In this link, you have complete information to understand what color temperature is and its relationship with human vision.
Here’s a summary:
Color temperature gives us an idea of the dominant color in the light emitted by a given light source.
Temperature to measure color?
Yes, it may seem quite strange, but now you will see that it has its logic.
The blackbody radiation model is used to study objects that emit light.
It is a mathematical model that characterizes the emission of electromagnetic radiation from the SunSun and other objects.
There is a direct relationship between the object’s temperature and the wavelength at which its emission peak is located in this model.
The typical example is when we heat a piece of iron: first, its maximum emission is in the infrared (heat) zone, we continue to increase its temperature, and we begin to see it red if we grow more, it reaches red hot, orange, white, bluish.
This model with a black body characterizes very well sunlight, incandescent filament bulbs, light generated by candles, bonfires, etc.
For example, how do we model the sunlight?
- Sun’s surface temperature: about 5700K
- The wavelength at which its maximum emission is located: 500nm
And that of a candle?
- Candle flame temperature: about 1800K
- The wavelength of its emission maximum: approx. 1600nm
Before moving on to color temperature, let’s take a quick look at how human vision works.
The human vision is optimized to work with the light of the SunSun.
Humans only see a narrow band of the electromagnetic spectrum, which spans roughly the wavelengths of 400 to 700 nanometers.
Notice that the SunSun has its maximum emission at 500nm, coincidence?
The color is an interpretation that makes the brain from the information sent three types of light-sensitive cells called cones, located in the retina:
- L cones – detect wavelengths that we interpret as ‘red.’
- M cones – detect wavelengths associated with ‘green.’
- K cone – detect wavelengths associated with ‘blue.’
In the image above, the shape of the Sun’sSun’s emission curve has been exaggerated to highlight where its maximum lies.
Actually, between 400 and 700nm, its radiation spectrum is relatively flat.
Under those conditions where the three types of cones receive a balanced combination of ‘red’ photons, ‘green’ photons, and ‘blue’ photons, the brain interprets that we see pure white light.
Note that white light does not exist physically (there are any photons of white light).
White light is a construction of the brain. A base setup. The configuration to which vision has been adapted over millions of years of evolution.
Color temperature (now yes)
Imagine that we have a remote control that allows us to change the temperature of the Sun’sSun’s surface.
If we lower the temperature, its emission peak shifts towards the long, less energetic wavelengths.
The radiation curve that the eye receives is no longer centered and balanced.
When scrolling to the right, the ‘red’ cones detect a higher level than the ‘blue’ cones.
We see a different light, and we interpret it as a more yellowish, orange glow.
The same happens if we increase the temperature: it moves towards the shorter (more energetic) wavelengths, and our vision would interpret a more bluish light.
Well, that’s the color temperature: associate a climate with a wavelength (color)
The color temperature tells us that the color dominates the light that a scene receives with a single parameter.
Does this model work well?
Almost all light sources can be characterized more or less well with this model.
There are exceptional cases, such as fluorescent lamps, which have a discontinuous emission spectrum and can give dominance in the green zone that would not fit the black body emission model and the color temperature.
We will see this case later.
Color temperature refers in some cases to the actual temperature of the object: combustion temperature, the temperature at which a filament is incandescent, etc. But in many other cases (most), it is not related to anything physical. It is merely a parameter to characterize the color cast of a light source.
Color temperatures of the most common light sources in photography and video:
Light, color, and digital cameras
Although there are different technologies to capture color, we will focus on the model used by practically all cameras.
Ideally, you would have three sensors. like the human eye: one sensor for red, one for green, and one for blue (some professional-grade video cameras work like this)
But that would be very expensive. Something similar can be achieved if we use a single sensor and divide it somehow so that some cells collect only red, others only green, and others only blue.
This is achieved by placing an RGB filter on the sensor.
It is an optical filter that forms a mosaic matrix so that each sensor cell only sees a specific color.
The color filters are distributed following different patterns: Bayer, X-Trans.
