A laser system comprising a laser configured to emit a laser beam wherein the laser beam is linearly polarized in a polarization plane and an optical assembly comprising a partial reflector having a refractive index and comprising a partially reflective surface. The partially reflective surface is arranged to receive the laser beam at an angle of incidence which lies in a plane of incidence and reflect a portion of the laser beam such that the reflected portion is output from the optical assembly. The partially reflective surface is arranged such that the plane of incidence forms a polarization angle with the polarization plane of the laser beam and the laser beam includes a p-polarized component and an s-polarized component. The angle of incidence and the polarization angle are arranged such that a fraction of the laser beam which is output from the optical assembly, following reflection from the partially reflective surface, is substantially invariant with changes in at least one of the temperature of the partial reflector and a thickness of a contamination layer disposed on the partially reflective surface. The partially reflective surface is arranged to reflect a fraction of the laser beam which is greater than or equal to approximately 0.5% of the laser beam which is incident on the partially reflective surface.