Getting average light level of player view

Is there any way to get the average level of light a player can see (i.e. from their camera view or in screenspace)? Ideally I’d like to get either a 0-1 ‘brightness’ value or RGB values.

By ‘average light level’ I mean, for example, if the player could see a completely black scene, the function would return a 0 or 0/0/0, and a completely white scene would return 1 or 255/255/255.

I want to use some HDR/eye-adaptation-like effects such that the player camera’s postprocess would automatically adjust based on the light level in the scene. Some of what I want can be done using the Auto Exposure settings, but I’d really like more precise control over things.

Ideally I’d like to do this in blueprints, but C++ is fine too.

Hope this makes sense, and thanks!