A phase plug for optimally matching a drive-unit’s diaphragm to the throat of a waveguide is disclosed. The combiner is optically located between a user and a scenery of interest, optically transmitting a scene image of the scenery of interest to the user. The regenerated cellulose can be viscose or lyocell. The reaction plate includes openings for ramp segments of the ramp plate, and the ramp segments extend into the plate openings to maintain angular locations of the segments relative to cooperating rollers of the centrifugal module. The cover defines an opening through which at least a portion of the screen protector is accessible.