Why does the wrong white balance create a blue or orange color cast?
Asked 5/3/2015
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2 answers
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I understand the purpose of white balance, but I’m trying to understand the mechanism behind the color cast you see when it’s set incorrectly.
For example, if I photograph a scene in bright daylight but set white balance to 3200K, the image looks cooler/bluer. Why does the camera’s correction produce blue specifically? What is it doing to the red, green, and blue channels that causes this result?
In other words, how does white balance processing work, and why does choosing the wrong color temperature shift the whole image toward blue or orange?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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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.
Originally by user28381. Source · Licensed CC BY-SA 4.0
user28381
11y ago
0
Generated from our catalog & community — verify before relying on it.
White balance is a processing step that scales the red, green, and blue channel data so that something neutral in the scene appears neutral in the image (roughly equal RGB for gray/white areas).
Different light sources have different spectra. Daylight contains relatively more blue than tungsten/incandescent light, while tungsten is much warmer and weaker in blue. Camera sensors also don’t respond equally in all channels, so the camera applies multipliers to compensate.
If you set the camera to 3200K, it assumes the light is warm tungsten. To correct that, it boosts blue relative to red/green, because tungsten light is normally deficient in blue. But if the scene is actually daylight, blue was already plentiful. Adding that tungsten-style correction makes the image too blue/cool.
The opposite happens if you use a daylight WB under tungsten light: the camera doesn’t add enough blue correction, so the image stays orange/warm.
So the color cast isn’t arbitrary: it comes from applying the wrong RGB multipliers for the actual illumination. White balance is essentially the camera trying to force neutrals to neutral, but based on the wrong assumption about the color of the light.
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