Russian Roulette

Approximately 50% of CPU time is consumed to track secondary and higher order photons and the electrons they set in motion. It is possible to remove a part of these photons by Russian Roulette. That is, such photons are simulated only if a uniformly distributed random number between $0$ and $1$ is smaller than the value $1/f_{weight}$ specified by the user. In this case the weight of the photon must be increased by the factor $f_{weight}$. Application of Russian Roulette to scattered photons reduces the CPU time but introduces an additional source of variance (because of varying weights). It is therefore not a priori clear whether this technique increases the efficiency.