1. Distribute photons more evenly, which means each photon carries approximately the same power.
2. Use disc(ellipsoid) instead of sphere to locate nearest photons, to reduce false photons in corners.
3. Use differential checking to stop adding photons when the estimate is either constantly increasing or decreasing.
4. Use a initial search radius with very low value, if the value turns out to be too low, another search is performed with a higher maximum radius[Kopp99].
