Progressive-multi-focus lens

BACKGROUND OF THE INVENTION

This invention relates to a spectacle lens and, more particularly, to an improvement in a progressive-multi-­focus lens having an area in which the refracting power progressively changes between upper and lower positions generally at the horizontal center of the lens.

A progressive-multi-focus lens is a kind of spectacle lens for presbyopia having a refracting power that gradually changes along a line on the lens (main fixation line) where a line of sight which moves as it changes from a long distance focus to a short distance focus passes through the lens. The main fixation line on a progressive-­multi-focus lens should be positioned in such a manner that it corresponds to a point through which the line of sight 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 meridian", and this umbilical meridian is usually employed as the main fixation line. The degree of astigmatism is zero on the umbilical meridian of a progressive-multi-focus 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-multi-­focus lens where the refracting power gradually changes, only that region which is in the vicinity of the umbilical meridian, 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 meridian (main fixation line) on a spectacle lens is very important. This line should be positioned in correspondence with the point where the line of sight most frequently passes on the basis of elaborate studies as to how the line of sight of the user wearing the spectacles moves.

Examples of conventional techniques relating to the disposition of the main fixation 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 disclose techniques relating to asymmetry of the refracting surface of the lens due to the disposition of the main fixation line based on convergence of the eyes (inward movement of the lines of sight of the left and right eyes when changing from a long-distance focus to short-distance focus). In particular, Japanese Patent Unexamined Publication No. 59-88718 discloses a technique relating to the relationship between the inward displacement of the main fixation line in response to convergence of the eyes and the changes that take place in the refracting power along the main fixation line.

These techniques, however, are not concerned with determination of the disposition of the main fixation line with respect to the strength of the lens. A person suited to wearing a progressive-multi-focus lenses of comparatively low strength is an incipient long-sighted person who has not inconsiderable ability to focus the eye and whose experience in wearing long-sighted spectacles is inadequate. Generally, such a person is unskilled in the basic manner of using progressive-multi-focus lenses by, for short-distance eye work, projecting the chin slightly forward and directing the lines of sight downward so as to see through the lower portions of the spectacle lenses (for short-distance eye work). He tends to look a close objects through portions of the lenses (designed for intermediate-­distance eye work) slightly above the portions actually intended for short-distance focusing. Complaints that "it is difficult to see a close objects" or "the short-­ distance view 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 short-­distance focusing means looking with the lines of sight from the left and right eyes passing through portions that inwardly deviate from the main fixation lines. As described above, this is short-sighted 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 short-distance focusing is shifted higher in the case of a lens of small strength. 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 direction.

In view of these problems, it is an object of the present invention to provide an easy-to-use progressive-­multi-focus lens in which the main fixation line is disposed in accordance with the characteristic tendency with which a particular person wearing the spectacles moves the line of sight for each of his or her eyes.

To this end, the present invention provides a progressive-multi-focus lens 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 generally called an "umbilical meridian" formed in this 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 displacement of the main fixation line toward the nose being varied depending upon the strength of the lens.

The present invention also provides a progressive-­multi-focus lens in which the position from which the displacement of the main fixation line toward the nose is started is shifted upward if the strength of the lens is reduced.

The strength of the progressive-multi-focus lens corresponds to the degree to which the person who will wear the progressive-multi-focus lens lacks the ability to focus the eyes.

Therefore, a progressive-multi-focus lens of comparatively low strength is used for a long-sighted person at a comparatively early stage of experiencing long-­sightedness while a progressive-multi-focus lens of comparatively high strength is used for a long-sighted person at a comparatively late stage of experiencing long-­sightedness.

There is a great difference between long-sighted persons in the early and late stages in terms of the manner in which the lines of sight move from long-distance focusing to short distance focusing.

Specially, a long-sighted person in an early stage of development of this condition tends to move (downward) the lines of sight to a smaller extent during the course of changing from long-distance focusing to short-distance focusing compared with a person who is experiencing a late state of long-sightedness.

In consequence, the main fixation line (indicated at 4 in Fig. 1) on a progressive-multi-focus lens of comparatively low strength (lens l in Fig. 1) is formed in such a manner that it extends downward from a position higher than that of the main fixation line (indicated at 6 in Fig. 2) on a progressive-multi-focus lens of a comparatively high strength (lens 5 in Fig. 2) while being displaced inward (toward the nose). The main fixation line is thus made to coincide with the movement of the line of sight of a particular long-sighted person.

