Sigma2 is a volumetric scattering material node. Volumetric scattering is an important addition to 3D computer graphics for the realistic rendering of such materials as marble, skin, and milk and other real-world semi-transparent substances. The image above shows the LightWave logo with an additional bright Spherical light behind it.
The surface color, often supplied by texture maps.
Specularity is the correct term for reflection. Normally all surfaces have some element of incident reflection and this setting is usually accompanied by a Roughness setting (in the Standard material this accompanying setting is called Glossiness and works in the opposite direction - 100 % Glossiness is the same as 0 % Roughness).
This setting is usually paired with Specularity and defines how tight the reflection will be. A high roughness will mean that the surface might be reflective but the reflection will be diffused and unrecognizable.
Transparency Weight (Scalar)
This is the distance at which the medium absorption color reaches Transmission Color.
Transparency Color (Color)
The transmission of a material is its effectiveness in transmitting light - how transparent it is - and what color light it transmits through the material it is paired with.
Scattering is a volumetric effect. To work properly, it needs to have Affect Volumetrics turned on in lights. Otherwise, it only absorbs light according to the scattering color.
How quickly light passing inside an object is extinguished. A low value here with indicate greater transparency. Based on Beer’s law of absorption, to give a more realistic and non-linear result.
Scales the Scattering and Absorption values.
Ratio between the amount of forward to backward scattering. With a positive value, more light is scattered from behind the object, with a negative value more light is scattered from in front of the object, relative to the observer.
With this example we have two lights, one in front, one behind our sphere. Both are very strong and the one behind is colored green, the one in front red. With Asymmetry set to 0.5, the light from the green light is scattered forward towards the observer, in this case the camera. When Asymmetry is set to -0.5 the light from the red light behind the camera is reflected toward the observer.
Step Size (Scalar)
This is the volumetric scale for your surface. Larger numbers will render faster but be less defined.
The Index of Refraction determines how much light is bent when it enters a material. Common values include 1.33 for water, 1.42 for olive oil, 1.52 for crown glass, 2.42 for diamond.
Bump Height (Scalar)
Specifies the bump height or “amplitude” of the Bump directional vectors.