LuxRender Materials Scatter
From LuxRender Wiki
Scatter is a diffuse material with a surface-scattering effect. In most cases, it is best combined with the null material and homogenous volume in a mix material for rendering things such as clouds, flames, gas halos, corona discharge, etc. When combined with null+homogeneous, it can give the volume more color and an extra scattering "kick" to light as it crosses the boundary of the mesh, mimicking the glow flames (for example) have when viewed edge-on, such as in these photos
This is the only color of the material, it is the color of the reflected and scattered light.
phase function (asymmetry)
The asymmetry value is a number between -1 and +1 that determines if the scattering is primarily forward (same direction as the ray was traveling) or backwards (back towards where the ray came from). Positive values are more forward scattering (best for clear particles), negative values are more backward scattering (best for opaque particles). 0 is isotropic, meaning the light is scattered evenly in all directions.