If you are new to photography, there is a good chance the word "Kelvin" means nothing to you. Even many advanced photographers only know the Kelvin System as a set of arbitrary numbers used to control white balance. In this article, you'll find out all about the Kelvin system, where it comes from and how to use it to make better photos.
Ready to be enlightened? (No more puns, I promise.)
The Deeper Science
History of Kelvin
The first definition of Kelvin actually correlates to heat, not light. "But," you say, "I'm a photographer, not a scientist (kudos if you're both), why do I need to know about heat?" The temperature of an object (heat) in Kelvin defines the color of the object in Kelvin (color).
Kelvin is a scale of measurement created by a physicist and engineer at the University of Glasgow named William Thomson, Lord Kelvin. First introduced in 1848, Lord Kelvin wrote On an Absolute Thermometric Scale, a paper that called for need of a cold "null" point. At this point in time we have the Fahrenheit scale, which was based on marking a bottle arbitrarily and seeing where a brine solution will freeze and boil, 32°F and 212°F, respectively. We also had the Celsius scale, based on the freezing and boiling points of water: 0°C and 100°C. Neither of these scales, however, had a "null" point, meaning it was possible to have a negative degree of heat, theoretically forever.
Lord Kelvin used the same scaling distance as Celsius, but reset the values to give a theoretical zero point. In the Kelvin System the amount of "heat" of a thing actually corresponds to the amount of movement in the particles on an atomic level. So 0 K means absolutely no movement atomically. This is technically impossible, but to date, scientists have been able to cool particles to less than 1 K.
The act of creating this system that has a null point anchors every measurement in science. Since then, many different things have been measured in Kelvins, including heat, pressure, and noise, but more importantly for this article, color. Color Temperature describes the specturm of light radiated from a "black body" at a given temperature. A perfect black body is an object that absorbs all incident light, meaning it does not reflect any light or allow light to pass through it. What we talk about when we measure something in Kelvin is the color the black body turns when it reaches super hot tempertures. Ever seen a movie where a blacksmith hammers a sword and when he heats it, the sword glows? That's what I'm talking about.
Black bodies don't really begin to show light on a spectrum we can see until they hit about 2000 K. From 2000-4000 K, they appear reddish. By the time they reach 7500 K, they change from reddish to bluish. The hotter it gets, the deeper the blue.
Defining Color Temperatures
Starting with the coolest things you can actually see, let's list a few real objects so you can understand the changes in heat, wavelength, and color. Note, all temperatures here are approximate.
- 1800 K – Candle Flame
- 3200 K – Incandescent (Tungsten) Lamps
- 4200 K – Xenon Arc Lamps
- 5000 K – Fluorescent Lamps
- 5778 K – The Sun*
* So why did I say earlier daylight is either 5500 K or 5600 K, when I very specifically just said the sun is 5778 K? The sun burns at 5778 K. By the time is reaches us and gets filtered through the atmosphere, depending on the angle (changed by time of day, location, and season) the refracted light we see is mostly just the red wave lengths, which lowers the color temperature we can actually see.
There are many other things with colors that you associate with light (as seen in the chart above) but the ones I listed were the ones that actually burn and create that color temperature. The rest of the items have a "correlated color temperature:" that is, close to the color of an object that would burn at that temperature.
Camera vs. Brain
To understand all of this, you have to look at with your camera, not your eye. Take a red apple. Hold it next to the lamp on your desk. Is it red? Of course it is. Take is outside. Is it still red? You bet. So if you just changed your light source color by over 2000 K, how can it possibly be the same color? Your brain has an extra mechanism your camera doesn't have called chromatic adaptation. You look at the red apple, know that it's supposed to be red, and your brain instantly remaps the colors that your eyes actually saw, reinterpreting them into the colors it thinks you're supposed to see. When that signal actually hits your brain and you recognize that you see the red apple, the "white balance" has already been changed without your knowledge, and you just see a red apple.
When you have a camera sensor instead of a brain, you have two options. You can set your camera to auto white balance, but what fun would that be? In all seriousness, there's nothing wrong with auto, but it's not really great either. It won't give you scenes the way you see them, it will give technically neutral scenes. When's the last time you were awe inspired by something technically neutral? In addition to making matching across many photos a pain, if your camera isn't giving you what you want in auto, your second option is to be the boss over your photos and tell the camera what color temperature it is supposed to see.
