Is there a way to catch the light ?

[henrib]

Like the process "shadow pass", i would like to catch the light with alpha channel to composite in Photoshop rendering with pictures.
Here is an example ...

I want to change the ground without changing the shadows and the light.
When I use only the "Shadow pass" process, i loose the color of the light.

Thanks,

Henri

[Pibuz]

Only method that comes into my mind is to isolate separately all the light sources in the scene, anda save an image out of each one of them. In Photoshop, it'd be sufficient to load the different images as layers and change the fusion option to "screen". That worked to me in different cases.