The Color Of Light
We talk about light temperature and light color in art and photography circles. We often have either an incomplete or incorrect understanding of it, even if we utilize it effectively. So, not being a scientist, I did my best to try and understand this and will attempt to explain what I found. I encourage any actual physicists to weigh in and offer corrections.
Light is the word we use to refer to that part of the electromagnetic wave spectrum that our eyes can see without special equipment. That word “spectrum” is important. It refers to electromagnetic waves within a range of lengths or frequencies that the acronym I learned in school corresponds to, “ROYGBIV”. I sometimes see this written out like a name, ROY G. BIV. This stands for “Red”, “Orange”, “Yellow”, “Green”, “Blue”, “Indigo”, “Violet”. Nobody knows why indigo is included, so just ignore it. At one end of the spectrum is violet, at roughly 400 nanometers in length. At the other end is red, at 700-780 nanometers in length.
These lengths can be converted to frequencies, which describe how often the wave peak crosses a point of observation in some time period at a fixed speed. This is usually a wave at the speed of light crossing a point of observation in one second, and we call it “terahertz”. The longer the wave, the fewer peaks of a wave at a constant speed will cross the point of observation, and therefore the lower the terahertz. The lower the frequency, the less energy a wave has, so longer wave lengths have less energy than shorter ones. This means blue, at 450nm, has more energy than red, at 700nm.
So, we can describe this in terms of nanometer lengths or terahertz frequencies, but we can also use kelvins as another way to label the energy of these waves. Kelvins are interesting and I encourage you to read about them, but to simplify, they are a measure of heat (or energy) that are rooted at absolute zero and have the same magnitude as celsius degrees. That is, zero kelvins is roughly equal to -237 degrees celsius. Kelvins are used to measure “the relative temperature of light as defined by the total radiative power per square unit”. So when we talk about light temperature, especially now that we have compact fluorescent and LED light bulbs with variable temperature, what we are really measuring is the energy (frequency, a function of the wavelength, etc.) of the electromagnetic waves emitted from the bulb.
We know that “blue” shifted white light has a temperature of greater than 5000K while “red” shifted white lite has a temperature of less than 2000K. This follows what I learned in school, ROYGBIV. Red light waves are longer than blue ones, and therefore have less energy. This also explains atmospheric perspective and its effect on light temperature. Red light has less energy than blue light, so over long distances, red light drops out, then orange and yellow, and finally green. We’re left with whatever blue or violet remains.
Note that the use of kelvins for measuring the temperature of the color of light seemingly contradicts what artists usually say about the “color temperature” of paint. Blue paint is cooler than red, etc. In terms of electromagnetic waves, blue light is higher energy and more kelvins than red. I sometimes hear people refer to flames to describe this: the blue part of the flame is hotter than the red. This is true, and tallies with this discussion of kelvins as a way to describe the energy of electromagnetic waves. See my earlier blog post about why this has nothing to do with the color temperature of paint.
Now, nobody needs to know any of the science to mix colors or paint well. It hasn’t helped me paint better landscapes. If we paint what we see, we can forget about all of the physics and just paint. This is the reason why the “primary colors” we use as artists are correct in our context, even though physicists use a different set of colors as primary. Their primaries are correct in their context, which is measuring electromagnetic waves. Our context is smooshing colored paste together to get the effect we want.
Here is an example of color temperature from one of the old masters, Rubens, who often used “cool” highlights and “warm” shadows.
Here are two examples of color temperature from Church. Note that in the first, Church uses warm highlights and in the second he uses cool. In both you can see the effects of atmospheric perspective in the distant mountains.
This is just my attempt to demystify the terminology we use, and since I’m not a physicist, it’s likely I got some of this wrong, so don’t take my word for it. If this interests you, do your own research and share it!
Hopefully, somebody finds this interesting or entertaining. Either way, I’ll just keep painting.