Today’s Question: As I understand it, the color temperature for light is rated by the Kelvin scale. The higher the number, the cooler or more blue. The lower the temperature, the warmer or more red. Yet we photographers with Lightroom and Photoshop seem to be using the Kelvin scale in reverse. Lowering the Kelvin temperature makes the image cooler and more blue, whereas increasing the temperature warms the image. What am I not understanding about how the Kelvin scale is applied to color and light temperature?
Tim’s Quick Answer: When it comes to the Temperature slider in most imaging software, the reason the adjustment seems backwards is that you are choosing a color temperature to compensate for, not choosing what color temperature the overall color in the image should look like.
More Detail: First off, your overall understanding of the Kelvin scale is correct. A low Kelvin value (temperature) reflects a warmer color tone (more yellow, for example). A higher value reflects a cooler color tone (more blue, for example). This scale is based on the behavior of a theoretical “black body” that radiates heat in the form of visible light as it is heated up.
Of course, it is worth noting that what is really backwards here is our definition of color values in what I refer to as a “psychological” way. We refer to colors in the red and yellow range as being “warm” colors, and we refer to colors in the blue range as being “cool” colors. But in reality, hotter objects emit what we would refer to as cooler colors. So our psychology of colors could be described as “backwards” to some extent.
When referring to the color temperature of visible light, the Kelvin scale is often used to describe the color of that light. So a warmer (or more yellow) color would have a relatively low Kelvin value, while a cooler (or more blue) color would have a relatively high Kelvin value.
In most imaging software, reducing the color temperature value cools off the image, while increasing that value warms up the image. This certainly seems backwards based on the actual Kelvin scale for color temperature.
However, the color temperature adjustment is actually allowing us to define what color temperature we want to compensate for. So, if the light illuminating a scene was very blue, we would shift the color temperature adjustment toward blue. In essence, we are saying, “The light was very blue, so please shift the image toward yellow to compensate”.
In other words, when you adjust the color temperature Kelvin value up or down, you’re really attempting to define what color the light actually was, so that an appropriate compensation can be applied.
So, if you think of the white balance adjustment as enabling you to define what color temperature the light was in order to produce appropriate neutral values in the image, I think the slider will make more sense.