# More on Color Temperature

In regard to the below message, how does one determine RGB values from a
point of choice in Radiance? I cannot use Radiance yet, but I am just
interested in the code, if it is possible. I would just like to use Radiance
to compare how different luminaires render color differently, and how
varying surfaces appear differently under those lighting conditions. So you
can see I am not interested in the RGB values of the light leaving the
source, but rather the RGB values of the surface which we see.

Thanks,
Nick Calcagni

I hope that makes sense...

Hi

It may be possible to estimate the color temperature from the RGB
1) transforming the RGB values into xyz trilinear coordinates

2) using an approximate formula relating xyz to color temperature.
See for instance this paper:
McCamy, Calvin S. (April 1992). "Correlated color temperature as an
explicit function of chromaticity coordinates". Color Research &
Application 17 (2): 142-144

http://dx.doi.org/10.1002/col.5080170211

Raphael Compagnon

Hi Nick,

You are better off multiplying the source spectra with the reflectance spectra by hand, then converting to the color space of your choice. Radiance does not really do this, and the approximation of three primaries (RGB) multiplied between source and surface is not very reliable. See the following paper for more information:

Ward, Greg, Elena Eydelberg-Vileshin, "Picture Perfect RGB Rendering Using Spectral Prefiltering and Sharp Color Primaries," Thirteenth Eurographics Workshop on Rendering (2002), P. Debevec and S. Gibson (Editors), June 2002.

Best,
-Greg

···

From: "Nick Calcagni" <[email protected]>
Date: April 15, 2008 2:19:40 AM PDT

In regard to the below message, how does one determine RGB values from a point of choice in Radiance? I cannot use Radiance yet, but I am just interested in the code, if it is possible. I would just like to use Radiance to compare how different luminaires render color differently, and how varying surfaces appear differently under those lighting conditions. So you can see I am not interested in the RGB values of the light leaving the source, but rather the RGB values of the surface which we see.

Thanks,
Nick Calcagni

I hope that makes sense...

Hi

It may be possible to estimate the color temperature from the RGB Radiance channels by:
1) transforming the RGB values into xyz trilinear coordinates

2) using an approximate formula relating xyz to color temperature.
See for instance this paper:
McCamy, Calvin S. (April 1992). "Correlated color temperature as an explicit function of chromaticity coordinates". Color Research & Application 17 (2): 142-144

http://dx.doi.org/10.1002/col.5080170211

Raphael Compagnon

Greg,

First, I apologise for starting a "new" topic rather than doing a reply to
your post. I know message board ettiquite but I haven't figured out how to

I'm just an undergrad in architectural engineering, and I haven't had any
courses that went into depth about color or light. So I don't quite follow
what you mean by, "multiplying the source spectra with the reflectance" and
"converting to color space". I've looked into it since you've responded and
now know what those terms mean, but not how to work with them. I was
wondering if you could simplify what that entails and what numbers I will
need to obtain or measure and then what calculation I would have to do. Is
there is an example of what you mean?

Thank you,
Nick Calcagni

Hi Nick,

Most mail systems have a "Reply" button that should work just fine.

I don't really have time to take you through basic colorimetrics, so I recommend using one of the many online resources available, or buying a book. Have a look at:

This should get you started.

-Greg

···

From: "Nick Calcagni" <[email protected]>
Date: April 15, 2008 10:50:49 AM PDT

Greg,

First, I apologise for starting a "new" topic rather than doing a reply to your post. I know message board ettiquite but I haven't figured out how to "reply" yet.

I'm just an undergrad in architectural engineering, and I haven't had any courses that went into depth about color or light. So I don't quite follow what you mean by, "multiplying the source spectra with the reflectance" and "converting to color space". I've looked into it since you've responded and now know what those terms mean, but not how to work with them. I was wondering if you could simplify what that entails and what numbers I will need to obtain or measure and then what calculation I would have to do. Is there is an example of what you mean?

Thank you,
Nick Calcagni