Green is usually given more predominance. The greens correspond more or less to the center of the visible spectrum and provide more information about the scene’s brightness.
Well, we already have our sensor mounted with the RGB filter.
When we take a photo with these types of sensors, we are getting three different images of lower resolution:
- A photograph gives us an idea of the brightness of the red tones of the scene (25% of the pixels)
- Another shot with the brightness of the green component (50% of the pixels)
- And another image with the blue detail (25% of the pixels)
Each of these images is called a color channel.
Therefore we have the red channel, the green channel, and the blue channel.
To obtain the final image, an image in which each point (pixel) has the complete color information, we have to combine the three channels’ knowledge.
This process is known as demosaicing in English (in Spanish, it would be color interpolation or chromatic interpolation), and quite complex algorithms are often used.
The ‘white’ color problem
Let’s imagine that we are in the street at noon, with neutral white sunlight (without any color cast)
We place a perfectly homogeneous white background (neutral white) on the ground and take a photo of it with the camera in such a way that the white background fills the entire frame (and there are no shadows, etc.)
What should the color channels be like?
Well, all the points of the three channels should have more or less the same intensity to give rise to a light gray (depending on the exposure that we have configured)
Let’s assume that we set for an exposure that achieves 75% gray.
For each sensor cell, zero would be pure black, and 100% would be pure white.
We should get three color channels with very similar values. All the sensor cells should give us a RAW deal of around 75%.
Does this happen in reality?
In real sensors, for example, the optical filters have different performances depending on whether it is a red, green, or blue filter. Also, the performance of the photosensitive cell can vary with wavelength.
The green channel has a higher performance than the red, and finally, the blue channel usually has a lower version.
In our example case, we will suppose that the green channel gives us an average value of around 75%, the red one around 60%, and the blue one an average value of 50%.
How is the final image, then?
As the background will not appear white, it will probably appear greenish or with some color cast, depending on the relationship in each color’s sensor performance.
How could it be solved?
Well, the solution is straightforward if we have the neutral white reference:
- We take as a reference the average level of the green channel, which is usually the one that collects more information about the brightness of the scene.
- We calculate the mean of the red channel and apply a correction factor to remain at the green level.
- We calculate the standard of the blue channel and apply its correction factor to equalize.
And that’s what doing the white balance of the image consists of.
Once the correction factors have been determined, they are applied to all the values, to all the channel’s pixels.
In our example, when applying the white balance, we will have the image we wanted with a neutral gray of 75%, without color casts.
The white balance consists of applying a correction factor to the color channels (usually red and blue) to make the neutral grays in the scene appear in the image as neutral grays, without color casts.
If we always had neutral white light, the white balance would be effortless: the camera itself would have the correction factors that would be applied automatically to the channels.
The point is that we are not always going to use neutral white light. Many times, the scene will be illuminated by the light of different color temperatures.
Color temperature and white balance
Imagine that you take an object with colors, a Rubik’s cube for example, and take pictures of it in different environments while maintaining the ‘neutral’ white balance (daylight) of the camera:
- In the light of a candle
- In a room with an incandescent lamp
- Outside at dawn
- At noon in full SunSun
- In the office, illuminated by fluorescent lamps
Then we take the photos and compare them on the computer.
Yes, indeed, the colors of the cube will be slightly different when comparing photo by photo. In some cases, the difference will be very noticeable.
What is the correct version, the most faithful to reality?
Probably the one you did outside at noon because we told the camera to use the white balance of sunlight (a color temperature of about 5500-5700K)
Can we correct the images to make them all have the same colors?
If we have the original color channels (for example, if we have the RAW file generated by the camera), we can apply a custom white balance to get all the images to show the same colors.
Especially if in the image itself we have a neutral white or gray object that serves as a reference to readjust the correction factors (in a development or editing program, it is achieved by clicking)
Why is white balance important?
White balance is essential when we want to have images (or videos) with true-to-life colors or recognizable colors that do not look unnatural.
In portraiture, for example, the skin tone must be as realistic as possible.
