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> There is sun, which is slightly yellowish The sun light is white during the day, otherwise, anything white would appear yellow.

gmueckl 5y ago
No, the white color of the sunlight is an illusion. It actually is yellowish, but our vision adjusts for that. We basically have a kind of adaptive white balance. The spectral intensity is mostly that of blackbody radiation, which peaks in the yellow part of the spectrum for our sun.
- pineaux 5y ago
  I dont think this is true. direct sunlight can be many shades of white. It differs in the morning and the evening and it is mostly yellow. The yellowness of the sunshine is a direct result of raleigh scattering and other atmospheric effects.the blue part of the spectrum that comes from the sun is scattered by the sky, making the sky blue. Taking away the blue makes the nonscattered sunlight yellowish. On photos of the moon, you see the true color of sunlight. Which is a pure white.
  - bnegreve 5y ago
    Of course there are different shades of white, what I am saying is that the color of the sun during the day is what people most would classify as "white" and not as "yellow" if it the color was printed on a sheet of paper. If the sun color was actually yellow, people wouldn't be able to tell the difference between a white object and yellow object under the sun light (they would both appear yellow). And clearly they are. On the other hand a white object doesn't get any "whiter" under another kind of light than it appears under direct sun light, this again is because the sun light is white, and not yellow. Of course there is some subjectivity here, but it is a misconception to call the sun light yellow. For example, this "bright sun" RGB [1] color, is clearly not the color of direct sun light. Hopefully, you will agree with this. [1] https://www.colorcombos.com/colors/FCD440 https://www.colorcombos.com/colors/FCD440
    - nine_k 5y ago
      Sunlight is wide-band (see "black body spectrum"). This is why a white object (that reflects all light equally) and a yellow object (which mostly reflects reds and greens) are discernible under such light, even under sunset light which is visibly yellow or orange. The sunset orange light still contains many other colors. A yellow object and a white object would be not discernible under a very narrow-band yellow light, or a mix of two narrow-band red and green lights of right proportions. This is how LEDs and lasers emit light, but totally unlike hot objects (like Sun) or fluorescent lamps (like "natural light" LED lamps) emit light.
      - bnegreve 5y ago
        Look, my point is simple: if you agree that snow and sugar are white, then the sunlight is also white. It has to be because white was historically described as the color of the snow (or whatever is white) under the sunlight. Now, if you want to adopt a more precise definition of white and other colors, based on (not so) recent scientific insights about the light, you can. Then the sunlight is not white, neither is snow or sugar. And is also certainly not yellow. Let's be honest, the sun light is almost perfectly white, and kids use yellow to draw the sun, because it wouldn't make sense to draw it with white.
  - gmueckl 5y ago
    The fundamental yellowness of sunlight is not Rayleigh scattering, but the sun's natural black body spectrum. See my reply to the sibling comment.
    - nine_k 5y ago
      There is another catch. Sun's surface is about 6000K. A light with such color temperature would be considered bluish by most people if they looked directly at a lamp with such a spectrum. Rayleigh scattering lowers it to 5600-5800K at noon time, but still most humans would call a moderately bright lamp of this color "bluish". The eye has a built-in white balance correction, and it moves the white point towards longer waves as the brightness lowers. This is why a single 5600K lamp in a room makes bluish light, while 10 of the same lamps make it lit by bright white. Same, a single 2500K lamp in a room makes a calming sort-of-white light. but 10 such lamps make a unmistakably yellow light. The sunlight outside at a cloudless summer noon is very bright (one usually even wants to wear sunglasses), so the sun looks white or yellowish, especially in contrast to the blue sky.
- dandellion 5y ago
  The sun is actually white. The yellow tint is due to Rayleigh Scattering in the atmosphere. When the sun is high in the sky there's less scattering and it appears whiter, when it's low there's more scattering and appears yellow/orange. Here's a better explanation: https://physics.stackexchange.com/questions/47694/why-is-the-colour-of-sunlight-yellow https://physics.stackexchange.com/questions/47694/why-is-the...
  - gmueckl 5y ago
    This is just wrong. The surgace of the sun is around 5700K and rhe emitted light follows the back body spectrum for that temperature. And that has a massive yellow peak. See https://en.m.wikipedia.org/wiki/File:EffectiveTemperature_300dpi_e.png https://en.m.wikipedia.org/wiki/File:EffectiveTemperature_30... for the data. This is measured outside the Earth's atmosphere. Rayleigh scattering does come into play, but it is way less important. As I said, human vision has a kind of adaptive white balance that cancels out peaks like this to some extent. It also does that for artificial light sources with uneven spectra (neon light, blueish white LED lights...). [Side note: this is also the reason why cameras have adjustable white balance. A picture taken by a neutral sensor under some artificial lights is perfectly correct, but looks color tinted and wrong to the human eye afterwards, because the eye under the same conditions has a shifted white point.] Why is this important? If you set up a light source in a renderer, you often get a pretty naive equal energy white as a default, which is simply not what you need for sunlight, even at noon under a clear sky.
    - bjelkeman-again 5y ago
      You see how the brain does white balancing quite clearly when working with LED lights optimised for salad, for example. The light has very little green in it and looks nearly pink. After a few minutes it looks less pink and nearly normal (depending on the lamps). Then go outside in daylight and everything suddenly look green, which takes a minute to go back to normal. The brain adds the green it perceives to be missing, and removes it again when it isn’t needed.
      - 5y ago

gmueckl 5y ago

No, the white color of the sunlight is an illusion. It actually is yellowish, but our vision adjusts for that. We basically have a kind of adaptive white balance. The spectral intensity is mostly that of blackbody radiation, which peaks in the yellow part of the spectrum for our sun.

