Thanks for your VERY quick reply!
Yes - that was also my initial approach. But in order to get the illuminance
falling on the sensor I need to know the reflectances of the surfaces in the
image in order to convert from luminance to illuminance. And the
reflectances might vary, so I was wondering if I could use another approach.
..hmm..and I am not that familiar with all the handy tools ;o)...but maybe
there is a chance that we can discuss this at the workshop next week.