The Diffuse, specular and Reflection terms have lead to a lot of confusion because they have been commonly used to describe different lighting processes along the CG history and sometimes diverge from their scientific usage.
to clarify this I is use my own vocabulary made up of different terms i picked up here and there:
1- Surface Reflectance:
- could correspond to specular map in the old system
- Correspond to the fresnel reflection part of the BRDF model for the dielectric materials and to the global reflection for the metallic ones.
The Surface-Relfectance process description: the light is "bouncing" off the surface without any transmission inside the material or micro subsurface scattering involved in the process (no refraction, no absorbtion).
Light color information remain inchanged during surface reflection process except for some precise cases (colored metallic reflection, shimmering)
1.1 - Rough surface reflectance: is light "bouncing" off a rough material (micro facets) in a more or less evenly distributed direction.
1.2 - smooth surface reflectance: is light "bouncing" off a glossy or smooth material in an more or less oriented direction.
2 - Body reflectance
The Body-Reflectance process description: The light hitting the surface that is not surface-reflected is first being transmitted in the interior of the object and then may be absorbed, further scattered and reflected, and in some cases exit the material again. It involve micro sub-subsurface scattering from internal irregularities.
Light color information is changed during the absorbtion steps of the body relfectance process. And if the light manage to get out of the material again, it will transmit its color information.
The body reflectance process is not applyable to metallic material as they only totally absorb or surface-reflect light depending on its wave lenghts.
Body-reflectance will not be influenced by material surface smoothness as there is scattering involved inside the material no matter the surface, except maybe for transparent materials where there is mostly absorbtion process involved (no light deviation) and very few scattering. Then when going out again, the surface roughness could really influence if these light rays are going out parallels or scattered.
Micro subsurface scattering is different than global subsurface scattering as, for a matter of simplification through approximation, the light is considered as going out of the material at the same exact point it went in. This is a just what makes that regular dielectric objects have color; there must be transmission, then absorbtion and micro scattering, then re-transmission outside of the material in order to get dielectric's color
Ok now, what i've understand from this naming confusion:
1 - concerning the diffuse reflection
What we call usually call diffuse reflection is the mechanism that include rough surface-reflectance and body-reflectance for a rough dielectric surface.
But in some case the term diffuse reflection can be used to describe only the surface reflectance part when opposed to the transmission process.
Concerning Metallic materials, diffuse reflection concern, as a matter of fact, only rough surface reflectance. In case of smooth metallic material, the term diffuse reflection is replaced by specular reflection or direct reflection (which add to the confusion as specular is used here to mean "sharp").
When talking about smooth dielectric material, there are still diffusing processes, in the sense that the light transmitted into the material is still scattered when goign out of it (body-reflectance), but the surface-reflectance part of it could be called specular or direct reflection.
2 - Concerning the albedo
In the physicist field, Albedo seems to be the ratio between reflected light intensity (surface-reflectance + body-reflectance) and incident light. So it is a one dimentionnal value.
In CG, in the other hand, we view albedo as a three dimentional value in RGB which correspond to the traditionnal "diffuse" of the old system and to the "baseColor" of the metall/roughness workflow. In this case albedo would be, for a metal/roughness workflow the body reflectance for dielectrics and and the Surface-reflectance for metals but without the fresnel composant of the fresnel surface reflectance.
But in the physicist way of the term, albedo also cover the surface-reflectance part of the light re-emmission (fresnel reflection).
In the metall/roughness workflow though, BaseColor doesn't have any incidence onto the fresnel reflection wich is directly embedded into the shaders. So BaseColor is basically the body-reflectance RGB value for the Dielectric material and the surface relfectance RGB value being surface-reflected by the metallic material (being "Surface-reflected", but in a colored way because of the conductive property of the metals and their cristalline organisation combined).
It is all really confusing indeed... and i'm not even sure that i entirely get it
One of the doc I'm refering to along with substance PBR guidelines: