Color temperature

Learning goal

You will be able to read the color of a light source on the Kelvin scale and explain where this scale comes from. You will describe why the color of a heated object shifts from orange to blue; you will observe, through the day and under different lamps, that a low Kelvin value means orange light and a high value blue light; you will see that the same Kelvin difference creates a much bigger color change at the warm end of the scale than at the cool end; you will know that Kelvin alone doesn't fully describe a color, and that the green–magenta shift enters the calculation too; and in a scene where light of two different colors falls on the same face, you will notice how light and dark skins are affected by this mix.

The question

In the evening, you are in front of a neighborhood corner shop. The inside of the shop is filled with a yellow-orange light, and outside the sky has turned deep blue. Your eye accepts both lights easily; a white carton of milk looks white to you both inside and outside. But when you take a photo, the inside turns into an orange box and the outside into a blue evening.

Does light have a color? Even though the eye usually doesn't notice this color, it does, and the camera records it much more honestly than the eye. In this lesson we will talk about color temperature, which describes the color of light, where the Kelvin scale comes from, why it seems to work backward, and why a single number isn't always enough to describe the color of a light.

Context

In Lesson 7.4 we met the temperature slider in the editing program, and saw the approximate Kelvin values of candlelight, warm white bulbs and shade. That lesson was about correcting color afterward. This module begins with understanding and managing color at the moment of shooting; the first step is getting to know the color of light itself.

Color temperature describes not how hot a light is, but what color it is. Its name, though, comes from a real temperature: as a heated object gets hotter, the color of the light it gives off changes. Kelvin is a unit that measures temperature starting from absolute zero. When we call a light 3200 Kelvin, we mean that its color looks like the color of the light given off by an object heated to about that temperature.

Simple explanation

Why is the color of light described with a temperature?

Think of an iron bar in a blacksmith's forge. As it heats up, it first starts to glow a deep red, then turns orange, yellow, and when it gets very hot it comes close to white. Every heated object, as its temperature rises, gives off more light of shorter wavelengths, that is, closer to blue. Physicists calculate this behavior using an ideal object that reflects no light at all, which they call a black body, and match a color to each temperature. Color temperature is based on this match.

Incandescent bulbs, candles and the sun really are hot objects, and their light fits this scale well; this is what we call incandescent light. LEDs and fluorescent lamps, on the other hand, produce light not with heat but in other ways; the Kelvin value given to them is an approximation describing which temperature's object light the eye finds them most similar to. That is why 2700 Kelvin written on an LED bulb promises not the same color as an incandescent bulb, but a color that resembles it.

How do you read the Kelvin scale?

The scale looks backward to everyday language. We call orange light warm, yet its Kelvin value is low; we call blue light cool, yet its value is high. The words warm and cool describe the feeling a color stirs in us, while Kelvin, written Kelvin (K) on settings, describes the temperature of the object giving off the light. A single sentence is enough not to mix the two up: a low number is orange, a high number is blue.

With approximate values, the scale lines up like this. Candlelight is around two thousand Kelvin. Warm white bulbs at home sit between two thousand seven hundred and three thousand two hundred. The rising and setting sun is orange, and climbing toward noon it reaches around five thousand, five thousand five hundred Kelvin. A cloudy sky goes into the six thousands, and a place in shade on a clear day, lit only by the blue sky, goes above seven thousand, sometimes above ten thousand. Within the same day, on the same street, the color of light keeps changing across this range.

Why aren't Kelvin steps equal?

The difference between Kelvin numbers doesn't describe the change in color in equal steps. At the warm end of the scale a small difference is a big color change; at the cool end a big difference is a small change. That is why lighting technicians use another unit called the mired: you divide one million by the Kelvin value. 3200 Kelvin is about 312 mired, 5600 Kelvin about 179 mired.

Looked at with this unit, the imbalance shows clearly. Going from 2700 to 3200 Kelvin, that is a step of only five hundred Kelvin, is a change of about 58 mired. Going from 5600 to 6500 Kelvin, even though it looks like a bigger step of nine hundred Kelvin, is a change of about 25 mired. So small differences between warm lamps can catch the eye more in a photo than big differences between different hours of daylight. When you put a colored filter on a light, too, the filter's effect is added not in Kelvin but in mired.

Is a single number enough to describe a color?

Kelvin describes color on a single line between orange and blue. But some lights sit a little off this line. Many fluorescent lamps and cheap LEDs shift toward green compared with incandescent light of the same Kelvin value; some lights, on the other hand, sit close to magenta, to pink. This second shift is called tint (green–magenta), and on cameras it is corrected with a separate setting. That is why two lamps can paint a face in different colors even if they carry the same Kelvin value.

