Patent Publication Number: US-4836659-A

Title: Lens

Description:
The present invention relates to lenses. More particularly, but not exclusively, it 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 plano-convex) 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. 
     According to a first aspect of the present invention there is provided a convex lens having a geometric axis (as herein before defined) spaced from its optical axis (as herein before defined). 
     Preferably the lens is of plastics material and may be manufactured by an injection moulding process. 
     According to the second aspect of the present invention there is provided a method of manufacturing a convex lens comprising the steps of providing 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, and injecting into the mould cavity through the 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 drawing, in which: 
     FIG. 1 is a cross-section through a bi-convex lens; and 
     FIG. 2 is a front veiw of the lens. 
    
    
     Referring now to the drawings, there is shown a circular bi-convex lens. As can be seen from FIG. 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 FIG. 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 generallly 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 FIG. 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 invention is, of course, not restricted to lenses manufactured by injection moulding but may be used in any lens, whether plastics or glass. 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 FIG. 1 is obviously somewhat exagerated in order to show clearly the difference in edge thickness. 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. 
     
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ANGLE  THICKNESS (mm)                                                     
                     ANGLE     THICKNESS (mm)                             
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 0     1.574          95       3.242                                      
 5     1.580         100       3.376                                      
10     1.597         105       3.507                                      
15     1.626         110       3.636                                      
20     1.666         115       3.760                                      
25     1.717         120       3.880                                      
30     1.779         125       3.994                                      
35     1.850         130       4.101                                      
40     1.932         135       4.201                                      
45     2.022         140       4.292                                      
50     2.120         145       4.374                                      
55     2.226         150       4.447                                      
60     2.339         155       4.510                                      
65     2.458         160       4.561                                      
70     2.582         165       4.602                                      
75     2.710         170       4.632                                      
80     2.841         175       4.649                                      
85     2.974         180       4.655                                      
90     3.108                                                              
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