I have a very specific problem.
Is there a way to get the the Value of light intesity which hits a surface?
Not at rendertime, no. Light and influencial (?) scene evaluation is done inside shader stage of a material node setup (diffuse, glossy node etc), and these output only the evaluated shading which cannot be used for anything but mix/add/material output nodes. So things like light vectors and intensities are only available to shaders. In some cases you can fake stuff like this using empties for “light location” and calculate shading effects manually, but often it’s not worth the hazzle. Example anim of using such technique to render color ramped highlights - very fake.
Then is there at the moment no chance to build a real light reflector with a simple node setup. So I have to calculate the reflection light source manually.
Thank you for the fast reply