# Predictive color temperature model

#### Matthew Fisher

##### New member
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 must 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)

#### Doug Kerr

##### Well-known member
Hi, Matthew,

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 must be placed in direct sunlight.

Most sunlight does not have a color temperature (only that whose chromaticity happens to lie on the Planckian locus).

Perhaps you mean the correlated color temperature (CCT).

Your project sounds very interesting and worthwhile. Good luck with it.

I'm not sure how you plan to determine the chromaticity of the light reflected from the "neutral" card from the RAW image data from the camera. To really do this requires calibration of the individual camera to light of known chromaticity.

One can approximate this from published data on the camera model (such as that published by DxO Labs,l, but you have not even asked for camera model information in the submissions (perhaps you intend to discern that from the Exif metadata).

If you do plan to infer the camera's transfer function from the DxO Labs data, I'd enjoy learning of your approach.

Best regards,

Doug

#### Asher Kelman

##### OPF Owner/Editor-in-Chief
Hi Matthew,

I's suggest you have folk specify what grey card they are using. Whibal™ is well characterized. You may have other favorites. Also have the pictures taken at different times of the day. A picture of a Gretag Macbeth color chart would seem to be a good idea too.

You might have the same "color temp" but differing appearance of colors on a color chart. Have you completed your experimental protocol? I'd even think of sending your own small piece of Whibal™ to each person so that you'd have one material that's constant!

Also one would want to know the cloud color, pollution and so much more to make head or tail of the data. But no doubt all that is taken care of in your modeling.

Good luck!

Asher