Progressive-multi-focus lens

Abstract: A progressive-multi-focus lens (1, 5) for constituting spectacles having long-distance and short-distance focuses, having an area in which the refracting power progressively changes (successively increases) from upper to lower positions generally at the horizontal center of the lens; and a main fixation line (4, 6) generally called an "umbilical meridial" formed in the progressive-change area, in which the degree of astigmatism along the main fixation line is substantially zero, and the main fixation line is gradually displaced toward the nose in accordance with the convergence of the eyes as it extends from an upper position to a lower position. The disposition of the main fixation line displaced toward the nose is varied depending upon the strength of the lens in such a manner that it is shifted upward as the strength of the lens is reduced, thereby optimizing the performance of the lens for a particular user.

Description: This invention relates to a spectacle lens and, more particularly, to an improvement in a progressive-power multi-focal lens having a zone along which the refracting power progressively changes between upper and lower positions generally along the vertical centreline of the lens. A progressive-power multi-focal lens is used for presbyopia, having a refracting power that gradually changes along a line on the lens (main fixation line or principal sight line) along which the line of sight passes when it shifts from far vision to near vision. The main fixation line on a progressive-power multi-focal lens should be located at a position where the sight line most frequently passes. Preferably, there is no astigmatism along the main fixation line. A point on the surface of the lens where the astigmatism is zero can be regarded as an infinitesimal spherical surface and is generally called an "umbilical point". A row of successive "umbilical points" is called an "umbilical line", and this umbilical line is usually employed as the main fixation line. The degree of astigmatism is zero on the umbilical line of a progressive-power multi-focal lens, but it increases at an ascending rate as it deviates from this line. There are therefore extremely inferior optical conditions. In other words, in the region of a progressive-power multi-focal lens where the refracting power gradually changes, only that region which is in the vicinity of the umbilical line has good optical performance (region of distinct vision). The other regions are usually treated as regions of non-distinct vision due to the existence of astigmatism. Consequently, the disposition of the umbilical line (main fixation line) on a spectacle lens is very important. This line should be located at a position where the sight line most frequently passes on the basis of elaborate studies as to how the sight line of the user wearing the spectacles moves. Examples of conventional techniques relating to the disposition of the umbilical line are disclosed in Japanese Patent Examined Publication No. 47-9626, and Japanese Patent Unexamined Publication Nos. 57-10113, 57-210320, and 59-88718. Japanese Patent Examined Publication No. 47-9626 and Japanese Patent Unexamined Publication Nos. 57-10113, and 57-210320 (the latter corresponding to GB-A 2 100 877) disclose techniques relating to asymmetry of the refracting surface of the lens due to the disposition of the umbilical line based on convergence of the eyes (inward movement of the sight lines of the left and right eyes when shifting from a far vision to near vision). In particular, Japanese Patent Unexamined Publication No. 59-88718 discloses a technique relating to the relationship between the inward displacement of the umbilical line in response to convergence of the eyes and the changes that take place in the refracting power along the umbilical line. These techniques, however, are not concerned with determination of the disposition of the umbilical line with respect to the addition of the lens. A person suited to wearing progressive-power multi-focal lenses of comparatively low addition is an incipient presbyopic person who has not inconsiderable ability to focus the eye and whose experience in wearing far-vision spectacles is inadequate. Generally, such a person is unskilled in the basic manner of using progressive-power multi-focal lenses by, for near-vision eye work, projecting the chin slightly forward and directing the sight lines downward so as to see through the lower portions of the spectacle lenses (for near-vision eye work). He tends to look at close objects through portions of the lenses slightly above the near distance portions (those portions being designed for intermediate-distance eye work). Complaints that "it is difficult to see close objects" or "near vision is restricted" are often made as a result of this tendency. This is because looking through the portions above the proper portions set in the spectacle lenses for near vision focusing means looking with the sight lines from the left and right eyes passing through portions that are closer together than the umbilical lines. As described above, this is near vision focusing with a high degree of astigmatism. Furthermore, the directions of astigmatism with respect to the left and right eyes (the directions in which the image is warped) differ from each other to such a degree that they meet almost at right angles, thereby making image fusion highly difficult. In a possible design method aimed at avoiding this defect, the portion for near-vision focusing is shifted higher in the case of a lens of low addition. However, this method is the same as a method aimed at reducing the length of the zone of progressive change, resulting in another problem of producing abrupt increase in the degree of astigmatism in the lateral area. In view of these problems, it is an object of the present invention to provide an easy-to-use progressive-power multi-focal lens in which the umbilical line is disposed in accordance with the characteristic tendency with which a particular person wearing the spectacles shifts the sight line for each of his or her eyes. The invention provides a method of manufacturing a set of progresssive-power multi-focal lenses each including an area having a refracting power which progressively increases from an upper to a lower region in the vicinity of a vertical center line of said lens, a predetermined addition, and an