Doug Kerr
Well-known member
The Story about the why and how of white balance color correction basically says at the end, "And that's why we want the illumination in the place where we view the image to have the same chromaticity as the white point of our color space." (That a teaser; you will be subjected to The Story - in an abridged form - shortly.)
But for the sRGB color space, the white point chromaticity is that of standard illuminant D65, while the recommended illumination of the viewing room has the chromaticity of standard illuminant D50. How come?
Many discussions of the sRGB color space address this with facile explanations, but upon closer examination they are like "Although the Moon is smaller than the Earth, it is farther away." But this is what I make of it.
I will start with a (fairly-concise) rendering of The Story of white balance color correction.
Imagine that we have a piece of note paper whose reflectance is uniform over the range of visible wavelengths. It certainly seems to deserve the description "white" paper. For such paper, the chromatically of the light reflected from the paper will be the same as the chromaticity of the illumination on the paper.
If we first observe the piece of paper at, say, the dining room table under incandescent lighting (remember incandescent lighting?), and later observe it at our patio table under sunlight, the color of the light reflected from it will differ appreciably. Yet our perceptual system in each cases probably judges the "color" of the paper to be the same.
This happens because of the extraordinary ability of the human visual system to outguess what is going on here. From the color of the light reflected from various surrounding objects (many of them familiar), the visual system in effect deduces what is the chromaticity of the illumination, and "backs that out" of the color of the light that strikes the retina to determine the "reflective color" of the piece of paper.
Now we move from direct visual observation of that piece of a paper to a photograph of the scene it is in. Suppose we first do that in the "outdoor" setting. And suppose that our end-to-end image chain results in the color of the "piece of paper" on the viewing screen having exactly the same chromaticity as the light reflected from the paper had. But further suppose that the image is viewed on-screen in a room illuminated by, for example, incandescent lighting.
The visual system sets up to deduce the reflective color of that item in the image assuming (incorrectly) that it is really right there, being illuminated by the light in the viewing room. The result will be is that to the viewer the paper will look "bluish", not white.
Outwitting this phenomenon is the purpose of white balance color correction.
[To be continued]
But for the sRGB color space, the white point chromaticity is that of standard illuminant D65, while the recommended illumination of the viewing room has the chromaticity of standard illuminant D50. How come?
Many discussions of the sRGB color space address this with facile explanations, but upon closer examination they are like "Although the Moon is smaller than the Earth, it is farther away." But this is what I make of it.
I will start with a (fairly-concise) rendering of The Story of white balance color correction.
Imagine that we have a piece of note paper whose reflectance is uniform over the range of visible wavelengths. It certainly seems to deserve the description "white" paper. For such paper, the chromatically of the light reflected from the paper will be the same as the chromaticity of the illumination on the paper.
If we first observe the piece of paper at, say, the dining room table under incandescent lighting (remember incandescent lighting?), and later observe it at our patio table under sunlight, the color of the light reflected from it will differ appreciably. Yet our perceptual system in each cases probably judges the "color" of the paper to be the same.
This happens because of the extraordinary ability of the human visual system to outguess what is going on here. From the color of the light reflected from various surrounding objects (many of them familiar), the visual system in effect deduces what is the chromaticity of the illumination, and "backs that out" of the color of the light that strikes the retina to determine the "reflective color" of the piece of paper.
Now we move from direct visual observation of that piece of a paper to a photograph of the scene it is in. Suppose we first do that in the "outdoor" setting. And suppose that our end-to-end image chain results in the color of the "piece of paper" on the viewing screen having exactly the same chromaticity as the light reflected from the paper had. But further suppose that the image is viewed on-screen in a room illuminated by, for example, incandescent lighting.
The visual system sets up to deduce the reflective color of that item in the image assuming (incorrectly) that it is really right there, being illuminated by the light in the viewing room. The result will be is that to the viewer the paper will look "bluish", not white.
Outwitting this phenomenon is the purpose of white balance color correction.
[To be continued]