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The manual says :
lwo2rayslimit
Can prevent problems with LW carpaint material. lwo2rayslimit 1,0.9; works as follows: rays above recursion level 1 of node evaluation will be fired only in 90% (0.9) tries. It prevents for very depth recursion (which can happend with car paint material)

And my question is : is it useful only with carpaint, or should it be used with others like conductor?

brownie wrote:The manual says :
lwo2rayslimit
Can prevent problems with LW carpaint material. lwo2rayslimit 1,0.9; works as follows: rays above recursion level 1 of node evaluation will be fired only in 90% (0.9) tries. It prevents for very depth recursion (which can happend with car paint material)

And my question is : is it useful only with carpaint, or should it be used with others like conductor?

Yes it can be useful with other nodes too. If you fins photon shooting phase taking very long or hanging then try this command.