BRIEF DESCRIPTION OF THE DRAWINGS

• Fig. 1 is a diagram of the disposition of a main fixation line on a progressive-multi-focus lens in accordance with the present invention;
• Fig. 2 is a diagram of the disposition of a main fixation line on another progressive-multi-focus lens in accordance with the present invention;
• Fig. 3 is a diagram of the composition of the main fixation lines shown in Figs. 1 and 2; and
• Fig. 4 is a diagram of the disposition of a main fixation line on a conventional progressive-multi-focus lens.

DESCRIPTION OF PREFERRED EMBODIMENT

An embodiment of the present invention will be described below with reference to the accompanying drawings.

Fig. 1 shows a disposition of a main fixation line on a progressive-multi-focus lens l for the right eye viewed from the side of the convex surface thereof.

The lens l has a geometrical center O through which a meridian L = L′ passes, a long-distance-focusing center 2A having a diopter of 0.00, and a short-distance-focusing center 3A having a diopter of +0.50. The strength of the lens is therefore 0.50. A reference character H in Fig. 1 indicates the horizontal direction.

The disposition of the main fixation line on the lens l is indicated by the reference character 4. The short-­distance-focusing center 3A is downwardly spaced apart from the long-distance-focusing center 2A by 16 mm while being displaced toward the nose by 2.5 mm.

The main fixation 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-multi-focus lens proposed by the applicant of the present invention in Japanese Patent Unexamined Publication No. 57-230320; the umbilical meridian is displaced toward the nose; and the distribution of astigmatism is asymmetrical about the umbilical meridian in the horizontal direction on the nose side and the ear side.

In comparison with the progressive-multi-focus lens 7 of the conventional art shown in Fig. 4, the position at which the displacement of the main fixation line on the lens l shown in Fig. 1 is started as this line extends downward is higher although the zones of progressive change of these lenses are equal in length. The main fixation line 4 is positioned above a straight line M which connects the long-distance-focusing center 2A and the short-­distance-focusing center 3A.

Since, as described above, an incipient long-sighted person tends to move the lines of sight downward to a smaller extent during change from long-distance focusing to short-distance focusing, it is easy for such a person to use progressive-multi-focus lenses formed in the same manner as the lens l shown in Fig 1 in which the displacement of the main fixation line is started 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-multi-focus 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-multi-focus lens 5 based on the same design as that of the lens shown in Fig. 1 except for the disposition of the main fixation line. The position of a long-distance-focusing center 2B and the position of a short-distance-focusing center 3B of the lens 5 are the same as the lens shown in Fig. 1.

The long-distance-focusing center 2B is of a diopter of 0.00; the short-distance-focusing center 3B is of a diopter of +3.00; and the strength of the lens is 3.00.

The short-distance-focusing center 3B is downwardly spaced apart from the long-distance-focusing center 2B by 16 mm while being displaced toward the nose by 2.5 mm.

A main fixation line 6 is positioned below a straight line M′ which connects the long-distance-focusing center 2B and the short-distance-focusing center 3B.

Therefore, this lens is used for a long-sighted person at a comparatively late stage of experiencing the long-­sightedness who is skilled in basic use of the progressive-­multi-focus lens. However, this lens is also easy to use because the extent of downward movement of the line of sight 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 with respect to the strengths of 0.5 and 3.00.

The main fixation 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 main fixation line is varied with respect to the strength of the lens not only every unit value thereof but also every range of the strength divided into more than two. In other words, the present invention is effective whatever the pitch of strengths or range of approximating strengths with which the disposition of the main fixation line is changed.

It may be said that the main fixation line is disposed in an upper or lower position relative to the position of a point of intersection (point c in the case of the lens l or point d in the case of the lens 5) of the main fixation line and a bisector Y of a line indicating a distance (a) between the long-distance-focusing center 2 and the short-­distance-focusing 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 long-distance-focusing center 2 and the short-distance-­focusing center 3 if the lens is of a weak type and the strength 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 strength 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 strength 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 long-sighted spectacles is inadequate and a person who is skilled in using long-sighted spectacles because the lens is design to change the disposition of the main fixation line depending upon the strength of the lens.

The length of the progressive-change zone is substantially constant although manner of displacing the main fixation line is changed depending upon the strength of the lens. Therefore, there is no possibility of increase in the degree of lateral astigmatism.