Abstract: An optical module for head mounted displays and other applications comprises: an optical substrate and optical superstrate having inter-engaging ridged surfaces. A reflective layer is formed on at least one of the surfaces. An index matching material may be located between the surfaces. A region that receives a projected image, from a projector, directs rays launched from the projector onto the ridged surfaces, so that in use a viewer perceives an augmented image. The augmented image comprising reflected rays from the projector and transmitted rays from an object located on an opposite side of the module to that of the viewer.