Patent GB-700748-A (GB)
Title: Improvements in and relating to luminaires
IPC: F21S8/08
Filed: 1950-06-02
Abstract: 700,748. Street lighting fittings. BRITISH THOMSON-HOUSTON CO., Ltd. June 2, 1950 [June 3, 1949], No. 13848/50. Classes 75 (2) and 75 (4). In a luminaire for street lighting comprising a reflector 1 with an ovate mouth directing light downwardly at a relatively low vertical angle and a refractor 2 closing the ovate mouth of the reflector, the refractor also having an ovate mouth and being ovate in sections parallel to its mouth, a group of prisms is provided on the refractor for raising one portion of the light to higher vertical angle and for raising and redirecting laterally another portion of the light, these portions forming two main beams enclosing an angle of less than 180 degrees opening towards one end of the ovate mouth openings, and another group of prisms is provided at the extreme end of one of the two ends of the refractor for bending, without raising, light rays so as to form a lesser beam with rays laterally redirected towards the main beams. The luminaire is mounted above one side of the roadway with its longitudinal axis crosswise of the roadway and the pattern of light beneath the luminaire consists of a rectangular portion extending longitudinally along the roadway and a triangular portion on the house side. The ends of the rectangular portion subtend an angle of 30 degrees at the point of the roadway beneath the luminaire. The reflector 1 is of generally ovate horizontal configuration, the sides converging more sharply near the rear or house end than at the front or street end. The reflector has a circular upper opening defined by a collar 1b which fits inside a mounting hood 4 to which the reflector is hinged ; a pair of latches 5 hold the reflector in position. An elongated arc discharge lamp 6 or a non-elongated filament lamp is mounted in the reflector so that the centre c of the source is well above the lower rim of the reflector and is off-set longitudinally towards the house end of the luminaire. The reflector in horizontal section, Fig. 8, comprises a central portion AE formed by an ellipse AB with foci at the light centre c and at f1 on the opposite side of the vertical plane of symmetry, a parabola BD with focus at c and axis at 70 degrees to the plane of symmetry and a curve DE composed of a plurality of parabolas with common focus c and axes varying between 70 degrees at D and 60 degrees at E to the plane of symmetry. At the house end beyond the point E the horizontal section is elliptical with foci at c and f2. At the street end the horizontal section comprises a plurality of ellipses each with one focus at c. In vertical section the central portion AE is parabolic with focus at c and axis at 53-56 degrees to the vertical. The vertical section at the house end comprises a lower parabolic or near parabolic portion and a small upper elliptical portion. The vertical section at the street end comprises a lower parabolic portion but at the extreme front the section is elliptical. In a modified form of reflector for use with a filament lamp the central portion in horizontal section is divided by indentations into three distinct sections each comprising ellipses with a common focus at the light centre and secondary foci located so that the beams from the three sections overlap. The refractor 2 is of generally ovate horizontal section similar to the reflector but the side portions of the refractor converge more sharply than the side portions of the reflector. In transverse outline these refractor sides are circular with centres at the level of the top of the reflector and vertically above the lower rim of the reflector. The refractor has a number of prismatic areas, Fig. 7, which are distinct or may be blended into each other to form continuous prisms. The area 20 which extends from the transverse plane through the source to the street-end portion of the refractor and from the rim downwardly for three quarters of the depth of the refractor is provided with horizontal prisms for raising direct and reflected light through approximately 17¢ degrees. The areas 21, 22, 23 midway of the depth of the refractor in its rear quadrants have oblique prisms, the slope of the prisms increasing from front to rear. These prisms raise and turn towards the main beams direct light and light reflected by the front of the reflector. Above the areas of oblique prisms are areas 24 of arcuate prisms blending into the horizontal prisms of area 20. These arcuate prisms raise and turn direct light. The extreme front and rear portions 25, 26 of the refractor have vertical prisms which redirect laterally both direct light and light from an auxiliary reflector 11 disposed above the main reflector the vertical prisms thus forming two lesser beams. The lower portion 27 of the refractor has oblique prisms which converge forwardly and downwardly and which raise and turn towards the main beams both direct light and light reflected by the auxiliary reflector. The auxiliary reflector is mounted on the hood 4 to close the upper opening in the main reflector and directs rays downwardly so that they do not pass through the light source. With an elongated arc lamp as source the auxiliary reflector 11 is of the form shown in Fig. 5 comprising two embossments 11a, 11b which merge into each other along the plane of symmetry of the luminaire. In longitudinal vertical section, Fig. 7, the embossments are elliptical, the rear portions directing light into the lower front portion of the refractor and the front portions directing light to the back end of the main reflector where it is redirected towards the bottom of the refractor. In transverse vertical section the embossments are also elliptical. In horizontal section the embossments are ovate. The centres of the embossments may be depressed. A pair of electromagnets 9, Fig. 1, are mounted in the hood above the auxiliary reflector to control the arc of the discharge lamp. With a non-elongated filament lamp as source the auxiliary reflector is of ovate dished formation and has an upper opening for the passage of the lamp and a circular flanged rim. The horizontal section of the reflector is made up of ellipses and parabolas. Each vertical section of the rear portion of the reflector is made up of a lower parabola and an upper ellipse and each vertical section of the front portion of the reflector is an ellipse. In order to reduce the amount of light transmitted to the house side of the street a further reflector of similar shape to the house end of the main reflector may be mounted below the main reflector at the house end of the luminaire. The surfaces of the reflectors may be specular or etched. The reflectors may be asymmetrical with respect to the major vertical plane of the luminaire wherebv each of the main beams is at a different angle to the street centre line. Specification 524,334 is referred to.