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How is the Color of Light Measured?

A simple way to describe the color characteristics of light is to use three descriptive categories: Warm (yellowish), Neutral and Cool (bluish). But on LED fixture Specification Sheets, the color of light is measured in degrees of Kelvin (°K).
Where did this Kelvin system come from?

A measuring system was needed for light and the measuring standard chosen was built around the temperature of a metal object.

Everyone has seen the color of a steel bar change as it is heated in a forge. The hotter it gets, the brighter the color of the bar. As it gets hotter, the glowing color will change from deep reds, to oranges & yellows, all the way up to white hot.

So, a system for light color measurement was developed that was based on the temperature of a metal object, and the different colors of light that were emitted as this metal got hotter.
Rather than using Celsius (C) or Fahrenheit (F) temperature scales, a new temperature scale was implemented. It was called the Kelvin (K) scale, which started at the coldest temperature that can be achieved, absolute zero, and went up from there.

The Kelvin scale was named in honor of William Thompson , Lord Kelvin, who determined the temperature of absolute zero.

Here are some examples of the color of light
as measured by the Kelvin Scale:

Less than 2000K:
gives off a dim glow of light, similar to what you might find from candlelight.
2000K-3000K:
gives off a soft white glow, often yellow in appearance.
3500K:
Warm white.
4000-4500K:
gives off a Neutral White light.
5000K:
Cool White.
6500K:
Blue-white light.
6500K and up:
gives off a bright bluish hue of light.
Bill Nagengast, Lighting Engineer
Solas Ray Lighting
Holds over 20 patents in the lighting industry.
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TJ-47 10.18.18