how to extract photometric data from images

Hello I have some confusions about extracting data from images and I know lots of image processing experts are in here. I would appreciate if someone can help me realize some concepts. how we can get some information,like intensity of a the light source from images? I know we can extract RGB value, but these values are associated with the surfaces and not with the light source spectra (I am talking about white light source with different spectra not monochromatic wavelength). is there a way to extract some information of the light source from images with matlab? should we convert color images to greyscale images? if yes, can you explain how grey scale giving us intensity (or other photometric data)? I know about HDRI so feel free to refer to them