1. Field of the Invention
The present invention relates to a method of manufacturing spectacle lenses and, in particular, progressive power spectacle lenses.
2. Description of the Related Art
As shown in FIG. 7, a conventional progressive power spectacle lens generally includes: a distance portion 11, which has a dioptric power for distance vision; a near portion 12, which has a dioptric power for near vision; an intermediate portion 13, which has a dioptric power at intermediate ranges between distance vision and near vision; and peripheral portions, i.e., right and left side portions 14 and 15.
A main meridian 16 can be defined on the spectacle lens, through which the line of sight of the lens-wearer frequently moves from near vision to distance vision (or vice versa). The main meridian 16 passes through the distance portion 11, the intermediate portion 13 and the near portion 12, approximately along the vertical center line of the lens. The lens is required to be designed so that aberrations along the main meridian 16 are well-correct ed. When the lens-wearer views an object at a near distance, the main meridian 16 approaches the nose (of the lens-wearer) because the line of sight of each eye moves inward (towards the nose). The horizontal displacement of the main meridian 16 towards the nose (i.e., the horizontal distance `H` in FIG. 7) in the near portion is known as the amount of "inset".
In a conventional progressive power spectacle lens, the amount of inset is generally determined in the range of approximately 2.5 mm through 3.0 mm regardless of the dioptric power of the lens. However, changing the amount of inset of the progressive power spectacle lens in accordance with the dioptric power of the lens has been recently proposed.
FIGS. 8 and 9 show the principle of the necessity of changing the amount of inset in accordance with the dioptric power of the lens. In Figs.8 and 9, right and left eyeball are designated by "RE" and "LE", respectively, and the amount of inset of each lens is designated by "H". FIG. 8 shows the case when viewing an object 17 at a near distance through a pair of spectacle lenses each having a negative dioptric power. Right and left lines of sight 18 and 19 through the lenses are shown by solid lines, while right and left lines of sight 18 and 19 without the lenses are shown by dotted lines. As can be seen in FIG. 8, the lines of sight 18 and 19 are bent through the lenses toward each other. FIG. 9 shows an example of viewing an object 17 at a near distance through a pair of spectacle lenses each having positive dioptric power. As can be seen in FIG. 9, the lines of sight 18 and 19 are bent through the lenses in opposite directions away from each other. Considering the effect the lenses have, in order for the lens-wearer to be able to view an object at a near distance naturally with both eyes, it can be appreciated in the case of the negative lenses that the amount of inset H should be reduced, whereas in the case of the positive lenses that the amount of inset H should be increased.
Accordingly, the dioptric power (the distance portion vertex power and the addition power) is one of the factors for determining the optimum amount of inset. However, the inventor of the present invention has found that conventional progressive power spectacle lenses are manufactured so that the amount of inset is determined only in accordance with the dioptric power without taking into account individual differences in wearing state of the lenswearers. wearers. Hence, there is still room for further improvement of progressive power spectacle lenses.