Page 168 - PowerPoint 演示文稿

158 Ophthalmic Lenses

The astigmatism across the pupil is inversely proportional to the length
of the progression zone. The longer is the progression zone, the smaller is
the surface astigmatism. In other words the rate of change in the surface
astigmatism in the periphery depends upon the addition. Higher addition
gives faster changes and hence peripheral astigmatism is more evident.
The reduction of peripheral distortion is also accomplished by using
the conic sections of changing asphericity rather than spherical sections as
power is increased in the vertical meridian. These varying sections reach a
relatively uniform power at a given peripheral portion of the lenses so that
prismatic and cylindrical effect at these edges are much the same through
out the peripheral vertical range of the lens. To achieve this in the upper
portion, the distance section is increased slightly in power towards the
periphery by + 0.30 D. This type of design also reduces the “optically pure”
zone for near in width by allowing the reduced remaining power variation
to spread over more area. As the curves are aspherical, the near zone reveals
a spherical area of only 12 mm wide. However, the power change is so
gradual that a much wider defective zone of practically equivalent power
exists, depending upon the power of addition.

EVOLUTION OF PROGRESSIVE ADDITION LENS
The first commercially successful progressive addition lens was introduced
by Essel under the name of Varilux in 1959. The design consisted of large
spherical distance and near zones, linked by a series of circles of ever
decreasing radii between the distance vision sphere and the near vision
sphere. More attention had been given to ensure larger distance and near
zones than to the quality of peripheral vision on either side of the
progressive corridor as shown in Figure 11. 38. Today such a design would
be described as very hard design.
The second generation of progressive lenses, Varilux 2 was introduced
in 1973. It too provided large distance, intermediate and near fields of vision,
but still it was at the cost of quality vision in the lateral regions of the
progressive zone. Conic sections of changing eccentricities from one section
of the lens to another replaced the circular section employed in Varilux 1
design. The effect of this was the reduction of power in the periphery of
the progressive zone. The new concept of “horizontal optical modulation”
was introduced which took care of the extra-foveal vision also. Binocular
vision was optimized as a result of an asymmetrical design. The overall
design of the lens is represented by a succession of conic sections as shown
in the Figure 11. 39.
During the next decade other manufacturer introduced progressive
addition lens design focusing specific optical characteristics. Some
emphasized on large distance and near vision zones, concentrating the