Approximate (or exact) relationship between illuminant color temperatures?
So, I have a photographic project in mind that involves a bit of remapping between illuminants, where the relationship between two illuminants is going to be more important, I think, than the actual illuminants themselves. Specifically, I'm thinking of using a pair of strobes, one in a softbox and another hard and directional to simultaneously simulate the overhead sky and the Sun. Gelling the softbox is likely to be impractical, so the thought is to leave it at its ~6500 native temperature and heavily gel the "Sun" correspondingly. I'd white balance the final image using the same sort of approach one would for a scene in direct Sunlight with shadows -- either neutral for the primary subject, or something that leaves the subject warmish and the shadows bluish, or some other variation along those lines. The actual temperature targeted would be much visually warmer / thermally cooler than outdoors, but I'm hoping that it'd still look as if it were shot outdoors. My obvious question: how red should the "Sun" get gelled to, and how do I know that without trial and error? (I'm expecting trial and error, of course, but I want to understand what's going on.) The sky is often north of 15,000 K, and horizon sunlight can be below 5,000 K. The most naive of math would suggest that, to pair with my 6500 K "sky" I need a "Sun" with a color temperature thousands of degrees below absolute zero -- obviously absurd and obviously nowhere near the correct approach. I'm guessing instead I'd want to compare distances on the Planckian locus and figure out x, y coordinates to target. Is that reasonable? And, yes, of course -- I realize that I'm seriously oversimplifying all sorts of things and that there probably isn't an actual answer. The SPD of the strobes isn't a good match for D series illuminants, for an obvious starter. Instead, I'm hoping for some sort of "close enough" approximation. If there happens to be an actual answer, fantastic, and it'd be a good exercise for me to work through the math. But some sort of a pointer to a general vague heuristic would be welcome, too. (And, yes, I can think of some other alternatives, such as covering the ceiling with an appropriate bluish color and bouncing a strobe off of it. Other alternate suggestions are fine and welcome, but the geometry of the studio setup is likely to work least miserably with a softbox and a directional light, and only the directional light is relatively easily gelled.) Thanks, b&
The concept you need is an old one in photography, called micro reciprocal degrees or mireds: https://en.wikipedia.org/wiki/Mired Express both direct sun color temp and blue sky color temp in mireds, subtract them, and that is the difference in mireds you need from any base color temperature to get the same relative color difference. Example: suppose direct sun is 5000K = 200 mired; and blue sky is 16000K = 63 mired . This is a difference of 137 mired If you start with 6500 to represent blue sky, that is 154 mired, and adding 137 gives 291 mired, or 3436 K to simulate direct sun against 6500 as blue sky. -----Original Message----- From: colorsync-users-bounces+waynebretl=cox.net@lists.apple.com [mailto:colorsync-users-bounces+waynebretl=cox.net@lists.apple.com] On Behalf Of Ben Goren Sent: Wednesday, March 30, 2016 7:29 PM To: ColorSync Subject: Approximate (or exact) relationship between illuminant color temperatures? So, I have a photographic project in mind that involves a bit of remapping between illuminants, where the relationship between two illuminants is going to be more important, I think, than the actual illuminants themselves. Specifically, I'm thinking of using a pair of strobes, one in a softbox and another hard and directional to simultaneously simulate the overhead sky and the Sun. Gelling the softbox is likely to be impractical, so the thought is to leave it at its ~6500 native temperature and heavily gel the "Sun" correspondingly. I'd white balance the final image using the same sort of approach one would for a scene in direct Sunlight with shadows -- either neutral for the primary subject, or something that leaves the subject warmish and the shadows bluish, or some other variation along those lines. The actual temperature targeted would be much visually warmer / thermally cooler than outdoors, but I'm hoping that it'd still look as if it were shot outdoors. My obvious question: how red should the "Sun" get gelled to, and how do I know that without trial and error? (I'm expecting trial and error, of course, but I want to understand what's going on.) The sky is often north of 15,000 K, and horizon sunlight can be below 5,000 K. The most naive of math would suggest that, to pair with my 6500 K "sky" I need a "Sun" with a color temperature thousands of degrees below absolute zero -- obviously absurd and obviously nowhere near the correct approach. I'm guessing instead I'd want to compare distances on the Planckian locus and figure out x, y coordinates to target. Is that reasonable? And, yes, of course -- I realize that I'm seriously oversimplifying all sorts of things and that there probably isn't an actual answer. The SPD of the strobes isn't a good match for D series illuminants, for an obvious starter. Instead, I'm hoping for some sort of "close enough" approximation. If there happens to be an actual