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When searching online about what the white balance setting in cameras does, you'll probably find an answer that goes something like this:

When taking a photo, the color of the lightsource might be different. There are yellowish light sources, like a candle, in the <3000k range that will give a strong yellow tint to the image, and there are blueish light sources, like the sun, in the >6000k range, that will give a strong blue tint to the image. With the white balance setting, you tell the camera what color temperature is considered pure white, and it accounts for that.

Ok, fine, I get that, and I know that the images turn out bad if the white balance was set incorrectly, but ... why? Why is it even necessary to account for the color temperature in the first place?

If I take a photo of a candle, I don't want the candlelight to be shown as pure white. The candle gives of a very yellowish light, and if I'm capturing a photo of it, the whole point of the image is to show this warm light. Similarly, if I'm getting heavy blueish light from an artifical light source, why would I want that to be pure white? I don't understand why, on a fundamental level, I need to tell the camera to manipulate the white point.

Why is there any interpretation involved at all? I'd expect that, if I took an image with a camera that literally records the wavelengths of light and send that information to a monitor, that monitor should be able to just reproduce the same wavelengths, and I should see the same image. Just like how, when a microphone records sound, we can just transmit the actual wave, and a speaker will reproduce it. There is no need to tell the microphone at what frequency middle C was on that day for a speaker to reproduce the sound correctly. The information is all in the waves themselves, no interpretation needed.

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  • $\begingroup$ A lot of the need to white balance is driven by how the eye and brain interpret color and less with the actual spectrum. $\endgroup$ Commented Jan 23, 2022 at 12:33

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The Q is not really much physics, it more relates to human physiology.

The reason is in the underlying mechanism of the human color vision.

Human eyes are very good at adapting to different light sources and "seeing" adequate colors when an object is lit by sources with blackbody-ish spectrum with color temperature from ~2000K (candle) and up to ~20000K (blue sky without direct sunlight).

If everything around you is lit by the same light source, this mechanism works pretty well.

On the other hand, you can pretty much fail the same mechanism by looking at a building with few lit windows at night.

Right now, different lightbulbs are available on the market and people have different preferences, this is why different rooms will be illuminated with light at 2700K, 3000K, 4000K and 6000K.

From the outside, these rooms will look like different hues of yellow and blue. Entering any of these rooms, you will consider them illuminated in white.

The same will happen if you exactly reproduce a picture taken in 2700K - in an environment lit at 4000-6000K it will look unnaturally yellow with a lot of blue lost.

In short, the whole "white ballance" is an adaptation to the human vision adaptation.

p.s.

You have pretty much romantic image of the sound reproduction as well. The sound engineering can also be called "the science of equalizers and echo cancelers". And, in the general case, the signal from the microphone and the signal fed into the loudspeaker (which microphone and which speaker in the whole setup?) are two pretty much different things. And don't even get me started on the magnetic recording or the vynil printing.

This is all done in order to get a "natural" sound.

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Perhaps the whole point is in the processing and interpretation, not the raw data itself. Our physical sensors do not measure the raw emitted data, they also filter before transmitting it to the brain, and our mind may construct a different version of this data that you experience.

The ear is a sensor that has a non-linear frequency characteristic what it records, and the brain also processes this before you actually hear something. For example, I have a tinnitus that fluctuates over time, and I can also train myself to hear a certain frequency better. Several hearing aids use this knowledge to improve the actual hearing (of voices or music etc).

The eye probably has the same. So although the data may be very yellow, you will not experience it like this (since your mind, or even your eye, already compensates the off bias.

Hope this helps

———

@fraxinus gives a better explanation below :-)

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