PRT quality based on the band numbers of SH

It is said that SH is good to approximate the low frequency lights like diffuse indirect lighting.

In Diffuse Precomputed Radiance Transfer (Jan Kautz) in SIGGRAPH 2005, there was a good figure which shows the quality of PRT based on the band numbers of the SH.

In this figure, the term degree specify the light-source degree. And if the light degree is small, more numbers of SH bands are needed to approximate them.

• Reference
  • http://www0.cs.ucl.ac.uk/staff/j.kautz/PRTCourse/Diffuse.pdf