Lighting and Rendering [2] Arnold Part 22: Sub Surface Scattering

Unaltered Image 

Assigned aiSurfaceShader to model. Arnold | Subdivision: Type- Catclark; Iterations- 2.0

Diffuse Layer plugged into ai_Head_SS: Base Colour 

aiStandardSurface [ai_head_SS] | Subsurface: Weight- 0.5; Subsurface Colour- Diffuse Source Image; Radius- Scatter Source Image; Weight- 'SubS_Weight' source image; File Attributes: Filter Type- OFF

aiStandardSurface [ai_head_SS] | Subsurface: Scale- 0.3

aiStandardSurface [ai_head_SS] | Specular: Weight- 1.0 (Plugged in Specular Source Image); Filter Attributes: Filter Type- OFF.

aiStandardSurface [ai_head_SS] | Specular: Colour- Darker Grey; Roughness- 0.4; IQR- Skin [1.4]

aiStandardSurface [ai_head_SS] | Specular: Colour- Lighter Grey; Roughness- 0.3

aiStandardSurface [ai_head_SS] | Geometry: Bump Mapping- 'Head Normal' source image; Filter Type- OFF

aiStandardSurface [ai_head_SS] | Specular: Colour- Lighter Grey; Roughness- 0.3

Shading Group Node | Attributes: Displacement Mat.- displacement source image.

Head Shape | Subdivision: Iterations- 3.0; Displacement Attributes: Height- 0.1

All layers made visible.

aiStandardSurface [ai_head_SS] | Specular: Roughness- 0.4

Render Settings | Camera (AA)- 4.0; Diffuse- 3.0; Specular- 3.0; Transmission- 3.0; SSS- 3.0; Volume Indirect- 0.0; Use Autobump in SSS- ON

Ambient Occlusion applied to model

Final composited image in Photoshop CC 2017