Patent GB-667085-A (GB)
Title: Improvements in and relating to street lighting units
IPC: F21S8/00
Filed: 1950-01-25
Abstract: 667,085. Light refractors and reflectors. HOLOPHANE, Ltd. (Holophane Co., Inc.). Jan. 25, 1950. No. 1982/50. Classes 75 (ii) and 75 (iv). In a street lighting fitting comprising an elongated light source mounted within a substantially parabolic reflector closed by a prismatic bowl refractor, the bowl (Fig. 3), is substantially oval in plan and bears on its inner surface a pattern of prism groups A ... K which is symmetrical about the vertical plane 7-7 through the major axis of the bowl and is asymmetrical about the vertical plane through the minor axis of the bowl so as to elevate and deviate laterally the light rays from the reflector. The fitting is mounted above the street with the axis of the elongated source, preferably a mercury arc lamp, and the major axes of the reflector and refractor disposed at right angles to the street. The specular reflector 22 (Fig. 1) is substantially oval in plan and comprises portions of different curvature bounded by the planes 22a, b, c, d. The mid portion is of parabolic contour with the foci of its two parts at a point 21 near the centre of the light source and with axes directed towards the " street " end of the fitting. The end portions are rounded, the curvature being less at the "street" end than at the "house" end. The portions between the end and mid portions are of different elliptical contour. The light source is mounted in the reflector with its centre offset towards the " house end of the fitting and is such that the angle subtended at the reflector by rays from the ends of the source is not greater than 60 degrees and the angle subtended by vertical depth of the source at the nearest point of the reflector is not greater than 6 degrees; the reflector rim shields the source over an angle of between 15 degrees and 30 degrees below the horizontal. In vertical section each of the two sides of the reflector is parabolic with focus at the light centre and with axis sloping downwardly. The mid portions of the reflector produce two major beams between 50 degrees and 60 degrees to the downward vertical and from 130 degrees to 150 degrees in azimuth and the elliptical portions of the reflector converge the light into the same azimuthal directions. The lower portions of the sides of the refractor (Figs. 3, 4, 6, 7) are spherical with centres 41 on opposite sides of the major medial plane 7-7 and preferably above the refractor rim as shown in Fig. 6. The lower portions of the refractor may be toroidal. The centre 48 of the bottom ridge 47 is at the same level as the centres 41 and is slightly offset from the centre of the refractor towards the "street" end thereof. The triangular end portions 49, 50, 51 of the refractor are toroidal in contour with centres lying in the medial plane 7-7. The upper portion of the refractor is of compound curvature and in vertical section it may be circular, the radii of curvature being less than that of the lower portion. All the refractor sections A... K, except section G have prisms for elevating the light or for deviating it laterally or for both as indicated in Fig. 7. The section G, which is at the " street " end of the fitting has light diffusing flutes. The prisms on the section F at the " house" end are of steeper profile than the prisms on the adjoining sections D. The prisms on the sections A, B, C, D refract the light so that it emerges at an angle of 70 degrees to 75 degrees to the downward vertical. The sections J have both vertical and horizontal prisms. The prisms on the lower sections K have active faces curved as shown in Fig. 9. The prisms are formed on the inside of the bowl by a mould and plunger and the mould may have an irregular surface to impress a diffusing pattern on the outside of the bowl. Specification 617,210 is referred to.