Color Temperature: The distortion / response of all colors when viewed under a particular light source.
One can achieve a good painting utilizing meaningful tones and exclude color temperature; however, one cannot achieve effective color temperature in the absence of good, solid tonal construction.
Artists classify light sources as either warm or cool. Sunlight and most incandescent light are considered warm; light from the sky and most fluorescent light are considered cool. All shadows assume an opposite temperature from that of the light source. Therefore, warm light produces cool shadows, and cool light produces warm shadows.
This approach, however, is highly simplified and does not attempt to relate the attendant phenomenon found with all light: alteration of color temperature. Strong light exerts the least amount of color distortion. Middle tones in warm light grow darker, color intensities grow more neutralized and, where there is less warm light, colors will seem to grow cooler.
In the diffusion edge, where light meets shadow, there is a prismatic color reversal. For example, in sunlight the diffusion shadow edge takes on the temperature of the warm light source. Graduating from the diffusion edge deeper into the actual shadow the lighter parts of the shadow (excluding any reflected or secondary light) will be perceived to be cool. The darker more protected parts of this "cool" shadow will be the warmest in tone. With a cool light source the reverse of these will be true.
Written and contributed by David Hardy, Oakland, CA., ed. Georgia Griffin. Further Reference: "Taking the Temperature" by David Hardy, American Artist Magazine, August 2002
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A more technical essay on the definition of Color Temperature by Bill Indursky:
Color Temperature: For the painter, the term color temperature is used to identify RELATIVE warmth and coolness in the color/light spectrum as it appears in the real world and is represented by paint.
For example, yellow pigment is usually considered a warm color but it can have relative levels of warmth and coolness. This same principle works for LIGHT and how we see it. Warm light can have relative warmth and coolness even if the light source is warm or cool. Simple rules like a warm light source = cold shadows is over simplistic and will not let the natural or realist painter to represent the real world situation accurately. The color of light we see depends on amount/strength/distance of light, color/saturation/temperature (warm or cool in the spectrum) of light, and the type/color/reflectivity/texture of the surface it falls on.
A real life situation can be broken down as follows:
Let's start with a theoretically NEUTRAL white light. It is not warm or cool but perfectly balanced, casting onto a 50% non-reflective non-textured material gray ball sitting on a 50% non-reflective non-textured material gray floor with the light at an angle to cast a shadow (fig. 1b) The only variable in this case is the AMOUNT/STRENGTH/DISTANCE of the light and the TYPE/COLOR/REFLECTIVITY/TEXTURE of the surface. The more light the more bright/white. The less light the more dark/black. (fig. 1a) This is what artist usually refer to as VALUE.
Now consider, not just the AMOUNT/STRENGTH/DISTANCE of light but the COLOR/SATURATION/TEMPERATURE of light. In this next example we take a warmer light source (in our case Yellowier) and exchange it for the white neutral one. We will keep the same gray ball on the gray floor. You will have to consider not only the AMOUNT of light, the surface COLOR but, the SATURATION/COLOR of light and the result when it "mixes" with the black/grays of shadow and surface color (fig. 2a & fig 2b).
Adding more complexity is the hard fact that the nature of light is different from the nature of pigment/paint. Color for both is achieved in different ways. Since painters do not paint in light but pigment this adds another level of challenge. It is like trying to represent apple with oranges. They are different. As a painter we can only try for the "illusion of light and observable color". Most realist or naturalist painters use a combination of pigments to achieve a more realistic "black"/shadow. Depending on the painter and their personal preferences or techniques they learned the mixture of the colors to achieve the "black"/shadow will by the nature of pigment be more cool or warm. Add the other layers of light and surface to the mix and now you got what most painters refer to "working out problems" of representation. Even if "black" from the paint tube is used it will usually have a more cool or warm inherent bias. See how this effects the mix in the next set of diagrams (fig.3a & fig.3b).
The next level deals with reflectivity. Light "bounces" off surfaces changing the property of the light when it is reflected back to the viewer. This reflectivity in life -- in painting is called a highlight. See the following diagram (fig 4b). Depending on the strength and color of the light and the reflectivity of the surface, highlights can also be both warm and cool and have several areas with both.
In summary, both shadow and highlight have various levels of warmth and coolness simultaneously regardless of the warmth or coolness of the light source. The general rule Warm light source = Cool shadows is correct but over simplistic.
Written and contributed by Bill Indursky
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