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Assuming the picture is taken at sea level, in direct sunlight (no shadow) under a cloudless sky, and knowing the latitude, day and hour, I can in principle determine the color temperature of sunlight (right ?).

Provided I am using the ICM profile for my camera in my favorite raw processing software, is knowing this color temperature enough to achieve the most accurate white balance ?

I am asking because Rawtherapee's white balance pick button also modifies other values, e.g. Curve and Green, hinting the color temperature may not be the single relevant parameter.

NB : My ultimate purpose is to automatize my workflow while getting as accurate colors as possible (painting photography).

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First off, color temperature is but one axis of what we call white balance or color balance. Color temperature is based on the light emitted by black body radiators at different temperatures expressed using the Kelvin scale. It runs from amber/orange on one end to blue/purple on the other. Roughly orthogonal to the amber ←→ blue axis is the green ←→ magenta axis. Two light sources with the same basic color temperature can have radically different "tint", which is how we often refer to the green ←→ magenta axis.

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With the sun, the only place it is always exactly the same in terms of white balance is in outer space. From anywhere on the surface of the earth atmospheric conditions and the angle of the sun in the sky will affect the exact color temperature and even the tint of daylight.

Even with a cloudless sky there are still factors such as particulates in the air that can alter how the atmosphere filters sunlight passing through it. Recent volcanic eruptions, even halfway around the world, can have anywhere from a subtle to a dramatic effect. So can local conditions due to man made or natural sources of dust or other small airborne particles. This also includes the amount of water vapor which has not condensed on particulates matter to form clouds. Likewise, one's position relative to the equator and poles will affect the amount of ozone in the upper atmosphere over one's observing position and ozone concentrations also change for a specific location over time.

Then there are color casts on any potential subjects due to the reflectively of other things around them. Light from the same sky will look very different on the same person standing on white or beige beach sand than it will if that person takes a few steps to a place where they are standing on lush green grass.

  • Indeed, quoting Rawtherapee's RawPedia : "White balancing works by multiplying each of the primary colors by a different amount, until a satisfactory result is reached. In order to make this operation more human-friendly, instead of operating on the three multipliers directly, the user is presented with an abstraction in the form of a temperature slider which adjusts colors along a blue-yellow axis, and a tint slider which adjusts them along the magenta-green axis." (emphasis mine). – Skippy le Grand Gourou Jun 16 at 10:35
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You can determine the colour temperature you would get from the sun if there were open sky between you and the sun. There are these pesky things called "clouds" which tend to show up every now and then (quite often if you're in the UK as I am...) which mean you won't get the same spectrum of light incident upon your scene - clouds don't radiate the same spectrum as they absorb. Also, you get a very different spectrum of light if you're in shadow than if you are in direct sunlight.

There are also a number of second order effects, for instance the amount of dust in the atmosphere, the colour of the ground on which your subject is standing and so on but clouds and shadow are going to be the big things which would make any attempt to automate this go rather wrong.

  • Yes, the question was edited to precise cloudless sky, and I'll add that it stands for direct sunlight. I believe the other effects are negligible (at least in comparison to the different rendering on different non-calibrated computer monitors, since this is for web usage), right ? – Skippy le Grand Gourou Jun 14 at 20:55
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There are several wrong assumptions. First you are confusing sunlight and daylight. Sunlight in space is pretty well described by black body radiation spectres with the temperature of the outer plasma, but the atmosphere makes quite a hash of that by dispersing a lot of the light (which is why the day sky is blue rather than black), absorbing some and so on. So while daylight is sort-of the definition of white, it is a mixture of bluish skylight and of what the sun's spectrum looks like after a lot of blue has been stolen for skylight and for making Earth appear like a "blue planet" in space.

This does not really have a "proper" well-defined color temperature (which is a single descriptive number rather than a function over wavelength) since its spectrum is a consequence of what the atmosphere did with the original brutish bluish-white color the sun exhibits in space.

So no.

  • Of course, but I seem to recall the transmittance curve is pretty stable, so I had assumed raw processing software would internally use this curve to transform the temperature color into a realistic white balance. However writing it down I realize it must not be the case since there is no reason for the software to assume the environment was daylight indeed, and if I choose Sunny Daylight as preset, it becomes Personalized as soon as I change the color temperature. Now I'm even more confused than before asking… – Skippy le Grand Gourou Jun 15 at 12:51
  • Anyway since this is the second "off" answer (out of two…) I figure the question is not asked properly. I'll try to think about it and do better later. – Skippy le Grand Gourou Jun 15 at 12:53
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Almost.

This is why the British Crown bestowed an OBE Knighthood on Bill Thompson for his seminal work to make all this practical with his brainchild Correlated Colour Temperature of the natural illumination called Photographic Daylight. It's measured in units named after his title of Lord Kelvin. The units are degrees Kelvin sometimes called Kelvins. Now, years later, Daylight's a mix of sunlight and skylight (and dust, ocean-bound plastic bags, kids helium balloons, 'n stuff.)

The original work specified Noon, June 21, 1939 in Washington, DC. as the standard for ASA Photographic Daylight.

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    Just to nitpick, Kelvin is an actual absolute unit, not a degree. It is not correct to say "degrees Kelvin" (except, perhaps, in a historical context). – Zeus Jun 17 at 0:51

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