answer, fantastic, and it'd be a good exercise for me to work through the math. But some sort of a pointer to a general vague heuristic would be welcome, too. (And, yes, I can think of some other alternatives, such as covering the ceiling with an appropriate bluish color and bouncing a strobe off of it. Other alternate suggestions are fine and welcome, but the geometry of the studio setup is likely to work least miserably with a softbox and a directional light, and only the directional light is relatively easily gelled.) Thanks, b& _______________________________________________ Do not post admin requests to the list. They will be ignored. Colorsync-users mailing list (Colorsync-users@lists.apple.com) Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/colorsync-users/waynebretl%40cox.net This email sent to waynebretl@cox.net
Am 31.03.2016 um 04:28 schrieb Ben Goren <ben@trumpetpower.com>:
The sky is often north of 15,000 K, and horizon sunlight can be below 5,000 K. The most naive of math would suggest that, to pair with my 6500 K "sky" I need a "Sun" with a color temperature thousands of degrees below absolute zero --
I’m afraid that would be more naive than math. ;-) Wayne Bretl’s suggestion is extremely simple and probably sufficient for practical purposes, but it’s not really precise from a colorimetric POV. Using the values from Wayne Bretl’s example calculation (sky = 16,000 K, sun = 5,000 K, fake sky = 6,500 K), the precise calculation would be as follows: 1. Calculate the xy values and from them the XYZ values for daylight with 16,000 K (using the CIE Daylight formula): D160xy = (x = 0.2594, y = 0.2676) D160XYZ = (X = 0.9693, Y = 1, Z = 1.7670) The XYZ values for D50 and D65 are well known. 2. Calculate the Bradford chromatic adaptation matrix for a white point shift from D160 to D65 (= from the real to the fake sky) (A matrix is hard to print in an email, so I omit it here.) 3. Chromatically adapt D50XYZ (= the sun) with this matrix. You’ll get: adaptedD50XYZ = (X = 0.9961, Y = 1, Z = 0.5010) 4. Calculate the correlated CCT for the adapted XYZ value: adaptedD50CCT = 3718 K So your fake sun has 3718 K. This should be as precise as it gets in colorimetry. The value is roughly 10% higher than the value calculated with Wayne Bretl’s method; the chromaticity difference (in uv(1976)) is deltaC = 9.79, so it’s relatively high and certainly visible. It would be interesting to know which CCT value achieves the better result artistically. Bye Uli _________________________________________________________________________ Uli Zappe, Christian-Morgenstern-Straße 16, D-65201 Wiesbaden, Germany http://www.ritual.org Fon: +49-700-ULIZAPPE Fax: +49-700-ZAPPEFAX _________________________________________________________________________
On Mar 30, 2016, at 8:49 PM, Wayne Bretl <waynebretl@cox.net> wrote:
Example: suppose direct sun is 5000K = 200 mired; and blue sky is 16000K = 63 mired . This is a difference of 137 mired
On Mar 30, 2016, at 9:33 PM, Uli Zappe <uli@ritual.org> wrote:
2. Calculate the Bradford chromatic adaptation matrix for a white point shift from D160 to D65 (= from the real to the fake sky)
Thanks much to both Wayne and Uli -- between the two of you, those were exactly the answers I was hoping for. In practical terms, it looks like I'll be buying the Rosco 1/2 CTO, 3/4 CTO, full CTO and double CTO filters to experiment with; they have mired shifts of, respectively, 81, 131, 167, and 320. And, of course, the bigger the shift, the less the transmission...stop losses are 0.5, 0.8, 1.1, and 2.1. One of those four should provide the look I think I'm imagining. The 3/4 CTO is as close a match to Wayne's number as is likely to be commercially practical. The 1/2 CTO goes in the direction of Uli's figures, probably overshooting. I think the full CTO might be what I'm envisioning, with an especially blue sky in the Golden Hour. The double CTO would let me experiment with some over-exaggeration. So, thanks again. I'll (eventually) report back with results. (And, of course, I'll also need to balance the luminosity of the two sources, but that's much more straightforward.) Hmmm...just occurred to me that I could maybe adapt this to have some fun simulating alien landscapes with more than one local star in the sky.... Cheers, b&
Am 31.03.2016 um 18:34 schrieb Ben Goren <ben@trumpetpower.com>:
I think the full CTO might be what I'm envisioning, with an especially blue sky in the Golden Hour.
Remarkably, a much bluer sky does not change as much colorimetrically as you might assume. For instance, if you take a blue sky with 25,000 K (the highest value the CIE Daylight formula is defined for), my approach will give you 3562 K, only 156 K less, and still higher than Wayne’s result. Now, if you assume a 25,000 K sky *and* a *really* golden sun (4000 K, the lowest value the CIE Daylight formula is defined for), then you’ll get 3071 K. Bye Uli _________________________________________________________________________ Uli Zappe, Christian-Morgenstern-Straße 16, D-65201 Wiesbaden, Germany http://www.ritual.org Fon: +49-700-ULIZAPPE Fax: +49-700-ZAPPEFAX _________________________________________________________________________
participants (3)
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Ben Goren
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Uli Zappe
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Wayne Bretl