Progressive power lens
A progressive power lens having a positive vertex power in a distance portion is provided. At least a back surface of the progressive power lens is configured as a progressive surface. The progressive power lens satisfies a condition: Pf + ADD 3 < D 1 < Pf + ADD ( 1 ) where Pf represents a vertex power in the distance portion along a vertical cross-section, ADD represents an addition power, and D 1 represents a surface power of a front surface.
1. A progressive power lens having a positive vertex power in a distance portion, at least a back surface of the progressive power lens being configured as a progressive surface,
given that an axis normal to a front surface of the progressive power lens at a prism reference point is defined as a z-axis, an axis which passes through an intersection of the z-axis and the back surface in a vertical direction in a state where the progressive power lens is worn by an wearer is defined as an y-axis, a sag amount which is a distance between a point on a curve, defined as an intersection line of the back surface and an y-z plane, at a height y and the y-axis is represented by z 2 (y), a curvature of the curve at a height y is represented by C 2 (y), a height of a distance reference point from the z-axis is represented by yf, a height of a near reference point from the z-axis is represented by yn, a height of a top end of the progressive power lens from the z-axis is represented by yt, and a height of a bottom end of the progressive power lens from the z-axis is represented by yu, the progressive power lens satisfying a following conditions:
C 2 ( yf )>0 (2),
C 2 ( yn )<0 (3), and
z 2 ( yt )+ z 2 ( yu )>2· z 2 ( 0) (4),
where a direction headed from the front surface to the back surface is defined as a positive direction of the z-axis, and a direction heading for an upper end of the progressive power lens along the vertical direction in a state where a wearer wears the progressive power lens is defined as a positive direction of the y-axis.
2. A progressive power lens having a positive vertex power in a distance portion, at least a back surface of the progressive power lens being configured as a progressive surface, the progressive power surface being configured such that a minimum one C 2 min of two principal curvatures obtained when the back surface is approximated by a toric surface satisfies a condition:
0<C 2 min<2 (5).
3. The progressive power lens according to claim 2 , wherein the back surface has a property of correcting astigmatism.
4. The progressive power lens according to claim 2 , wherein the progressive power lens is provided with an addition power in such a manner that distribution of a vertical component of the addition power among front and back surfaces thereof and distribution of a horizontal component of the addition power among the front and back surfaces are separately defined.
5. The progressive power lens according to claim 2 , wherein the progressive power lens is provided with an addition power in such a manner that one of vertical and horizontal components of the addition power is allocated to one of front and back surfaces thereof and the other of the vertical and horizontal components of the addition power is allocated to the other of the front and back surfaces.
6. The progressive power lens according to claim 2 , wherein the progressive power lens is provided with an addition power in such a manner that each of vertical and horizontal components of the addition power is distributed among front and back surfaces thereof.