Principle of the correction. Before traversing the birefringent plates, each beam is plagued by a differential delay between the horizontal and vertical polarizations. This differential delay can be canceled by suitably adjusting the inclination of each birefringent plate. The beam-to-beam delay is generally affected, but is easy to equalize once birefringence is taken care of. This figure is drawn as if the correction is performed after the unwanted birefringence has occurred; in the case of Pionier, the correction is in fact made upstream of the point where birefringence occurs.