IMPROVED LENS

Abstract: A convex lens has a geometric axis, defined to be an axis passing through the lens at a central point normally to the general plane of the lens, spaced from its optical axis defined to be an axis passing through the lens between its focal points. The lens may be manufactured from a mould, at least one concave surface of which has a centre point spaced from a deepest point, said mould having a feed point at an edge point substantially in a plane extending from said centre point through said deepest point to the edge of the concave surface. A transparent plastics material may then be injected into the mould cavity through the feed point.

Description: The present invention relates to lenses produced from plastics material by an injection moulding process. It is well known to produce lenses by injection moulding, but one difficulty has always been apparent. A convex lens (which term includes bi-convex and planoconvex) is by its nature, thicker at the centre than at its edges. In order to feed the plastics material in the injection moulding process, a feed point or gate must be provided at an edge so as not to impair the optical surfaces of the lens. The injection process feed point is of finite thickness and in fact needs to be sufficiently large to assure good lens quality. In order to accommodate the size of the feed point, it has hitherto been necessary to increase the thickness of the lens overall. It is an object of the present invention to provide a design of lens which enables a feed point of conventional size while reducing the overall thickness of the lens. By decreasing the thickness, the lens uses less material; is lighter, which may be important for hand-held magnifying lenses, and most importantly, it allows a decrease in the manufacturing time and hence the manufacturing cost. Looking at, or rather through, the face of a lens, it may be circular, elliptical, square, rectangular or occasionally any other shape. Whatever the shape there will be a central point. The geometric axis is defined as an axis passing through the lens at this central point normally to the general plane of the lens. The optical axis is defined as an axis passing through the lens between its focal points. It is known from CH-B378559 to provide non symmetrical lenses for use in a multiple lens system. According to the present invention there is provided a method of manufacturing a convex lens comprising the steps of providing a mould the inner surface of which has the shape of the lens to be manufactured, said mould having a geometric axis corresponding to the geometric axis of the lens to be made, which axis is defined as the axis passing through the lens at the central point of the lens normally to the general plane of the lens, characterised in that at least one concave surface of said mould has a deepest point which is spaced from said geometrical axis, said mould having a feed point located at a side edge of the mould, said feed point being located at the side edge of the mould that is located nearer to said deepest point than to said geometric axis, and injecting into the mould cavity through said feed point a transparent plastics material. An embodiment of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which: FIGURE 1 is a cross-section through a bi-convex lens; and FIGURE 2 is a front view of the lens. Referring now to the drawings, there is shown a circular bi-convex lens. As can be seem from Figure 2, the centre of the lens is marked by point C and an axis passing through this point C and projecting out of the page, or as shown in Figure 1, is termed the geometric axis. It is conventional in lenses that this axis is also the optical axis, by which is meant the axis joining the focal points of the lens. Conventionally, these two axes are coincident and the lens is generally symmetrical about point C. However, in order to avoid or minimise distortion, the lens need only be symmetrical about the optical axis and it has been found that if the geometric axis is displaced from the line of the optical axis, no undue distortion results. Since the thickness of the lens is determined by the thickness of the maximum edge, which must accommodate the injection process feed point, the lens may be overall made thinner. This is shown clearly in Figure 1. The feed point should always be on that size of the lens nearer the optical axis than the geometric axis. Lenses embodying the invention may be made by conventional moulding techniques, with the exception that the mould surfaces are shaped to correspond with these offset lens surfaces. Since the lens is less thick, the manufacturing time is reduced and hence the manufacturing costs are reduced. The surfaces of the lens may be spherical or aspheric, in either case being symmetrical about the optical axis. The amount of offset between the axes may be determined by the size of the maximum edge thickness required, the curvature of the lens, the overall dimensions of the lens, and the degree of thickness reduction desired. Quite small offsets between the axes can make considerable differences to the overall lens thickness. The cross-section of lens shown in Figure 1 is obviously somewhat exagerated in order to show clearly the difference in edge thicknesses. In fact, in two presently preferred examples, the minimum edge thickness is 1.574 mm and the maximum edge thickness is 4.655 mm, in each case for a lens of overall diameter 84 mm. In one lens the offset distance is 1.9445 mm, in another 2.56 mm. In the former case, figures for edge thickness at various points on the circumference (as defined by the angular position relative to the minimum thickness point at 0°) are given in the Table below.

Claims: A method of manufacturing a convex lens comprising the steps of providing a mould the inner surface of which has the shape of the lens to be manufactured, said mould having a geometric axis corresponding to the geometric axis of the lens to be made, which axis is defined as the axis passing through the lens at the central point of the lens normally to the general plane of the lens, characterised in that at least one concave surface of said mould has a deepest point which is spaced from said geometrical axis, said mould having a feed point located at a side edge of the mould, said feed point being located at the side edge of the mould that is located nearer to said deepest point than to said geometric axis, and injecting into the mould cavity through said feed point a transparent plastics material.

Inventors: FISHER, PAUL JAMES, PASCO, IAN KENNETH, ROOTE, ROBERT JOHN, SMITHERS, IAN JOHN
Applicants: COMBINED OPTICAL INDUSTRIES LIMITED
Application year: 1988
Main classification: G02B3
IPC codes: G02B3