Thanks Andy,
Here are the results of my photometry:
Have taken consequtive shots of the same (northern) area of the sky with equal exposure and without expocompensation. At the same time, I've noted the EV values displayed by camera (the ones that could probably be used).
(NB: as the Sun rised from 10° to 17°, the atmosphere extinction changed from 4х to 2.5х)
First and third point from the graph above have been used for making this animation.
I said, I had some doubt whether the decrease of illumination was visible visually, but with help of camera it is evident!
