Predictive color temperature model

[mattstevenfisher]

I’m a student at Cornell University working on a design project aimed at creating a formulaic model to predict the color temperature of sunlight at any time and geographic location in the world. Our algorithms are well on their way and we need empirical data to verify our work. Thus, we are turning to the online photography communities to help us out!

We are asking people from all around the world to capture the color temperature of direct sunlight on a, preferably, sunny day by photographing (in RAW) a white balance card pointed toward the sun. From that image we’ll determine the color temperature and add it to our growing dataset. While we want pictures taken at any time of day and sun position, the white balance card musts be placed in direct sunlight.

Please send all photos (in any RAW format) to msf245@cornell.edu and include the following information in the email:
-Location (lat/long, if possible)
-Time
-Weather description (brief: cloudy, clear, overcast, a picture would work as well)

[neuroanatomist]

Interesting. 'White balance card' - there are many on the market, some true neutral or close to it, some far from it. Some people use index cards, and the bluing agents used in paper can lead to incorrect WB, especially in sunlight (due to fluorescence). Some people use gray cards, which while the luminance is QC'd to 18% gray (they're an exposure tool), the color is not QC'd and varies from lot to lot.

Would these variables affect your results, and if so, do you have a recommendation for an appropriate WB card?

[mattstevenfisher]

So far, all of the data I have collected was with a white balance card and not a gray card. I would prefer people to use a white as opposed to gray. There are variation between cards, but there are variations within a single card of a given picture. I acknowledge that this isn't a precise science (as calibrated photospectrometer would have been use otherwise), but I'm more interested in extracting general trends that tie in what I've learned about solar position, atmospheric effects, etc.

[mattstevenfisher]

So I looked into different body/lens/white balance card configurations to see how much the calculated color temperature varies between setups. The maximum variation is 100-200K, which in the scope of my project, is irrelevant. I know within the lighting business, fluctuations of ±125K are considered to be the same color temperature.

Here are the results of my test for anyone who is curious: