Accessing per pixel luminance value in nits

Hello everyone,

I’m trying to write a postprocess function based on per pixel luminance in nits so I’m thinking about how I can access this value or calculate it by myself.
I’m talking about the NIT Value you can see in HDR (Eye adaptation visualizer) or the luminance value in pixel inspector.

Is there a way to have access to this value via Blueprint of C++. I tried looking in the pixel inspector source code and calculating the value myself but the result is a bit off.

So if anyone have some advices about how I can achieve this please tell me.