Patent GB-837607-A (GB)
Title: Improvements in or relating to refractor members for use in lighting fittings for street lighting and like purposes
Inventor: BEGGS STANLEY SUTTON
IPC: F21V5/00
Filed: 1956-09-14
Abstract: 837,607. Street lamps. GENERAL ELECTRIC CO. Ltd. Dec. 4, 1957 [Sept. 14, 1956], No. 28214/56. Class 75(2) A refractory member for use in lighting fittings suitable for street and the like lighting and of the type having at least two sets of prismatic light directing prisms, extending in radial planes on a surface of revolution 1, and arranged each set on a differend side of a central radial plane OC and designed so as to concentrate about each of two preferred radial planes OM, . ON<SP>1</SP> making an angle A e.g. COM with the same direction of and on each side of the central plane OC of between 60 degrees and 90 degrees inclusive, light which is incident on the prisms when a light source is placed on the axis O of said surface, has the tip angle P, Fig. 2, of the operative apex of the prisms in a set the same for substantially all the prisms or has one value P for substantially all the prisms within the region of angle A and a different value Ps for substantially all prisms of the set which lie outside the angle A, i.e. 180 degrees-A, where P and Ps lie between 60 degrees and 100 degrees and the prisms are so oriented that the angle T, Fig. 2, between the internal bisector FE of the tip angle and the prism plane OX (the radial plane through the centre of the prism base) is related to the angle A and to the angle # between the prism plane and the said direction of said central plane substantially in accordance with the formula T=1/3 # [(#/A )<SP>2</SP> - 1 + 3(90‹-P/2)/A] where A=A or (180‹-A); #=# or (180‹-#) P=P or Ps (and P may equal Ps) according as the prism lies within or without the angle A, and the sense of the angle T is such that the tip of the prism lies between the prism plane and the preferred radial plane for the set. Deviation from the above formula which are included in the word "substantially" are given in the Specification. To produce such a refractor by moulding, for each set of prisms most or all of the prism recesses are cut in the plunger mould surface by the use of a single cutting tool of tip angle equal to P or by use of a first cutting of tip angle P for the region of angle A and a second cutting tool of tip angle Ps for the supplementary region of angle 180-A, the cutting tool being held at progressively changing inclinations during the cutting of successive prism recesses for the set so that the angle T<SP>1</SP> corresponding to the angle T of the prism to be formed is related to the angle A for the refractor member to be made and the angle #<SP>1</SP> coresponding to # of the prism to be made substantially by the formula T<SP>1</SP>=1/3 #<SP>1</SP> [(#<SP>1</SP>/A)<SP>2</SP>-1+3 (90-P/2)/A] where A = A or (180-A) #<SP>1</SP> = #<SP>1</SP> or (180-#<SP>1</SP>) P =P or Ps according as the tip of the tool lies within or without the angle A and the sense of the angle T<SP>1</SP> is such that when the prism recess is cut the tip of the tool lies between the prism plane OX and the radial plane. OM. In an example the tip angle of a single cutter used is 90 degrees the axis of the tool being held in a fixed position and the mould plunger traversed relative to the tool so as to cut the prism recesses, the plunger being presented to the tool which is given a progressive tilt about the line of cut when the plunger is rotated from one. cutting position to the next so that the line of the tool corresponding to the bisector EF of the prism' angle assumes the required angles of inclination to the radial planes through the axis of the plunger and the centre points of the mould surface elements to be cut. Where'the inclination of the cutter requires an undercut prism face in compliance with the formula for T<SP>1</SP> the cutter is fed with its axis transverse to the radius so that the secondary face of the prism cavity is cut by the tip of the tool and not by a face of the tool and the undercut face avoided, the substitute face being relatively short and formed at right angles to the mould surface and the plunger can be withdrawn without damaging the corresponding surface of the moulded refractor. Specifications 818,401 and 820,868 are referred to.