GB873833A - Improvements in or relating to optical beam-splitting arrangements - Google Patents

Abstract

873,833. Optical beam splitting devices. PYE Ltd. Dec. 2, 1957 [Aug. 31, 1956], No. 26711/56. Class 97(1) An optical beam splitting arrangement comprises a block of transparent material consisting of at least two prisms in surface contact, there being provided at the or at least one of the interfaces between the prisms a layer of dichroic material capable of reflecting light of one colour while transmitting light of another colour or colours, said block being capable of splitting white light incident thereupon into two or more different colour components, a lens of adjustable focus for focusing the incident light beam so that irrespective of the distance of the object or source from the arrangement the light from said lens passes through the block as a parallel beam, and an exit lens for focusing each component of the split beam and set at infinity focus. As shown in Fig. 3 of the Provisional Specification, an afocal adaptor 9 is placed in front of the transparent block made up of four prisms with dichroic interfaces so that parallel light rays incident on the block are split up into the three primary colours and are respectively focused by exit lenses 5, 6 and 7, each set at infinity focus. Interference filters 10, 11 and 12 may be provided to limit the band width of the split beam incident on the separate exit lenses and a quarter-wave plate 13 may be used to circularly polarize the incoming light to avoid polarization changes across the dichroic layers. Marginal masking of the three exit faces of the block may be provided. One form of afocal adaptor 9 is shown in Fig. 3 of the Complete Specification and comprises a fixed lens 19 and a lens 18 which is carried in a front mount 21 externally screwthreaded to engage a tube 23, rotation of which in a housing 20 results in axial movement of the lens 18. The adaptor is mounted in a housing 25 containing the transparent prism block 26 held between retaining members 27 and 28 of which the member 28 is slidable upon a base plate 31 for adjusting the position of the prism block 26 as a whole. The exit lenses may be mounted in a turret containing lenses of different focal lengths and a continuously variable angle of view can be obtained by using a lens attachment to the afocal adaptor of the zoom type. The beam splitting arrangement may be employed in a colour television camera having three pick-up tubes. Specification 759,063 is referred to.