Abstract:
A progressive spectacle lens design has clear focal properties in the entire lens, and progressive variation in focal properties from the center of the lens down into a reading area. All current conventional progressive lenses have areas of distortion to the side of the reading area. The present invention provides undistorted optical viewing throughout the entire lens, although the focal length varies.

Description:
FIELD OF THE INVENTION 
     The present invention relates to wearable lenses, and more particularly, the invention relates to multifocal wearable lenses having reduced distortion. 
     BACKGROUND OF THE INVENTION 
     Progressive lenses are lenses typically used in eyeglasses to assist wearers who have difficulty with accommodation. The lenses are typically divided into upper and lower portions, wherein the upper portion is designed to improve long-distance vision, and the lower portion is designed to improve intermediate and short distance vision. The lower portion of a progressive lens, the progressive portion, generally comprises a gradually reducing focal length relative to the upper, long-distance portion. 
     A common problem with progressive lenses is that the vision through the sides of the progressive portion of the lens is distorted, and can cause discomfort and headaches for the wearer. The distortion can cause discomfort for the wearer of the lens, particularly when the wearer moves and the wearer senses the kinetic distortion. 
     Several types of lens have been devised with the intention of minimizing the problem of distortion. 
     U.S. Pat. No. 5,455,642 (Kato) discloses a progressive lens having a radius of curvature that varies along the vertical direction. While Kato discloses that the lens provides a wider field of view for astigmatism, relative to lenses of the prior art, the two lower areas of distortion remain a problem for the wearer of the lens. 
     U.S. Pat. No. 4,426,139 (van Ligten) discloses a progressive lens with a near vision portion, an intermediate vision portion and a far vision portion, employing a cosine function in determining the lens curvature in the transition corridor between the far and near vision portions, so as to reduce the distortion in the peripheral area of the lens. 
     However there still exists an ongoing need for a lens construction that provides the wearer with an undistorted view through regions of the lens corresponding generally to the wearer&#39;s peripheral vision. 
     SUMMARY OF THE INVENTION 
     In a first aspect, the present invention provides a multifocal ophthalmic lens having a distance optical center, comprising a first portion having focal properties, said first focal properties being substantially constant throughout said first portion; and a second portion having second focal properties extending downwardly from a location adjacent the distance optical center and having a focal length that varies progressively downwardly in said second portion; and two regions on either side of said second portion, each of said regions having focal properties selected to avoid optical distortion. 
     The regions, on either side of the second portion, are in conventional lens regions of visual discomfort due to varying focal properties leading to distortion. In the present invention these regions have constant focal properties and these can be one of: the same as the first portion; and different from the first portion. It is preferred for these two regions to have constant focal properties. 
     In a further aspect, the invention relates to a method of construction of lenses for a wearer with a field of view without regions of potential visual discomfort, wherein the method comprises the steps of: 
     constructing a long-distance lens piece; 
     constructing a progressive lens piece that mates with the first lens piece along peripheral edges of both lens pieces; 
     mating the lens pieces such that the progressive lens piece is substantially outside the regions of potential visual discomfort. 
     Preferably this technique is used simply to produce a master lens, for creating a mold, with final, production lens being molded in one piece. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: 
     FIG. 1 is a plan view of a lens in accordance with a first embodiment of the present invention; 
     FIG. 2 is an isometric exploded view of the lens in FIG. 1; 
     FIG. 3 a  is a side elevation cross-sectional view of a joint between portions of the lens in FIGS. 1 and 2; 
     FIG. 3 b  is a side elevation cross-sectional view of an alternate joint to that shown in FIG. 3 a;    
     FIG. 4 is a plan view of a lens in accordance with a second embodiment of the present invention; 
     FIG. 5 is an isometric exploded view of the lens in FIG. 4; and 
     FIG. 6 is an elevation view of an apparatus for making a lens in accordance with a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference is first made to FIG. 1, which illustrates a multifocal lens  10  made in accordance with a first embodiment of the present invention, and which will be used for the purposes of describing the operational aspects of the invention. Lens  10  is for use in eyewear, such as eyeglasses or monocles and the like, providing the wearer with improved vision at all ranges of distance: long-distance, intermediate-distance and short-distance. 
