Page 157 - PowerPoint 演示文稿

Progressive Addition Lenses 147

location in the vision field defines the power value needed at each point of
the progression. The horizontal eye and head movements determine the
field of gaze and defines the width of the lens zone used for foveal vision.
And to maximize wearers visual acuity in the lens central area, the
unwanted induced cylinder of the progressive lens must be kept to a
minimum and be pushed to the periphery of the lens.
Although, extra-foveal vision do not provide sharp vision but the wearer
do locate the object in space, perceive their forms and detect their
movements through extra-foveal vision. The prismatic distribution on the
progressive addition lens surface and its magnitude introduces slight
deformation of horizontal and vertical lines, thus altering the wearers visual
comfort. The whole of the retina is almost homogeneously sensitive to
motion. The variation of prismatic effects plays a role in the wearer’s comfort
where it must be slow and smooth across the whole lens to ensure
comfortable dynamic vision (Fig. 11.24).

Fig. 11.24: Horizontal eye/ head movement coordination and width of field

Binocular vision refers to the simultaneous perception of the two eyes
(Fig. 11.25). For perfect fusion, the image produced by the right and left
lenses must be formed as corresponding retinal points and display similar
optical properties. The eyes naturally converge when the wearers gaze is
lowered for near vision. The power progression must be positioned in the
lens in such a manner that it follows the eyes path of convergence
downwards in the nasal direction. Both right and left lenses must offer
approximately equal vertical prism on each side of the progression path.
The retinal images formed in both eyes must be similar in all directions of
gaze. For that purpose, the power and astigmatism encountered on
corresponding points of right and left lenses must be approximately equal.