I read Cycles sun is physically correct (radiant flux at sea-level is 1050 W/m² according to this source).
With a clear sky luminous efficacy of 100 lm/W, this should, with the support of a World surface texture, lead to the expected 110-120 klx global illuminance.
However on the images the indoor scene is very dark, and the rendering noisy (in spite of using two light portals):
The scene is using Blender units, openings are 2 units high. Switching to SI units doesn't improve. In addition a large portion of the color values is nearly unused (5 pixels only in the upper quarter of the histogram):
Question 1: What can explain this relatively low brightness indoor, compared to a real room.
Did I miss something in the use of the sun and environment lighting?
Question 2: Previously I solved this by adding lights, outdoor or indoor, but this leaves a visible effect on the walls (brightness that can't come from the sun; natural shadows weakened or lost, etc). Is it possible to prevent this from happening. How is this dealt with usually?
Additional questions, in case someone wants to shed more light on the topic:
Is there a mean to measure the final illuminance of a given area?
How does Blender compensate for different dynamic ranges. Does it adjust the sensitivity of its renderer (like adjusting the pupil of an eye, or changing the ISO number of a sensor)? Is there a way to influence this setting? Should it be used?