Patent GB-775007-A (GB)
Title: Improvements in or relating to lenses
IPC: G02C7/02
Filed: 1953-09-21
Abstract: 775,007. Spectacle lenses. JEFFREE, J. H. Sept. 16, 1954 [Sept. 21, 1953], No: 25990/53. Class 97(1) In a multifocal lens, the positive power of which increases gradually from the top to bottom in the vertical meridianal plane, the horizontal curvature is greater than the vertical curvature for each point on both its surfaces. Preferably both surfaces of the lens are horizontally concave towards the eye and the radii of horizontal curvature, at the middle of the lens, are approximately equal to the distance between the lens and the centre 11 of rotation of the eye. The thickness of the lens may also increase from top to bottom. The surfaces of the lens may be surfaces of revolution, the axes of which lie in the vertical meridianal plane and at an angle between 40 and 90 degrees to the horizontal. As shown in Fig. 2 the horizontal curvature of both surfaces increases from top to bottom of the lens, the sections 32, 33 and 34 being taken at 29, 30 and 31 respectively An arc 22 of predetermined radius, drawn through points 16 and 21, on normals 13 and 19, at points 14 and 18 on curves 12 and 17, intersects the curves 12 and 17 at the poles 23, 24 of the surface of revolution. This construction determines the axes of revolution which thus pass through the poles 23, 24 and the respective centres 15, 20 of horizontal curvature at points 14 and 18. Point 16 is the virtual image of the eye centre 11 produced by refraction in the horizontal plane and determined by the horizontal curvature at 14. Point 21 and the curvature of arc 22 are determined from the horizontal and mean vertical curvatures of the lens surfaces by the formulµ given. If the curves 12, 17 which are arbitrarily determined, are far from circular, it is preferable to take as the poles the interesections 25, 26 of the arc 22 with fictitious sections 27, 28 which are circular and of curvatures equal to the mean curvatures of curves 12, 17 respectively. Either curve 12 or 17 may be a straight line so that the resulting lens surface is conical. It is stated that the location of the axis of revolution in this was ensures that the performance of the lens in the meridianal section is, to a first approximation, the same as that outside it, so that it is only necessary to calculate the performance for this section. When no astigmatic correction has to be added to the surface of revolution the dies for mouding the lenses can be made by known methods. When an astigmatic correction is necessary, a cylindrical lens made in flexible material, such as moist gelatine is cemented to the surface and the die may be made from the resulting corrected surface by electroforming or similar processes. The geometrical construction which determines the general form of the cylindrical lens is described. In some cases the astigmatic correction may be introduced by displacing one die relative to the other. The lens according to the invention may have joined to its lower edge a lens segment of constant power in the vertical meridinal plane and the power may be markedly greater in this lower part than in the upper part. It is stated that there may be no discontinuity in the apparent field of view at the joint if the surfaces of the upper part approximate to surfaces of revolution and the surfaces of the lower part approximate to spheres with centres on the axes of the surfaces of revolution, or one centre may be displaced horizontally from its respective axis to compensate for the discontinuity approximately.