an incandescent light whose color is “closest”. But, “closest” doesn’t necessarily mean close. The strong, distinctive yellow glow a 2000 K HPS lamp doesn’t feel anything like the golden-amber of a 2000 K candle. To better distinguish the “quality” of different lights of the same color temperature we need to dig a bit deeper. 

Pass light through a prism and you’ll get a rainbow: red to orange to yellow, green, blue, indigo, and violet. The intensity of the different colors in this continuum is another way of describing the character of the light. Scientist and engineers call the intensity of light at the different rainbow colors the light’s spectrum. Water droplets in the air act like prisms to create a rainbow. The intensity of the light along the rainbow is sunlight’s spectrum. 

Sunlight is incandescent light with a temperature of 5700 K. 

A HPS Lamp, A Candle, and An LED lamp

A 2000 K HPS lamp looks very different than a 2000 K candle, or a 2000 K LED street lamp. We can see why this is by looking at the spectrum of each. 

The spectrum of the HPS lamp is very different than that of the candle of the same color temperature. The HPS lamp may be “closest” in appearance to the candle, but that doesn’t mean it is close.

The spectrum of the LED lamp, on the other hand, is very similar to the spectrum of the incandescent candle. The color of the light very similar to that of the candle.

If you compare the 2000 K LED or candle to the 5700 K sunlight you’ll see that sunlight has a lot more light toward the blue-indigo-violet end of the spectrum than does the 2000 K candlelight or 2000 K LED. That greater contribution of blue-indigo-violet light, relative to red-orange-yellow light, is what makes noonday sunlight look and feel differently than candle light. 

What’s my Take-Away?

The color temperature of a light tells us about the “color” of the light by identifying the incandescent lamp that is “closest” in appearance. But, “closest” does not mean close. An HPS lamp has the same “color temperature” as a candle, but would never be mistaken for candlelight. Beyond “color”, lights whose spectra are similar, will look similar. And, lights whose spectra are very different, will look very different.