     Lens  10  comprises a long-distance lens piece  12  and a progressive lens piece  14  and has a distance optical center  11 . The progressive lens piece  14  extends from a position adjacent the optical center  11  and, in use, provides a reading area or zone. All progressive lenses include two critical regions  16  and  18  which are located on the sides of the lenses, below center and which are delineated by dashed lines  20  and  22 . Having either a progressive region or a transition region of a lens anywhere within critical regions  16  and  18 , causes a distortion in images seen through critical regions  16  and  18 , and can therefore cause discomfort and difficulties for the wearer. Critical regions  16  and  18  are also known as regions of potential for visual discomfort 
     While regions  16  and  18  are high distortion areas in typical prior art lenses, regions  16  and  18  are low distortion areas in lenses in accordance with the present invention such as lens  10 , because lens  10  is made so that regions  16  and  18  are outside of progressive piece  14 , and have the same focal properties as long-distance piece  12 . 
     Reference is now made to FIG.  2 . Long-distance lens piece  12  has front and rear surfaces  24  and  26  which together have focal properties for viewing objects at a relatively long distance. Lens piece  12  is surrounded by an outer edge  28 , which includes a mating portion  30  for mating with progressive lens piece  14 . A groove  32  for holding an O-ring  34 , extends along the length of outer edge  28 , outside of mating portion  30 . 
     Progressive lens piece  14  has front and rear surfaces  36  and  38  which together have a range of focal properties for viewing objects at intermediate and short distances. Similarly to lens piece  12 , lens piece  14  is surrounded by an outer edge  40  which includes a mating portion  42  for mating with mating portion  30  of long-distance lens piece  12 . A groove  44 , which aligns with groove  32  on lens piece  12  for holding O-ring  34 , extends along the portion of outer edge  40  outside mating portion  42 . Mating portion  30  of lens piece  12  may be made by first making a standard lens piece that is initially round, and then cutting out a portion to form lens piece  12 . The progressive lens piece  14  can similarly be cut from a larger piece, with corresponding properties. The “cutting out” step may be accomplished using mechanical means, or by a waterjet cutter or a laser cutter, while ensuring that the lens piece is not harmed by the cutting process itself. 
     FIGS. 3 a  and  3   b  illustrate different methods of joining lens pieces  12  and  14  together. As shown in FIG. 3 a , mating portions  30  and  42  are simple edges that are aligned with radial lines  46  from the eye  48  of the wearer of lens  10  so as to minimize interference with the field of view of the wearer. An adhesive may also be used to bond mating lens pieces  12  and  14  together, providing strength to the joint. Alternately, as shown in FIG. 3 b , mating portions  30  and  42  may contain a mechanical joint such as a tongue-and-groove, whereby mating portions  30  and  42  have mating male and female portions  50  and  52  which may also be reinforced with an adhesive. As shown in FIGS. 3 a  and  3   b , transparent films  54  and  56  may cover the front and rear surfaces  24 ,  26 ,  36  and  38  of lens pieces  12  and  14  to provide further strength at the joint. Transparent films  54  and  56  may be used with any type of mating portions  30  and  42 . Transparent films  54  and  56  can be applied in any suitable way and could, for example, comprise heat shrink films and/or could be applied by a vacuum technique. The use of lens portions with different focal lengths will inevitably generate sagittal differences where the different lens portions meet. The use of known manufacturing techniques would smooth these out, without significantly affecting the optical properties. Note that in general the rear surface will be smooth and often will be surfaced to adjust for the overall prescription for a particular individual. 
     Referring back to FIG. 2, the O-ring  34  is fitted into grooves  32  and  44  to surround lens  10  and help hold lens  10  together. Reference is now made to FIG. 4, which shows a lens  100  in accordance with another preferred embodiment of the present invention. Lens  100  is a progressive lens, and comprises a primary lens piece  102 , a nose-side lens piece  104  and an ear-side lens-piece  106 . Primary lens piece  102  contains a long-distance focal region  110  in the upper portion and a progressive focal region  112  in the lower central portion. 
     Critical regions  16  and  18 , which are defined by dashed lines  20  and  22 , are the areas that, as discussed above, typically have high distortion and cause discomfort to the wearer because of their peripheral location in the field of view. Distortion is eliminated in critical regions  16  and  18  by making nose-side lens piece  104  and ear-side lens piece  106  with the constant focal properties and preferably the same focal properties as long distance lens portion  110  so that no discomfort is caused to the wearer, and also making them slightly larger than areas that can cause discomfort and distortion. All of the critical regions  16 ,  18  and the pieces  104  and  106  are partially defined by radii of the lens  100  and terminate short of the center of the lens  100 . The separate pieces  104 ,  106  enable separate focal properties to be selected. 