umbilical line which becomes gradually closer to the nasal side in accordance with convergence of the eyes of a user in said progressive-change area as it extends from the upper to the lower region, wherein the degree of astigmatism along said umbilical line is substantially zero, wherein the lenses have additions in their lower regions that vary from lens to lens, characterised by so positioning the umbilical line of each lens in the area between the upper and the lower regions in dependence upon the addition of that lens that the nasal gradual displacement of the umbilical line is greater in those of said lenses with lower additions. The invention further provides a set of progressive-power multifocal lenses having an area in which the refracting power progressively increase from an upper to a lower position in the vicinity of a vertical centerline of said lens, a predetermined addition, and an umbilical line in said progressive-change area wherein the degree of astigmatism along said umbilical line is substantially zero, and said umbilical line becomes gradually closer to the nasal side in accordance with the convergence of the eyes of a user as it extends from an upper position to a lower position, characterised in that the disposition of said umbilical line between the said upper and lower portions varies from lens to lens in dependence upon the addition of the lens, being further towards the nasal side in those of said lenses of lower addition. The addition of the progressive-power multifocal lens corresponds to the degree to which the person who will wear the progressive-power multifocal lens lacks the ability to focus the eyes. Therefore, a progressive-power multi-focal lens of comparatively low addition is used for a person at a comparatively early stage of experiencing presbyopia while a progressive-power multi-focal lens of comparatively high addition is used for a person at a comparatively late stage of experiencing presbyopia. There is a great difference between presbyopic persons at the early and late stages in terms of the manner in which the sight lines move from far vision to near vision. Specially, a presbyopic person at an early stage of development of this condition tends to move (downward) the sight lines to a smaller extent during the course of changing from far vision to near vision compared with a person who is experiencing a late state of presbyopia. In consequence, the umbilical line (indicated at 4 in Fig. 1) on a progressive-power multi-focal lens of comparatively low addition (lens 1 in Fig. 1) is determined in such a manner that it extends downward from a position higher than that of the umbilical line (indicated at 6 in Fig. 2) on a progressive-power multi-focal lens of a comparatively high addition (lens 5 in Fig. 2) while sloping inward (toward the nose). The umbilical line is thus made to coincide with the movement of the sight line of each individual presbyopic person. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram of the disposition of an umbilical line on a progressive-power multi-focal lens in accordance with the present invention; Fig. 2 is a diagram of the disposition of an umbilical line on another progressive-power multi-focal lens in accordance with the present invention; Fig. 3 is a diagram of the composition of the umbilical lines shown in Figs. 1 and 2; and Fig. 4 is a diagram of the disposition of an umbilical line on a conventional progressive-power multi-focal lens. An embodiment of the present invention will be described below with reference to the accompanying drawings. Fig. 1 shows a disposition of an umbilical line on a progressive-power multi-focal lens 1 for the right eye viewed from the side of the convex surface thereof. The lens 1 has a geometrical center O through which a meridian L = L' passes, a far-vision region center 2A having a diopter of 0.00, and a near-vision region center 3A having a diopter of +0.50. The addition of the lens is therefore 0.50. A reference character H in Fig. 1 indicates the horizontal direction. The disposition of the umbilical line on the lens 1 is indicated by the reference character 4. The near-vision region center 3A is downwardly spaced apart from the far-vision region center 2A by 16 mm while being displaced toward the nose by 2.5 mm. The umbilical line 4 is formed of an umbilical meridian, and the degree of astigmatism thereon is substantially zero. It is based on the same design as the progressive-power multi-focal lens proposed by the applicant of the present invention in Japanese Patent Unexamined Publication No. 57-230320; the umbilical line is offset toward the nose; and the distribution of astigmatism is asymmetrical about the umbilical line in the horizontal direction on the nasal side and the temporal side. In comparison with the progressive-power multi-focal lens 7 of the conventional art shown in Fig. 4, the position at which the displacement of the umbilical line on the lens 1 shown in Fig. 1 starts as this line extends downward is higher although the zones of progressive change of these lenses are equal in length. The umbilical line 4 is positioned above a straight line M which connects the far-vision region center 2A and the near-vision region center 3A. Since, as described above, an incipient presbyopic person tends to shift the sight lines downward to a smaller extent during change from far vision to near vision focusing, it is easy for such a person to use progressive-power multi-focal lenses formed in the same manner as the lens 1 shown in Fig 1 in which the displacement of the umbilical line starts at a correspondingly higher position. Also, the length of the progressive-change zone of this lens is substantially the same as that of an ordinary progressive-power multi-focal lens (e. g., a conventional lens shown in Fig. 4). Therefore, this lens can be designed to avoid steep concentrations of aberration in side portions. Fig. 2 shows a progressive-power multi-focal lens 5 based on the same design as that of the lens shown in Fig. 1 except for the disposition of the umbilical line. The position of a far-vision region center 2B and the position of a near-vision region center 3B of the lens 5 are the same as the lens shown in Fig. 1. The far-vision region center 2B is of a diopter of 0.00; the near-vision region center 3B is of a diopter of +3.00; and the addition of the lens is 3.00. The near-vision region center 3B is downwardly