White Balance & Color Temperature

One of the most important controls on your digital camera is the White Balance setting. This setting helps us keep skin tones looking natural and keeps a white piece of paper white.  Essentially it digitally adjusts the overall color of an image to compensate for light sources that may not be white light.  For example, if you photograph someone in open shade, the light that falls upon your subject tends to be quite blue. You use the Shade white balance setting to compensate for this blue light.  This shifts all color away from the blue and toward the amber - again, making the white piece of paper white. 

White Balance controls help keep whites white and skin tones natural.

You don’t need a deep understanding of color theory to know how to use the white balance control. But, it may be helpful if we lift the veil ever so slightly get a better understanding on what is really happening. 

The retina of our eye has two different types of photoreceptors, rods and cones.  The rods are by far more numerous with the average person having about 120 million of them compared to only 6 million cones. Rods are more sensitive than cones but cones do have one major advantage. Cones are sensitive to color - rods are not. 

CIE Chromaticity Diagram

Turns out, there are three types of cones and researchers have been able to successfully model the human perception of visible colors. The model is mapped out on what is called a CIE chromaticity diagram. Look familiar?  This is the graphic representation of the total human visible gamut. You’ve probably seen this chart with some graphic shape overlaid on top. For example, the sRGB color space only uses a fraction of this total scale.  

Now, let’s get back to White Balance. When we set the white balance control on our camera or adjust white balance in Photoshop or Lightroom, you may notice a temperature measurement.  That’s because white balance colors are measured in degrees Kelvin (K).   By definition, white balance color temperature is the temperature, measured in degrees Kelvin, of an “ideal black-body radiator that radiates light of comparable hue to that of the light source.” Bottom line is that this is a theoretical measurement of a theoretical thing but the concept can be seen easily enough in the coil on an electric stove. As it heats up the black coil begins to glow red. If you could get it hot enough it would even change color from red, through white and then blue. 

Planckian Locus

Now you’d think that any color could be defined by color temperature but that’s not the case. In physics there’s something called Planckian Locus which defines the path along which the color of our so called “ideal black body” would travel when heated. As you can see by the overlay on the second chart, the lower temperatures start in the red/amber portion of the chart and arc over to the blues.  It passes through white at about 5,500 degrees Kelvin. Sound familiar - that’s the color temperature for bright daylight. 

If you’ve ever made adjustments to white balance in Photoshop or Lightroom you may have noticed that there are two controls, Temp and Tint. The first adjusts your image along the temperature scale - the Planckian Locus line. But for images that don’t fall right on that line, you have a Tint slider too. Look on the chart at the colors that are above and below the Planckian Locus line and compare those on the tint slider.  Voila - now you know what that control really does. Tint adjusts your image above and below this line. But remember that the tint adjustment is subtle. It does not allow you to move that far above or below the temperature line.  The fact is that if you had a bright green light or a purple light illuminating your subject, you just can’t correct that with white balance. 

Color theory is one of those areas in photography that is deep in science and anchored by the rigid laws of physics. But don’t forget that we are also dealing with human perception. When we talk about warm and cool colors, we are talking about our emotional reaction to those colors and not color temperature. In coming issues of our blog we will discuss many topics about color. Even if you create black and white images it’s important to understand color.