If there is more than one light in the scene, each carries its own color to where it falls. If bluish daylight from a window lights one side of a face and a warm lamp on the table lights the other side, the face splits into two colors. This mix shows on every skin, but in different ways. On a light-skinned face, the warm lamp's side can shift to pink-orange, and the window side to a pale gray. On a dark-skinned face, the warm side stays a deep, glowing brown, while the blue shadow side can take on an ashen, dull look. Knowing the color of light is the first step to seeing these differences before the shot.

Visual evidence

You go back to the corner shop. The shop's owner, a dark-skinned, middle-aged man, stands on the doorstep: the side of his face looking in is lit by the warm bulbs on the shelves, the side looking out by the blue evening sky. When you leave the camera on the daylight setting and shoot, the shelves inside and the man's cheek turned inward are yellow-orange, and his cheek turned outward and the door frame are bluish. On the cheek turned inward his skin is a deep, warm brown; on the cheek turned outward it looks as if covered with a gray, dull layer.

You ask the man to take one step inside; now his face is lit only by the bulbs on the shelves. The frame is still orange, but this time both sides of the face are the same color; the brown of his skin has spread evenly across the whole face, and the lines on his cheek and the curve of his eyebrow can be read in a single light. How you correct the color is another decision; first you saw how many colors of light there were in the scene.

Common mistakes

Summary

Color temperature describes the color of a light in Kelvin, through the temperature of an ideal object giving it off: a low value is orange, a high value is blue, and through the day it changes across a wide range from candlelight to blue shade. The same Kelvin difference is a much bigger color change at the warm end of the scale, and that is why lighting technicians use the mired. Kelvin alone doesn't describe a color; the green–magenta shift and mixed light enter the calculation too, and a light split into two colors affects light and dark skins in different ways.

Mini assignment

In this assignment you will observe the color of light with a fixed camera setting.

1. Fix your camera's white balance on the daylight setting, and photograph the same white sheet of paper under a candle, a warm bulb, window light and the shade of a clear day in turn.
2. Write down the Kelvin values of two different lamps in your home, from their boxes if you have them, and compare their colors on the same paper; does one of them shift toward green or magenta?
3. Photograph a face with one side toward a window and the other side toward a warm lamp; compare the color of the two sides.
4. Move the same person to a place lit by a single light only, and shoot again.

Check yourself

• Were you able to line up the four paper frames in the right order by Kelvin value, from the most orange to the most blue?
• Did you see a color difference between two lamps carrying the same Kelvin value?
• In mixed light, did you notice, on every skin, that the two sides of the face took on different colors?

Glossary

Mini quiz

1. How does the Kelvin value of orange candlelight compare with that of blue shade?

• A) It is higher
• B) It is lower
• C) It is the same
• D) It can't be measured in Kelvin

2. About how many mired is 5600 Kelvin?

• A) About 560
• B) About 56
• C) About 179
• D) About 1790

3. Why can an incandescent bulb and a fluorescent lamp carrying the same Kelvin value paint a face in different colors?

• A) Because the fluorescent gives less light
• B) Because the incandescent bulb is always bluer
• C) Because Kelvin only describes brightness
• D) Because fluorescent light can shift toward green or magenta, and Kelvin doesn't describe this shift

4. On a dark-skinned face with one side toward a window and the other toward a warm lamp, if the window side looks ashen, what is the cause?

• A) The two sides of the face are getting light of different colors; the bluish light makes that side look cool and dull
• B) The camera's focus has slipped
• C) The aperture is too narrow
• D) The shutter speed is too long

Answers

1. B — Orange light has a low Kelvin value, blue light a high one; a candle is about two thousand, blue shade above seven thousand.
2. C — One million divided by five thousand six hundred comes to about one hundred seventy-nine mired.
3. D — Kelvin describes color only on the orange–blue line; the green–magenta shift is a separate dimension.
4. A — In mixed light, each source paints where it falls in its own color; the cheek getting blue light stands apart from the warm cheek.

Summary card

• The color of light is described in Kelvin, through the temperature of the object giving it off.
• A low number is orange, a high number blue: candle ~2000, bulb ~2700–3200, noon sun ~5500, shade 7000+.
• Mired = 1,000,000 ÷ Kelvin; at the warm end a small Kelvin difference is a big color difference.
• Kelvin isn't enough: fluorescent lamps and LEDs can shift toward green or magenta.
• Mixed light splits a face into two colors; it looks different on light and dark skin.

Bridge to the next lesson

In front of the corner shop, your eye counted both lights as white, while the camera recorded the inside as orange and the outside as blue. The eye adapts to the light in front of it on its own; the camera, on the other hand, either guesses which light to count as white or learns it from you. The answer to the question of why your photos come out orange or blue is hidden in this guess. The next lesson talks about how the camera finds white, and how the automatic setting, the presets, entering a Kelvin value and a gray card are used for this job.