     Nose-side lens piece  104  is on the side of lens  100  closest to the nose of the wearer, while ear-side lens piece  106  is on the side of the lens closest to the ear of the wearer. While nose-side lens piece  104  is shown in FIG. 4 on the right side of lens  100 , and ear-side lens piece  106  is shown on the left, their positions are selected for illustrative purposes only, and their positions would be reversed for a lens worn over the other eye of the wearer. 
     Also, the progressive focal region is preferably not exactly centrally located, but instead is slightly nasal, corresponding to sight lines when the eyes are in a reading position. Thus, the progressive lens piece  14  can provide a rear optical center  15  that is offset nasally. The lines  20 ,  22  delineating the critical regions are, as indicated, generally radial (except close to the distance optical center  11 ). For the nose-side or nasal lens piece these radii could be at an angle 5° above the horizontal and 60° below the horizontal, while for the ear side or temporal lens piece they could have angles of 5° and 70° below the horizontal, with the radii centered on the distance optical center  11 . The radii could also be centered on location displaced away from the distance optical center, typically downwardly and nasally displaced relative to the center  11 . The lens piece  14  then provides the near optical center  15  offset nasally, as shown. 
     Reference is now made to FIG. 5, which shows an isometric view of lens  100 . Primary lens piece  102 , nose-side lens piece  104  and ear-side lens piece  106  mount together in a similar fashion as lens pieces  12  and  14  in lens  10 . Primary lens piece  102  is surrounded by an outer edge  120 , which includes two mating portions  122  and  124  for mating with nose-side and ear-side lens pieces  104  and  106 . A groove  130  for holding an O-ring  132 , extends along the length of outer edge  120 , outside of mating portions  122  and  124 . 
     Similarly to lens piece  14 , nose-side lens piece  104  is surrounded by an outer edge  134  which includes a mating portion  136  for mating with mating portion  124  of lens piece  102 . A groove  138 , which aligns with groove  130  on lens piece  102  for holding the O-ring  132 , extends along the portion of outer edge  134  outside mating portion  136 . The ear-side lens piece  106  is surrounded by an outer edge  140 , which has a mating portion  142  for mating with mating portion  122 , and a groove  144 , which aligns with the groove  130 . 
     The lens pieces in the above embodiments are shown as having symmetrical shapes about a vertical axis. It is preferable to have a vertically symmetrical lens for cost and for ease of manufacture, however it is alternately possible to have asymmetrical designs. 
     While it has been disclosed that the focal properties within the critical regions of the lenses described match the properties of the long-distance viewing portion, the lenses may alternately have other focal properties, that are different from those of the long-distance portion, but are still substantially constant throughout the critical regions. 
     As mentioned above, after assembly of the lens pieces, they can be covered with a film or the like to smooth out edges and ridges. Such a lens is then used to form a mold. From the mold individual lens are made. In known manner, these individual lens are used to fill prescriptions for each customer as required. For example, each lens would be cut to fit a particular eyeglass frame and, if required, the back or rear surface can be resurfaced to alter the focal length thereof. 
     Reference is now made to FIG. 6, which shows an alternative manufacturing technique. FIG. 6 shows an edge view of a lens  200  manufactured in a single piece, in accordance with this aspect of the present invention. Lens  200  includes a long-distance viewing portion  202 , and a progressive portion  204 . Lens  200  is manufactured by the application of an excimer laser  210  to adjust the focal properties of lens  200 , particularly in critical regions  16  and  18 , so that lens  200  does not possess change in focal length in these regions. Excimer laser  210  is operated automatically by controller  212  to vaporize material from lens  200 , and does so without damaging the remaining lens material. 
     While lenses have been disclosed in the above embodiments to have constant focal properties in critical regions  16  and  18 , it is alternatively possible to have a progressive portion located substantially within one of the critical regions  16  and  18 , thus leaving one of the critical regions  16  or  18  substantially undistorted. This provides a smaller improvement in visual comfort for the wearer, but is nonetheless an improvement within the scope of the present invention. Fabricating a lens with constant focal properties in the regions of high potential distortion reduces visual discomfort for the wearer. 
     As will be apparent to persons skilled in the art, various modifications and adaptations of the systems and methods described above are possible without departure from the present invention, the scope of which is defined in the appended claims.