Patent GB-1233333-A (GB)
IPC: G02B5/10
Filed: 1969-08-20
Abstract: 1,233,333. Lamp reflectors. S. L. PERRET. 20 Aug., 1969 [21 Aug., 1968], No. 41492/69. Heading F4R. [Also in Divisions G2 and H4] A reflector is defined by a system of conies C, C 1  . . . C n  which are described relative to a rectangular cartesian co-ordinate system OXYZ in the following manner: (a) the conic C is drawn in the OXY plane Q (Fig. 1) and has a focus F and a vertex S both lying on the axis OY and a chord AB lying along the axis OY; (b) a conic D (Fig. 2) of a second system D, D 1  . . . D n  is drawn in the OYZ plane R so as to pass through the point S, this conic having a first focus F coincident with the focus of the conic C and a second focus F<SP>1</SP> which defines with the axis OX a plane T perpendicular to the plane R and forming a given dihedral angle with the plane Q; (c) the axes OX, OZ are rotated through a small angle in the OXZ plane P to positions OX 1 , OZ 1  and the conic D is simultaneously rotated through the same angle about the axis OY to form the second conic D 1  of the second system; (d) the second conic C 1  of the first system is then drawn through the points A 1 , B 1  on the axis OX 1  which correspond to the new positions of the points A, B, this conic having a vertex S 1  which lies on the conic D 1  and is spaced from the point corresponding to the vertex S. (e) successive conies C 2  ... C n  are drawn by successively rotating the axes OX, OZ and the conic D through further small angles and successively moving the vertices S 2  . . . S n  along the conies D 2  . . . D n . The respective conies of each system are of the same type and may be ellipses, parabolas or hyperbolas, but preferably the first system (c) is composed of parabolas and the second system (D) of ellipses. Rays of light emanating from the focus F and reflected from points on a conic C n  pass through points on a conic G n  of the same type as C n  and located in the plane T n  containing the focus F<SP>1</SP> n  and the axis OX n , this conic passing through the points An, B n  and having its vertex at F<SP>1</SP> n . A lamp having a source at F and a reflector of this type therefore produces a light pattern, on a plane below and parallel to the plane P, having the form of an elongated curved band. In a modification (Figs. 5-7, not shown) the reflector is constructed without rotating the plane R so that each conic of the second system lies in a plane parallel to R. In a further modification the axis FF<SP>1</SP> of the conic D is made to coincide with the axis OY, in which case the focus F<SP>1</SP> may coincide with O. For street lighting purposes a reflective screen (8, Figs. 18, 19, not shown) may be mounted inside the reflector to prevent direct rays from the source from leaving the lamp. The reflector may be formed in two or more sections of different shapes so long as the sections have the same defining parabola at their contiguous edges. Each lamp may be designed to illuminate a particular carriageway and to throw a longer beam in the direction of traffic flow to reduce dazzle. At bends in the road the reflector may be designed to produce a curved strip of light of corresponding shape.