Abstract: A security viewer to be mounted through a non-transparent partition, e.g. a door (7), comprises a housing (1) which extends through the door. The housing includes an inlet aperture (8), a screen (13) and a lens system (9, 10, 11, 12). The lens system forms a real image on the screen (13) of a field of view outside the door. Thus, an observer on the inside of the door can observe the real image without the need to place his eye against the viewer. The viewer is held in place by bezels (18, 19) which are glued in place on opposite sides of the door. The bezel (18) is slidable along the housing (1) to accommodate different door thicknesses.