A means is described for converting a single laser beam into two beams, of which one is polarised and the other unpolarised, with variable angle separation. This is done by using an optical train containing a polariser, a polarisation beamsplitter, a 45 degrees quartz depolariser and a pair of wedges. This system is said to be relatively insensitive to maladjustments because of the invariant optical characteristics of the prism, polariser and beamsplitter.