W. Jarosz, D. Nowrouzezahrai, R. Thomas, P. Sloan, and M. Zwicker, Progressive photon beams, ACM Transactions on Graphics, vol.30, issue.61 1, 2011.

J. Spanier, Two Pairs of Families of Estimators for Transport Problems, SIAM Journal on Applied Mathematics, vol.14, issue.4, pp.702-713, 1966.
DOI : 10.1137/0114059