Kelvin is one way to measure white light spectral output, but it isn’t technically a measurement of light energy at all. Much like Fahrenheit and Celsius, this is a metric of temperature. Over the years, Kelvin has been adopted by the lighting industry to describe what is referred to as the “color temperature” or hue of a white light source. Color Temperature is based on the rule that a filament or radiator will emit different colors of light depending on the temperature of said filament. The hue shifts from red at lower temperatures to orange, then yellow, then blue.