Hot 'n' Cold
No, I don't mean the song. But "hot" and "cold" on the Kelvin scale can quickly become a topic of confusion. Why do we call blue a cool color when it is a hot temperature? And why do we call red a warm color when it is actually cooler temperature than blue? The point this time around goes to psychology. In the 18th century, painters referred to reds, yellows, browns and tans as "warm" colors because they were associated with "warm" things, like summer afternoons, fireplaces, and sunsets. They referred to blues, greens, violets, and most grays (though as we will soon learn, even grays have warm or cool color shifts) as "cool" colors because those are associated with overcast skies, gloomy landscapes, and dark sulky rooms. "Warm" stimulates the viewer while "cool" relaxes or calms them. Thus, completely disregarding all logic and science, we now know red as "warm" and blue as "cool". Thanks a lot, painters!
How the Camera Remaps Colors
Since your camera doesn't have the same chromatic adaptation ability we do, here's how it handles colors: it knows that a certain Kelvin will have a certain color cast to it, be it red or blue. Your camera will add the same amount of the opposite color to try and create a neutral, or "white balanced" image, to make whites look as white as possible. Now that we know tungsten is 3200 K, and very orange, your camera is adding a lot of blue to balance it out. If you are shooting in the shade with the sun out, we already know the sun is very hot, which is blue, so the camera is going to add red (or orange/yellow) to balance it. When you are setting your own Kelvin settings, the lower number settings are adding more blue to your photos, "cooling" them, and the higher number settings are adding more red to your photos, "warming" them. Yet another reason we confuse how the Kelvin scale really works.
Why is Fluorescent Green?
I can't tell you how many times I've seen a green picture and thought "What the heck? It looked white in person?" You may have noticed, I haven't talked about green at all. That's because fluorescent lights do not get their color by heating a black body object like all the other light sources I mentioned, but instead use gases and phosphors.
If you only ever have to shoot in non-daylight balanced fluorescent light (oh dear), I hand you a "get out of this tutorial free" card, since you'll need the green/magenta shift that Kelvin doesn't account for. But besides shooting in awful old office buildings, Kelvin is incredibly useful for everything else, so keep reading.
Why You Should Care
But I Use Auto White Balance
Auto white balance is great for casual shooting when you are only taking a few pictures in a lot of varied environments.
Auto white balance is awful if you are trying to get the same color across a lot of different photos. It often also cools off skin tones way too much and makes people look dead. Or, if you are shooting in tungsten light and it skews neon orange, or worse, overcompensates and pulls all the color out of the room. If you have any of these problems, or just want to save a lot of time editing your images, choose a color temperature yourself.
But I Use Tungsten or Daylight White Balance
The default color temperatures (or white balance settings) on your camera are certainly a great place to start. The problem comes when you like a look, and none of the given settings give you the look you are looking for. Let's look at an example. It's a typical situation: picture of a person with daylight outside and tungsten light inside.
But I Shoot RAW so this Doesn't Matter
Think again. Yes, shooting RAW gives an amazing amount of flexibility, but in addition to making your photos look good on location, did you ever think that your choice in white balance can affect your exposure? Let's look at an example. Here's a candid photo of my sister during her engagement session.
This is a simple photo with a neutral color temperature. Let's take that same exact photo, and show you what happens when you change the white balance. If you can't see the exposure shift on the left, on the right are the exact same photos in black and white, done by hitting CTRL+SHIFT+U in Photoshop (CMD+SHIFT+U on a Mac).
Not enough to sway you? Trust me, this one is important. Shooting RAW shouldn't be a crutch to save you from taking bad photos, RAW should be a tool that can take good photos and make them better.
Let's look at a still from a music video I made. Remember, this is a music video. You only have the same flexibility with your footage that you would have with a jpeg. We don't have the flexibility of "changing it in post" like you can with RAW. There is a passion and intensity there and it all comes from the brilliant orange light. How did we do this? The simple explanation might have been "we set up a lot of lights with orange gels", but that would have been too easy. We didn't have enough orange gels to cover all of our lights on set. I didn't want to introduce the headache of having footage with multiple color temperatures, so I left all the lights bare. Our lights were your basic stage can-lights, measuring at 3200 K. The rest of the music video features ripped muscles, a sexy red car, and a fire dancer. The instrumental scenes needed to have that same intensity and hot look. If I wanted "neutral", I would have set my camera to 3200 K. Instead I set it as high as I could, which was 10,000 K.
Here's a shot with a "proper" color temperature.
Here's a shot using one of the camera's default color settings:
Here's the same shot set to a color temperature of 10,000 K:
How crazy is that?! I did not change any other settings in photoshop except for the color temperature. I told you the exposure shift would be important.
Thanks for coming along on this journey exploring everything you ever needed to know about Kelvin and color temperature. There are a million articles out there that can help you with the specifics of how to use this information to make your photos better, and many of them on here on tuts+. If you can think of more creative ways to use color temperature or can think of something I missed, let me know in the comments.