By combining four mirror blocks into a pair or periscopes it is possible to produce an in-line unit where the beam in and beam out are collinear. Put a ¼-wave phase retarder into the first mirror block and normal zerophase mirrors in the other three and you have an in-line circular polariser unit. The C-CM-PR4 has to be orientated correctly with respect to the linearly polarised beam entering the device; 45° to the S and P planes
Usually the plane of polarisation will be vertical or horizontal so that the unit itself will be
tilted over at 45°.