That is not an easy task. When you have two or more illuminants (light sources) illuminating a scene, it's not easy to figure out the relative illuminations each pixel took from each light source. There was a paper at the last Color Imaging Conference (Los Angeles 2012) where they showed an office scene illuminated by both overhead fluorescent lights, and outside daylight coming through the windows. Through some algorithm that I don't remember they were able to make such an illuminant map. You'd have to look up the paper but it's certainly not a few lines of code that I could share here.
Illuminant Estimation
Graham Finlayson, University of East Anglia
The Role of Bright Pixels in Illumination Estimation
Hamid Reza Vaezi Joze and Mark S. Drew, Simon Fraser University (Canada); Graham D. Finlayson and Perla A. Troncoso Rey, University of East Anglia (UK); pages 41-46.
Scene Illuminant Estimation of Multiple Light Sources
Shoji Tominaga, Takahiko Horiuchi, and Yu Kato, Chiba University (Japan); pages 47-51.
Reducing Worst-Case Illumination Estimates for Better Automatic White Balance
Milan Mosny and Brian Funt, Simon Fraser University (Canada); pages 52-56.
