In Project 12, the Heyney-Greenstein Phase Function was implemented in the renderer. The Heyney-Greenstein Phase Function (HGPF) is an imperical formula to simulate diffuse and specular reflection for a variety of materials using only two parameters. This function uses these these two parameters stored in each object and takes the incidence angle as input. This provides a much greater flexibility for different materials than using the simpler Lambertian reflection.
Finding little data as to proper values, I rendered an image using a wide range of values. From the image generated, it appears a high g-value seems important to produce a good image. This component supposedly relates directly to the angle where most of the light is leaving. It seems natural that a g-value closer to 1 will produce better pictures where light bounces off at a 90-degree angle, while a 0 value gives very splotchy results. The w-value used in the function scales the function and seems to have less of an effect after tone mapping is appled.
In the image below, 64 spheres are rendered. All spheres share the same color component, but have differing w and g parameters in the HGPF. From left to right, the g-component ranges from zero to one. From top to bottom the w-component ranges from zero to one