Human vision is compassionate when it comes to face recognition, and although the color is not that important, we can detect when a skin tone does not look natural.
For product photography (catalogs, advertising …) and fashion photography, it is essential to achieve the most accurate colors possible to reality.
The same happens when we photograph works of art, paintings, etc.
At other times the opposite may be of interest.
For example, if we are photographing in the golden hour or the blue hour, we may be interested in preserving or even highlighting those warm tones in the scene’s objects.
If we want to convey a particular sensation through photography or the video scene, we can play with the dominant colors to feel warmth or coldness.
White balance on a camera
If we take photos in RAW format, the white balance can be correctly done in the development phase once we have the computer photos.
If we make a video or take photos in JPEG format, the white balance in editing is more limited because, in creating the JPEG file, the camera has already made decisions for us.
For example, the camera has already done a white balance, the chromatic interpolation (demosaicing), has applied contrast, has made a tonal mapping, and has made the color correction.
Finally, it has used destructive compression algorithms (part of the information is lost, which is not essential for the visual result, but it is necessary if we have to do a more in-depth edit afterward)
Therefore, in these conditions, the camera must know in advance to make the correct balance.
Auto White Balance – AWB
All cameras include an automatic white balance mode (AWB – Auto White Balance)
When we configure this mode, the camera looks for the most appropriate white balance settings from the global scene.
The automatic white balance works pretty well in most situations, especially working with natural light.
Automatic white balance may fail, for example, when two or more different light sources are illuminating the scene.
In lighting situations further away from neutral white light: golden hour, blue hour, artificial lighting with some color cast …
Also, in scenes in which some color predominates, for example, autumn landscapes in which warm tones dominate, ice landscapes in which blues predominate, objects or buildings in which there are only variants of one color …
If you shoot and save in RAW format, white balance is delicate for 99.9% of situations because it can then be adjusted in the development process just like the camera would.
White Balance – Preset Modes
For video and photography in JPEG, it is best always to do a custom white balance. Later we see how it is done.
However, when we do not have time or for whatever reason, we cannot do it.
In those cases, we can choose a predefined white balance.
Most cameras include settings for the most common light sources:
- Natural light (noon)
- Shaded area outdoors
- Tungsten / incandescent
Note that each of these presets only works well in those specific situations and only correctly covers a certain range of color temperatures.
Therefore, my recommendation is to use them only if there is no choice.
Custom white balance
Most cameras with manual controls (SLR, EVIL, compact mid and high range…) allow you to customize the white balance.
Custom white balance is the best option for video (and for photography if we use JPEG directly from the camera instead of RAW)
How do you do a custom white balance?
1.You need a neutral gray/neutral white letter
You can use a white sheet if you don’t have anything else at hand, but keep in mind that the paper’s white varies a lot between brands, qualities, etc.
2.Place the gray card right where the main subject of the scene will be so that it receives the same lighting
3.In your camera, choose one of the ‘Custom’ options in the white balance option.
Some cameras only include customization, including several customizations that we can save for similar situations, such as taking a photo in the studio with the same lighting type.
4.Set the custom white balance using the neutral gray chart.
This step depends on each camera model.
Usually, the camera tells you to fill in the frame with the gray card. You only have to fill a central part of the frame with gray/white in other models.
The image does not need to be in focus.
In some models, it is finished by pressing the shutter button. In others, by pressing the ‘OK button to confirm the settings, etc. The camera usually indicates step by step what you have to do.
5.As long as the scene’s lighting conditions do not change, you can use that custom white balance for your photos or video shots.
IMPORTANT: when you finish the session, return the white balance to automatic (AWB).
For the next session, if you forget to set the white balance, you will have a more or less reasonable balance. But if you leave it in the custom configuration, your next session will likely be useless.
Manual color temperature adjustment
Some cameras also include the option to set an exact color temperature.
This option is usually indicated by the letter K (for Kelvin)
We can indicate the temperature to the camera by moving any of the dials or the arrows on the rear crosshead (depending on the camera)
Through the screen or the electronic viewfinder, we can see the effect on the scene, or we can take different images to adjust precisely the color temperature we want.