pineaux 5y ago
I dont think this is true. direct sunlight can be many shades of white. It differs in the morning and the evening and it is mostly yellow. The yellowness of the sunshine is a direct result of raleigh scattering and other atmospheric effects.the blue part of the spectrum that comes from the sun is scattered by the sky, making the sky blue. Taking away the blue makes the nonscattered sunlight yellowish. On photos of the moon, you see the true color of sunlight. Which is a pure white.
- bnegreve 5y ago
  Of course there are different shades of white, what I am saying is that the color of the sun during the day is what people most would classify as "white" and not as "yellow" if it the color was printed on a sheet of paper. If the sun color was actually yellow, people wouldn't be able to tell the difference between a white object and yellow object under the sun light (they would both appear yellow). And clearly they are. On the other hand a white object doesn't get any "whiter" under another kind of light than it appears under direct sun light, this again is because the sun light is white, and not yellow. Of course there is some subjectivity here, but it is a misconception to call the sun light yellow. For example, this "bright sun" RGB [1] color, is clearly not the color of direct sun light. Hopefully, you will agree with this. [1] https://www.colorcombos.com/colors/FCD440 https://www.colorcombos.com/colors/FCD440
  - nine_k 5y ago
    Sunlight is wide-band (see "black body spectrum"). This is why a white object (that reflects all light equally) and a yellow object (which mostly reflects reds and greens) are discernible under such light, even under sunset light which is visibly yellow or orange. The sunset orange light still contains many other colors. A yellow object and a white object would be not discernible under a very narrow-band yellow light, or a mix of two narrow-band red and green lights of right proportions. This is how LEDs and lasers emit light, but totally unlike hot objects (like Sun) or fluorescent lamps (like "natural light" LED lamps) emit light.
    - bnegreve 5y ago
      Look, my point is simple: if you agree that snow and sugar are white, then the sunlight is also white. It has to be because white was historically described as the color of the snow (or whatever is white) under the sunlight. Now, if you want to adopt a more precise definition of white and other colors, based on (not so) recent scientific insights about the light, you can. Then the sunlight is not white, neither is snow or sugar. And is also certainly not yellow. Let's be honest, the sun light is almost perfectly white, and kids use yellow to draw the sun, because it wouldn't make sense to draw it with white.
- gmueckl 5y ago
  The fundamental yellowness of sunlight is not Rayleigh scattering, but the sun's natural black body spectrum. See my reply to the sibling comment.
  - nine_k 5y ago
    There is another catch. Sun's surface is about 6000K. A light with such color temperature would be considered bluish by most people if they looked directly at a lamp with such a spectrum. Rayleigh scattering lowers it to 5600-5800K at noon time, but still most humans would call a moderately bright lamp of this color "bluish". The eye has a built-in white balance correction, and it moves the white point towards longer waves as the brightness lowers. This is why a single 5600K lamp in a room makes bluish light, while 10 of the same lamps make it lit by bright white. Same, a single 2500K lamp in a room makes a calming sort-of-white light. but 10 such lamps make a unmistakably yellow light. The sunlight outside at a cloudless summer noon is very bright (one usually even wants to wear sunglasses), so the sun looks white or yellowish, especially in contrast to the blue sky.
dandellion 5y ago
The sun is actually white. The yellow tint is due to Rayleigh Scattering in the atmosphere. When the sun is high in the sky there's less scattering and it appears whiter, when it's low there's more scattering and appears yellow/orange. Here's a better explanation: https://physics.stackexchange.com/questions/47694/why-is-the-colour-of-sunlight-yellow https://physics.stackexchange.com/questions/47694/why-is-the...
- gmueckl 5y ago
  This is just wrong. The surgace of the sun is around 5700K and rhe emitted light follows the back body spectrum for that temperature. And that has a massive yellow peak. See https://en.m.wikipedia.org/wiki/File:EffectiveTemperature_300dpi_e.png https://en.m.wikipedia.org/wiki/File:EffectiveTemperature_30... for the data. This is measured outside the Earth's atmosphere. Rayleigh scattering does come into play, but it is way less important. As I said, human vision has a kind of adaptive white balance that cancels out peaks like this to some extent. It also does that for artificial light sources with uneven spectra (neon light, blueish white LED lights...). [Side note: this is also the reason why cameras have adjustable white balance. A picture taken by a neutral sensor under some artificial lights is perfectly correct, but looks color tinted and wrong to the human eye afterwards, because the eye under the same conditions has a shifted white point.] Why is this important? If you set up a light source in a renderer, you often get a pretty naive equal energy white as a default, which is simply not what you need for sunlight, even at noon under a clear sky.
  - bjelkeman-again 5y ago
    You see how the brain does white balancing quite clearly when working with LED lights optimised for salad, for example. The light has very little green in it and looks nearly pink. After a few minutes it looks less pink and nearly normal (depending on the lamps). Then go outside in daylight and everything suddenly look green, which takes a minute to go back to normal. The brain adds the green it perceives to be missing, and removes it again when it isn’t needed.
    - 5y ago