spaced apart from the far-vision region center 2B by 16 mm while being displaced toward the nose by 2.5 mm. An umbilical line 6 is positioned below a straight line M' which connects the far-vision region center 2B and the near vision region center 3B. Therefore, this lens is used for a presbyopic person at a comparatively late stage of experience of presbyopia who is skilled in basic use of the progressive-power multi-focal lens. However, this lens is also easy to use because the extent of downward movement of the sight line is large. Fig. 3 shows a comparison between the dispositions of the main fixation lines shown in Figs. 1 and 2. It is understood that the manner in which the main fixation line is displaced toward the nose greatly differs between the additions which are 0.5 and 3.00. The umbilical line corresponding to the convergence of the eyes as described in the specification of the present invention includes not only a type of line which is curved as viewed from the side of the concave surface, such as those shown in Figs. 1 and 2, for example, those disclosed in Japanese Patent Unexamined Publication Nos. 59-88718 and 57-210320 but also another type of line having straight and bent portions. The disposition of the umbilical line is determined with respect to the addition of the lens not only every unit value thereof but also every range of the addition divided into more than two. In other words, the present invention is effective whatever the pitch of additions or range of approximating additions with which the disposition of the umbilical line is changed. It may be said that the umbilical line is disposed at an upper or lower position relative to the position of a point of intersection (point c in the case of the lens 1 or point d in the case of the lens 5) of the umbilical line and a bisector Y of a line indicating a distance (a) between the far-vision region center 2 and the near-vision region center 3, as shown in Fig. 3. Preferably, the positions of these points of intersection (c and d) are above a point of intersection 7 of the bisector Y and the straight line M which connects the far-vision region center 2 and the near-vision region center 3 if the lens is of a weak type and the addition thereof is smaller than 1.50; these positions are below the point of intersection 7 if the lens is of a strong type and the addition thereof is greater than 2.50; and these positions are in the vicinity of the point of intersection 7 if the lens is of a medium type and the addition thereof is 1.75 to 2.25. The lens in accordance with the present invention is easy to use for both a person whose experience in wearing spectacles for presbyopia is inadequate and a person who is skilled in using spectacles for presbyopia because the lens is designed with the disposition of the umbilical line depending upon the addition of the lens. The length of the progressive-change zone is substantially constant although the manner of displacement of the umbilical line depends upon the addition of the lens. Therefore, there is no possibility of increase in the degree of lateral astigmatism.

Claims: A method of manufacturing a set of progresssive-power multi-focal lenses (1, 5) each including an area having a refracting power which progressively increases from an upper to a lower region (2, 3) in the vicinity of a vertical center line of said lens, a predetermined addition, and an umbilical line (4, 6) which becomes gradually closer to the nasal side in accordance with convergence of the eyes of a user in said progressive-change area as it extends from the upper to the lower region, wherein the degree of astigmatism along said umbilical line is substantially zero, wherein the lenses have additions in their lower regions that vary from lens to lens, characterised by so positioning the umbilical line (4, 6) of each lens in the area between the upper and the lower regions (2, 3) in dependence upon the addition of that lens that the nasal gradual displacement of the umbilical line (4, 6) is greater in those of said lenses with lower additions. A method of manufacturing a set of progressive-power multifocal lenses as set forth in claim 1, which comprises positioning said umbilical line (4) above a straight line (M) joining optical centers (2A, 3A) of the upper and lower regions (2, 3) everywhere between those centers in the case of any lens in which the addition lens is less than 1.5 dioptres. A method of manufacturing a set of progressive-power multifocal lenses as set forth in claim 1 or claim 2, which comprises positioning said umbilical line (6) below a straight line (M) joining optical centers (2B, 3B) of said upper and lower regions (2, 3) everywhere between these centers in the case of any lens in which the addition is greater than 2.5 dioptres. A set of progressive-power multifocal lenses (1, 5) having an area in which the refracting power progressively increases from an upper to a lower position (2, 3) in the vicinity of a vertical centerline of said lens, a predetermined addition, and an umbilical line (4, 6) in said progressive-change area wherein the degree of astigmatism along said umbilical line is substantially zero, and said umbilical line becomes gradually closer to the nasal side in accordance with the convergence of the eyes of a user as it extends from an upper position to a lower position, characterised in that the disposition of said umbilical line (4, 6) between the said upper and lower portions varies from lens to lens in dependence upon the addition of the lens (1, 5), being further towards the nasal side in those of said lenses of lower addition. A set of progressive-power multifocal lenses as claimed in claim 4, including at least one lens in which the addition of the lens (1) is not more than 1.5 dioptres and the umbilical line (4) is above a straight line (M) joining optical centres (2A, 3A) of the upper and lower regions (2, 3) everywhere between those centre. A set of progressive-power multifocal lenses as caimed in claim 4 or claim 5, including at least one lens in which the addition of the lens (5) is not less than 2.5 dioptres and the umbilical line (6) is below a straight line (M') joining optical centres (2B, 3B) of the upper and lower regions (2, 3) everywhere between those centres.

Inventors: KITANI AKIRA, KITANI, AKIRA
Applicants: HOYA CORP, HOYA CORPORATION
Application year: 1988
Main classification: G02C7
IPC codes: G02C7