This option can be useful if we want to give a particular effect to the photograph (JPEG) or video, for example, to make the scene a little warmer or more relaxed.
If we want to make an exact white balance, this option is perhaps riskier because taking the image on the camera screen as a reference, we will not have enough information to adjust by eye and nail the balance.
There are devices called colorimeters that show the exact color temperature that the scene receives. In that case, we would have an accurate reference to configure that color temperature in-camera.
How does the white balance affect the lights in the scene?
Once we set a white balance on the camera:
- All lights whose equivalent color temperature is smaller – will look warmer, yellower / orange.
- All lights whose temperature is higher: will look colder, more bluish.
For example, imagine we have a scene lit by:
- Natural light through a window in the morning. Temperature approx. 5700K
- An incandescent light bulb that illuminates the interior environment. Temperature approx. 3200K
- The screen of a computer lights up a person’s face. Temperature approx. 6500K
If we choose a temperature of 3200K in the chamber:
- Interior items will appear in true colors and neutral grays.
- The exterior and the area lit by the window will have a bluish cast (cold tones)
- The person’s face will have very bluish lighting.
If we choose a temperature of 5700K in the chamber:
- Interior objects will appear in warm, orange tones.
- The exterior and the area near the window will appear in true colors and neutral grays.
- The person’s face will have slightly bluish lighting in part facing the screen and somewhat warm in the areas where the incandescent bulb dominates.
If we choose a temperature of 6500K in the chamber:
- Interior items will appear very orange.
- The exterior will have a slightly warm, yellowish hue.
- The face facing the screen will appear in actual color, while the areas of the head illuminated by the bulb will have an orange cast.
The same happens if we make these adjustments a posteriori in editing.
White balance with various light sources
As we have seen in the previous example, white balance can get quite complicated when we have several light sources in the scene, each one with a different color temperature.
Under those conditions, the automatic white balance will probably make an overall assessment of the scene and apply some between them.
As it is a decision that depends on the camera’s internal algorithms, we cannot be sure how it will behave.
Furthermore, any small change in the frame or the exterior lighting (a cloud that passes in front of the SunSun, for example) can produce a significant shift in colors.
For these types of situations, it is usually preferable to make a custom white balance, deciding what kind of light is predominant or what environment we want to recreate in the scene from an artistic point of view.
If we can modify one of the light sources, we can try to equalize the dominant one concerning the other birth we cannot control. For example, if we illuminate with flash or continuous light bulbs, we can use gels (tinted sheets) to change the color temperature.
But in many cases, it will be impossible to do a global white balance of the entire scene.
If we understand how light works and its properties, we are the ones who decide the white balance to achieve a particular aspect.
If we delegate that decision to the camera, the colors and balance will sometimes turn outright and sometimes go wrong.
Beware of reflected light.
Even if we have a single light source (or several sources with the same characteristics), it may happen that the walls or objects located around the scene are not white and reflect light with an inevitable dominance.
For example, you often work with flashes bouncing the light off the ceiling to achieve soft or fill lighting.
If the roof is painted a non-neutral color, the reflected light will include that color cast, and we will indeed have a complicated mix to balance.
There are many situations in which we can find this type of effect.
And in other cases, we can look for them ourselves to give a special touch to the scene.
The problem with fluorescents
Some fluorescent lamps introduce color casts (e.g., in the green zone) that do not fit the black body model and cannot be characterized only by their color temperature.
It can also happen with some LED lamps, depending on their quality and the technology they use to produce the ‘white’ light.
The green cast of fluorescent tubes can only be compensated for by modifying the blue-red axis’s complementary colors, which would be those of the green-magenta axis.
Thus, (also because we can find all colors’ dominance due to reflected light), the editing and development programs include both axes.
- The White Balance axis: red – blue
- Tint axis: green – magenta
Some cameras also include options to adjust the two axes. They are usually indicated as A-axis ( Amber-Blue) and G axis ( Green-Magenta )