Physically-based rendering library

[Anasky]

I was wondering if there is an existing library to perform physics-based rendering. So far I've found engines that do (such as Unreal Engine 4), but haven't yet found a library to do pbr in a project without using the entire engine. I was wondering if any exists at all, and if not if there is a valid reason as to why it was never created (such as f.e. performance being too much impacted).