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I understand what colour balance is and how it is meant to be used. However I don't fully understand why we see what we see if an incorrect White Balance is used.

If for example we take a photo out in bright daylight, but have the white balance set to 3200K, that photo will appear "cooler" or to have more blue.

What is the camera trying to do that results in the blue colour? Why not any other colour?

The fact that there is more blue to me says that is either trying to reduce the other colours, or it's trying to increase the level of blue, but I can't reason why it would be trying to do either.

I have looked at this question but either I didn't understand it, or it didn't answer quite the same question I have: What *exactly* is white balance?

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White balance is applied while processing raw data. The purpose of white balance is to reach R = G = B for neutral (grey, non-colored, achromatic) areas of the image. The whole problem arises from the fact that color channels of the sensors (for typical Bayer, those are R, G1, B, G2) have different "sensitivities", and the responses also depend on the spectrum of the light. Typically, for daylight green channels are about half a stop to one stop more sensitive than red and blue; for incandescent light the responses in green and red channels are close to each other, while the response in blue channel is lagging behind. So, to equalize the responses white balance coefficients are applied through the multiplication of the linear raw data in respective channels. To put some numbers to it, here are the white balance coefficients for Olympus E-M5 camera, in R, G1, B, G2 order:

// Olympus E-M5 CameraGroup= 6
{"Olympus", "E-M5", "Tungsten", {1.296875f, 1.0f, 3.265625f, 1.0f}},
{"Olympus", "E-M5", "3300K CCT", {1.546875f, 1.0f, 2.578125f, 1.0f}},
{"Olympus", "E-M5", "3600K CCT", {1.640625f, 1.0f, 2.367188f, 1.0f}},
{"Olympus", "E-M5", "3900K CCT", {1.734375f, 1.0f, 2.203125f, 1.0f}},
{"Olympus", "E-M5", "FL-W", {2.000000f, 1.0f, 2.601562f, 1.0f}},
{"Olympus", "E-M5", "4300K CCT", {1.851562f, 1.0f, 2.125000f, 1.0f}},
{"Olympus", "E-M5", "4500K CCT", {1.921875f, 1.0f, 2.148438f, 1.0f}},
{"Olympus", "E-M5", "4800K CCT", {1.976562f, 1.0f, 1.945312f, 1.0f}},
{"Olympus", "E-M5", "Daylight", {2.078125f, 1.0f, 1.820312f, 1.0f}},
{"Olympus", "E-M5", "Cloudy", {2.281250f, 1.0f, 1.640625f, 1.0f}},
{"Olympus", "E-M5", "6600K CCT", {2.304688f, 1.0f, 1.734375f, 1.0f}},
{"Olympus", "E-M5", "Shade", {2.476562f, 1.0f, 1.437500f, 1.0f}},
{"Olympus", "E-M5", "Flash", {2.351562f, 1.0f, 1.617188f, 1.0f}},

To convert to photographic stops / EV, calculate log2 of the numbers. From above you can see that if the image is taken in daylight, the blue channel needs to be multiplied by 1.8. If the white balance is set to Tungsten it will be multiplied by a much larger amount, 3.3 times. That is why it will appear bluish.

  • Thanks so much, this has been bothering me all day but this made it click. If I understand you correctly this would mean the sensor doesn't have a base or native white balance at all? – James May 3 '15 at 16:28
  • @James : No, sensors do not have native white balance under any normal light sources. Daylight or flash with a magenta filter like CC40m result in something close to native white balance. So is incandescent with a strong blue filter. – Iliah Borg May 3 '15 at 16:31
  • Most sensors do not record any color information at all. They record monochromatic luminance values for each pixel. The pixels are filtered for Red, Green, or Blue light, but there is only one luminance value for each pixel. The concept is similar to using color filters when shooting with B&W film. A red filter will cause the red light passing through the filter to have a higher tonal value (brighter gray) on the film than equally bright blue or green objects. – Michael C May 3 '15 at 17:27
  • To get color from that information, the demosaicing algorithm used by your camera (JPEG) or computer's photo conversion application (raw) compares the relative brightness of adjacent pixels filtered with the different colors and interpolates R, G, & B values for each pixel. – Michael C May 3 '15 at 17:29
  • @ Michael Clark - they do record information about colour. The representation of colour as intensities is no different from colour TIFF. TIFF with three channels, each of which is "monochromatic", or Bayer raw with 4 channels, each of which is also "monochromatic", differ only spacially. – Iliah Borg May 3 '15 at 17:46

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