The base color weight (default is 0.8).
The base_color sets how bright the surface is when lit directly with a white light source (intensity at 100%). It defines which percentage for each component of the RGB spectrum which does not get absorbed when light scatters beneath the surface. Metal normally has a black or very dark base color, however, rusty metal's need some base color. A base_color map is usually required.
Diffuse file texture -> Base Color
The base component follows an Oren-Nayar reflection model with surface roughness. A value of 0.0 is comparable to a Lambert reflection. Higher values will result in a rougher surface look more suitable for materials like concrete, plaster or sand.
This switch in the standard_surface shader specifies whether specular or transmission bounces behind diffuse bounces are enabled or not. As caustics can be noisy, these are disabled by default.
To control noise from caustics, the global indirect_specular_blur setting may be increased, which blurs out caustics to reduce noise at the cost of accuracy.
Arnold will require a high number of diffuse samples to achieve a clean image when using caustics. Care should be taken when enabling this feature.
Reflective caustics (left), refractive caustics (right). Rollover images to view without Caustics.
Notice that the areas in the scenes below are darker inside the glass. This is because the illumination inside the glass is essentially all caustics. Enabling caustics for the glass shader fixes this issue.
Rollover images to view with caustics enabled
You may experience fireflies when using high specular_weight, and low Specular Roughness with Caustics enabled. To reduce this type of noise, increase the specular_roughness of the shader or indirect_specular_blur globally.
Enabling caustics can add more believability when rendering eyes.
Rollover image to view caustics enabled for outer cornea shader
This will cause the standard_surface shader to trace a ray against the background/environment when the maximum GI reflection/refraction depth is met and return the color that is visible in the background/environment in that direction. When the option is disabled, the path is terminated instead and returns black when the maximum depth is reached.