Abstract: Back-projection display system including a screen which can be moved away from or, conversely, placed up against a support such as a wall, for example. A projector located at some distance away, on the ceiling for example, illuminates a back-projection mirror located at 45.degree. in relation to the illumination beam and in relation to the screen. The back-projection mirror itself is also articulated. In addition, the mirror may be made in the form of a holographic mirror.

Abstract: Method and apparatus for separating a mixed frequency light beam, particularly a laser light beam, into its different frequency components by impinging it on a first surface of an optical slab which reflects one component but transmits another, and re-directing the reflected component via a roof prism to a second surface of the slab which is adjacent to and positioned 90.degree. to the first surface, to obtain a reflected beam which is co-directional with the mixed beam, but stripped of the transmitted component.

Abstract: A rectangular prism includes an entrance surface for receiving light, an exit surface from which the light emerges, two reflecting surfaces orthogonal to each other for directing the light ray incident from the entrance surface to the exit surface, and a groove of V-shaped cross-section disposed in the boundary between the entrance surface and the exit surface.

Abstract: An optical signal band-pass filter module permitting a light which is transmitted through a band-pass filter twice so that the central wavelength in a transmittance bandwidth thereof is prevented to be varied.

Abstract: A viewfinder optical system including an objective lens for forming an image of a subject, a rooflike reflecting portion for reflecting an incident bundle of rays so as to deflect the incident bundle of rays through 90.degree., which is provided between the objective lens and the image of the subject formed by the objective lens, a double reflecting portion for twice reflecting a bundle of rays reflected by the rooflike reflecting portion so as to deflect the bundle of rays through 270.degree. and an eyepiece for enlarging the image of the subject.

Abstract: A constant-deviation device is disclosed which comprises a first reflective element in the form of one or more roof prisms and one or more second reflective elements providing one or more reflections. The first and the second reflective elements are mutually adjustable by rotation about one or more axes thereby to set an angle between the incident and the exiting beam. In one or more planes the angle is substantially unaffected by changes in the direction of incidence of the incident beam.