using the previous figure this distance d is given by the following relation where e is the thickness of the glass plate and u03b81 and u03b82 are the air plate and plate air angles of incidence respectively as shown on fig2 b illustrating the deviation of a beam through a glass plate these glass plates are fixed at their bases to a motor 13 the axis of which is preferably parallel to the z optical axis of the system the height of these plates is equal to or similar to the height of the light valve it is possible to fix several sets of three glass plates in order to reduce its rotation frequency for example for a device using three glass plates the motor frequency may be 50 hz with three plates covering one disk each plate occupying an angle of 120 u00b0 and for a device using nine plates the angle occupied by each plate will be 40 u00b0 and its frequency could be 16 66 hz the motor frequency in other words the frequency at which each plate passes in front of the light valve must be synchronous with the line and frame frequency of the light valve and the addressing circuit multiplexer 27 after passing through the glass plates the.