GB650352A - Improvements in light projectors - Google Patents

Abstract

650,352. Light projectors. HOLOPHANE, Ltd., and ENGLISH, S. Sept. 7, 1948, No. 23524. [Classes 75(ii) and 75(iv)] A light projector for floodlighting or street lighting comprises a composite reflector having a central part 14 of elliptical section with its near focus F at the light source and an outer part 15 of parabolic section with its focus also at the light source and a lens 18 equal in width to the elliptical part of the reflector and centred at the remote focus P of the elliptical part of the reflector. The projector may be trough-shaped to accommodate an elongated fluorescent tube or it may be circular for use with an incandescent lamp in which case an aperture is provided at the top for a lamp holder. The converging lens 18 is integral with or attached to a preferably flat glass or transparent plastic plate 17 closing the mouth of the reflector and is formed with a series of prisms which are either straight and parallel to the the light source when the projector is trough-shaped or circular when the projector is circular. In either case the apical angles of the prisms increase from the outermost prism 18a to' the central prism. Rays striking the lens directly are refracted to form a beam 62-70 which may be made parallel by arranging the focus of the lens to lie at the light source; rays reflected by the central elliptical part 14 of the reflector pass through the centre of the lens where the prisms either cause only slight deviation or are omitted and the rays are transmitted in diverging directions 84-90; rays reflected by the outer parabolic part 15 of the reflector pass undeviated through the outer part 21 of the closure plate 17 to form a parallel beam 76-82 of annular section or two distinct parallel beams according to whether the fitting is circular or troughshaped; and rays which strike directly the outer part 21 of the closure plate pass therethrough with slight displacement and form a divergent beam 72, 74. Diffusion of the various beams may be effected by forming the prisms of the lens 18 with curved surfaces or by providing flutes on the inside of the lens 18 and by forming convex or concave flutes on the outer part 21 of the closure plate or by stippling the outer part 21 or the whole of the closure plate. The closure plate may carry parallel prisms to deviate laterally the emitted light. The reflector may be made of prismatic glassware as described in Specification 315,114, [Class 75(ii)], or it may be specular and made of metal. A street lighting fitting may be formed by mounting two of the above described projectors and a third lighting unit of a different kind giving a wide beam, within a single lantern framework, the two projectors directing beams along the street in opposite directions and the third unit illuminating the street area below the lantern. The axis of the projectors may be tilted to divert the beams below the horizontal or the cover plates of the lantern may be provided with horizontal prisms deviating the beams downwardly.