Access exposure/eye adaptation data in code

Hey there! The question is straightforward. How do I access the input data for auto exposure procedures in my code (C++, for blueprints no any hope). Even simpler: how do I access the amount of lighting the same way the auto exposure algorithm does? The purpose is simple as is, an accurate graphic iris contraction based on the amount of lighting, so the ideal would be getting actual brighness multiplier(s) without applying them to the image (the latter would deserve less radical post-processing). The workaround with iterating through light sources is a bad idea, very bad, I think. On the other hand the present auto exposure is too complicated to be fully shader-based, so I assume the data is accessible in CPU/RAM.