Patent GB-710127-A (GB)
Title: Lighting systems and luminaires and refractors for use therein
IPC: F21S2/00
Filed: 1951-09-17
Abstract: 710,127. Light refractors; street lighting. HOLOPHANE, Ltd. (Holophane Co., Inc.). Sept. 17, 1951, No. 21752/51. Classes 75 (2) and 75 (4) The invention consists in (1) a system for lighting an extended horizontal surface of predetermined width (2) a luminaire for use in the system producing a light distribution concentrated into one half of a right circular cone and (3) a refractor for the luminaire. (1) The system comprises a row of aligned luminaires disposed at a common elevation above the surface to be illuminated and facing across the width of the surface from the side thereof, the spacing between luminaires being twice the width of the surface whereby adjacent illuminations meet without overlapping, each luminaire having an inwardly concave refractor of greater radius than depth for intercepting a hemisphere of direct light from a concentrated light source and provided with refracting prisms intercepting light at angles farther from the nadir than the highest angled undeviated light which can fall on the surface and bending such light into the direction of the undeviated light, the power of the prisms above the horizontal plane through the source decreasing at least stepwise with increase of azimuth angle from the median vertical plane to compensate in part for decrease in vertical angle of corresponding incident light, the refracting power of the prisms below said horizontal plane increasing at least stepwise with increase of azimuth angle from the medianplane to compensate at least in part for increase of vertical angle of corresponding incident light. (2) The luminaire comprises an inwardly concave refractor of greater radius than depth intercepting a hemisphere of light to one side of a vertical plane and having juxtaposed prism systems refracting downwardly all light above a predetermined angle from the nadir into substantially uniform angles throughout all azimuth angles, the prisms varying vertically and at least step-wise horizontally in accordance with the variation of the vertical angle of the light falling thereon. (3) The refractor is in the form of a spherical segment terminating in a vertical plane and is arranged with the centre of the sphere at the level of a point-light source but laterally displaced therefrom so that the refractor intercepts a hemisphere of light from the source, the refractor having juxtaposed horizontal prisms of varying refracting power vertically to deviate direct light above a predetermined angle from the nadir into substantial parallelism at the predetermined angle in the median vertical plane, the power of the prisms above the horizontal at wide azimuth angles being less than at less azimuth angles and of those below the horizontal at wide azimuth angles being greater than at less azimuth angles, the differences being to compensate for the variation of vertical angle with azimuth angle at corresponding elevations. As shown the luminaire comprises a drum-shaped metal housing 20 adapted to be secured against the side of a building and having a light emitting aperture on its front wall. A lens ring 28 hinged to the front wall supports two pressed-glass refractors 31, 32 in the form of segments of concentric spheres with centres at SC, the segments being less than hemispheres. An incandescent or mercury lamp 26 is supported within the housing so that the centre LC of the light is in the vertical plane of the edges of the refractors. The inner refractor has an outer face with horizontal prisms 32c on its upper half and a smooth lower half and its inner face, Fig. 7, has a band of horizontal prisms 32a below the horizontal extending wholly across the face and, on its upper half, horizontal prisms 32b extending between meridians of 60 degrees on either side of the central meridian. The outer refractor has an outer surface wholly covered with shallow vertical flutes 33e and an inner surface, Fig. 9, provided with horizontal prisms 33a extending from the top to the equator between meridians 331 at 60 degrees on either side of the central meridian and extending for a distance below the equator between meridians 33<SP>11</SP> at 45 degrees from the central meridian. The prisms 33a are flanked above the equator by less powerful horizontal prisms 33b and below the equator by more powerful horizontal prisms 33c. The lower part below the horizontal prisms is provided with horizontal flutes 33d. In the central meridian plane, Fig. 2, the horizontal prisms are located above a ray 50 at 63 degrees to the nadir and bend the light into substantial parallelism with the ray 50, the prisms for this purpose varying in refractive power vertically. Since the light centre LC and the centre SC of the refractor do not coincide the vertical angles of the rays intercepted by any one horizontal prism varies with azimuthal angle from the central meridian plane and although it would be possible to make the power of the prisms vary in azimuth in a continuous fashion exactly to compensate for the variation of vertical angle it is sufficient to make each prism of uniform power and provide a step-like compensation by means of the flanking prisms 33b, 33c. A reflector 27 is mounted behind the lamp to receive light over the hemisphere opposite to the refractor and to redirect this light clear of the source. The reflector comprises an outer part of toroidal shape extending from the mouth to a break 100 and an inner part also of toroidal shape extending from the break 100 to a flate rear part. A row of luminaires may be provided on each side of a street, the distance between the rows being greater than the distance between the curb lines and the luminaires being opposite each other or staggered. The remote curb line is at a uniform angle below the luminaires equal to the highest angle undeviated light and the spacing between the luminaires equals twice the width of the street.