First of all, what is the difference between biased and unbiased rendering?
The discussion of what constitutes unbiased rendering versus biased rendering can get technical pretty quickly.
We want to avoid that, so I’ll try to keep it as basic as possible.
Unbiased – Unbiased renderers like Maxwell, Indigo are typically hailed as “physically accurate” render engines. Although “physically accurate” is something of a misnomer (nothing in CG is truly physically accurate), the term is meant to imply that an unbiased renderer calculates the path of light as accurately as is statistically possible within the confines of current-gen rendering algorithms.
In other words, no systematic error or “bias” is willfully introduced. Any variance will manifest as noise, but given enough time an unbiased renderer will eventually converge on a mathematically “correct” result.
Biased – Biased renderers, on the other hand, make certain concessions in the interest of efficiency. Instead of chugging away until a sound result has been reached, biased renderers will introduce sample bias, and use subtle interpolation or blurring to reduce render time. Biased renderers can typically be fine-tuned more than their unbiased counterparts, and in the right hands a biased renderer can potentially produce a thoroughly accurate result with significantly less CPU time.
http://3d.about.com/od/A-Guide-To-3D-Software/tp/Rendering-Terminology-Explained.htm
