Physically Based Rendering
Physically based rendering (PBR) is an approach in computer graphics that seeks to render graphics in a way that more accurately models the flow of light in the real world. Many PBR pipelines have the accurate simulation of photorealism as their goal. Feasible and quick approximations of the bidirectional reflectance distribution function and rendering equation are of mathematical importance in this field. Photogrammetry may be used to help discover and encode accurate optical properties of materials. Shaders may be used to implement PBR principles.
In Source
Source does not ship with PBR by default. However, it is possible to add PBR shaders to a 
Source 2013 mod.
Additionally, there are five third-party Source Engine branches or games that supports and use PBR:
Half-Life 2 RTX (in development)
Hunt Down the Freeman
Portal with RTX
Strata Source
Titanfall branch
In Source 2
The default shaders in 
Source 2 utilize PBR.