Subject: Photography
Objective: Students will understand the core concepts behind colour temperature and have a grasp of a few key baselines as expressed in Kelvin
Colour Temperature and White Balance
Physicist William Thomson - Lord Kelvin, father of the kelvin colour temperature system, ironically here seen in black-and-white. (Image Credit: https://www.sil.si.edu/DigitalCollections/hst/scientific-identity/fullsize/SIL14-T002-07a.jpg)
){kind=link}
History
In 1892 the first British scientist became Baron Kelvin of Largs, that scientist was William Thomson. Aside from being a renowned mathematician, he was an avid and competitive rower, a professor of natural philosophy at the University of Glasgow, a passionate advocate for the arts, and a pioneer in such areas as hydrodynamics, electrodynamics, thermodynamics, and geophysics. Among other achievements, it was his implementation of an absolute scale of temperature which we’re going to be exploring today; but we should not overlook that in 1872 he managed to create a machine which theorized the revolution of the earth’s poles - something not confirmed until the 1950’s by computers. He also completed the design of the electric cable, and his work showing that electricity could travel underwater led to the linking of India to Europe through a cable which traveled along the bottom of the Persian Gulf. I could go on, but this remarkable man’s achievements seem endless, so let’s just get to the reason we’re here.
Why IS his ABSOLUTE SCALE contribution so important?
The Kelvin scale, named after Lord Kelvin (who chose his name after a river which flowed past his university), is an exact absolute scale of temperature. According to the kelvin scale, 0 degrees represents the coldest temperature which is theoretically possible. The benefit of such a system would be obvious in that it could then never move into the negatives - unlike other arbitrary systems, or ones which lacked a fuller understanding of the coldest temperature like Fahrenheit, or which were more arbitrary like Celsius whose goal was more to divide the difference between water’s boiling point and freezing point by 100.
“The wavelength of the emitted radiation is inversely proportional to its frequency,
or λ = c/v”
Given that light represents radiation
We need to remember that Planck’s Law concerns itself with electromagnetic radiation which would be emitted by some theorized “black body” at a specific relative temperature. Coupling this standard with Lord Kelvin’s standard, the relative temperature becomes easy to assign along the visible spectrum as the colour of that black body would change. While we would only be capable of seeing the portions between infrared and ultraviolet, the spectrum would obviously extend well beyond those on either side, down to absolute zero (the lowest theoretical temp Kelvin made his standard) and conversely there is no known limit to the range of electromagnetic wavelenths, so theoretically - to quote the movie Mean Girls, “the limit does not exist.”
How does your camera guesstimate WB?
If you’re set to shoot in RAW then it really does not apply directly, as RAW reflects the purest form of the data which has hit the sensor, and thus white balance can be adjusted after the fact. If you are shooting to a JPEG or other compressed format, the automatic white balance settings (AWB) generally scan the image to determine an indication of what its best representative of neutrals in the black, white or grey range. If it’s simpler for you to understand, it’s very similar in approach to how the camera estimates exposure.
While people and living subjects tend to suit warmer tones better, imagery which features technology such as camera gear or cars is generally more favorably cast in cooler tones. There are exceptions to every rule however.
WHITE BALANCE TEMPERATURES
Understanding now where the numbers in the camera’s white balance come from - generally most professional cameras offer options as broad as a white balance range of 2,500k-10,000k, we should establish a few standards. Here’s what most consider acceptable kelvin white balance settings for common situations.
Candlelight: 1800k - 1900k
Incandescent lights: 2700k
Sunset/golden hour: 2750k - 3250k
High Noon Sun: 5000k - 5500k
Camera Strobe: 5500k (Set to model High Noon Sun)
Cloudy/Overcast Conditions: 6000k - 7500
Shade: 8000k
How we can use WB to our advantage
Colour matters. Certain colours carry psychological ties and can trigger subconscious associations when we see them. It’s so well known in fact that many organizations trademark or otherwise copyright specific pantones to ensure that they will be remembered wholly as tied to them (among other reasons).
Beyond that, there’s the consideration of warmer and colder tones relative to the subject. People generally associate life with the warmer tones along the kelvin scale, as if you cast a person in more cool or blue tones they tend to look less alive. Technology on the other hand is often highlighted with cooler tones to emphasize it in a way that we have become very accustomed to (largely through repetitive exposure in our cultural marketing).
It can also be used to draw colours towards a more pleasing range. For instance, say you were to capture an image where the hues were just a hint off - imagine you’re taking a photo of Kermit the frog and he’s correctly balanced and is the lovely green we all remember. However, if the image is intended to be associated with a brand logo which carries a similar colour, you can adjust the slider subtly to draw the colour of Kermit more in one way or another. You certainly shouldn’t push Kermit towards forest green where the viewer would be distracted by Kermit’s new species identity, but just something subtle. One thing to remember is that Kermit has white eyes, and if you adjust the white balance too much then it will quickly show.
While inside the shade the bride is standing at might register between 8,000k-10,000k, it might be more prudent to consider that the light is falling through the window and adopt a WB setting to reflect the source outside.
Devices like this X-Rite Colorchecker Passport are phenomenal tools which I always traveled with on deployments and to weddings. For the record, there are no paid links on this site, and I am not drawing revenue from anything herein.
Examples
Much like we discussed kelvin and the visible range, talking about it might not make it very real to you. Here is one photo adapted to be too far to the cold temperatures, balanced effectively, and adapted too far to the warm. Take a look and see how the impacts can make the same image look VERY different.
Colours as a Form of Creative Expression
A studio image of me which we took at All Saints High School as a part of their studio theory, and to practice processes like frequency separation and retouching.
Though we’ve explored colour along the kelvin scale in white balance contexts is a bit of a subtle art, you can still experiment through the use of gels or coloured lights to create creative and interesting images to engage your viewers. For instance this is a photo I created with some students a few years ago at All Saints. See how only a hint of it is warm? Even though the rest of me is cast in a cooler purple, it’s still a interesting balance which takes advantage of 2/3 of a split complimentary to create a draw. The process of adding or removing colour from light to create image effects or employ the psychology of colour effectively is something we’ll explore in later lessons.