Kelvin Warm versus Cool


Today’s Question: [Following up on a question about the target color value for monitor calibration in the November 3rd edition of the Ask Tim Grey eNewsletter,] isn’t a higher Kelvin number warmer (more red) versus a lower number cooler (more blue)?

Tim’s Quick Answer: Actually, a lower Kelvin value does indeed relate to what we think of as a “warmer” color (more red), and a higher Kelvin value relates to what we think of as a “cooler” color (more blue).

More Detail: Our psychology of color is essentially reversed relative to the Kelvin scale. At the lower end of the Kelvin scale are “warm” colors such as red, orange, and yellow. At the higher end of the scale are “cool” colors such as blue.

The Kelvin scale is based on a theoretical black body radiator. In other words, when you heat something up, the color of light emitted as it glows with heat relates to a value on the Kelvin scale.

As you heat up an object, it will glow first with a “warm” color, such as orange. As it gets hotter and hotter, it will eventually glow with a “cool” blue light, until becoming “white hot”.

So, the Kelvin scale goes from “warm” colors to “cool” colors based on our psychology of color, but those color values actually relate to the color temperature of light given off by an object as it is heated.

Adding further confusion, the Temperature scale also seems “backwards”. You drag the slider to the right for a higher Kelvin value, but that also results in a warmer appearance in the image. But this is because this slider value relates to the color temperature you want to compensate for. So a high value represents a “cool” color, and the software compensates for that cool light by warming up the image.