Page 6 - Cahier-Progressive Lenses-GB.indd

1.Concept of progressive lenses Concept A Basic design of Progressive lenses. A progressive lens is a lens whose power increases conti- 1 nuously from top to bottom, between an upper zone designed for distance vision and a lower zone designed for near vision. The increase in power is obtained by a continuous change in curvature of one or both surfaces of the lens. We will consider how this occurs by compa- ring the design with a single vision and a bifocal lens. Progressive lenses near vision only (Figure 2a). Because the lens focuses the Fig. 2a : Single vision The convex surface of a single vision reading lens consists of a single sphere of appropriate radius provi- ding, in combination with the back surface, correction for eye for near, when the wearer tries to look into the dis- tance, vision is blurred. Furthermore, there is no specific correction for intermediate vision which the subject will only be able to enjoy whilst there is sufficient accommo- dation (Figure 3a). In a bifocal lens, a distance vision sphere is placed above a near vision sphere and linked by a single step creating a visible dividing line (Figure 2b). Note in Figure 3b, which represents a subject whose amplitude of accom- modation is 1.50 D and who has been given a near addi- tion of 2.00 D, that there is a gap in the intermediate range of vision at arm’s length between 50cm and 67cm. In a progressive lens the curvature increases at a prede- termined rate between the distance and near portions providing clear intermediate vision at all distances in between. This is obtained from a succession of uninter- rupted horizontal curves between distance and near, with no visible separation, so that the lens power increa- ses smoothly from the distance zone at the top of the Fig. 2b : Bifocal. lens through an intermediate zone in the middle, to the near zone at the bottom of the lens (Figure 2c). The wea- rer benefits from continuous clear vision from distance to near (Figure 3c). Fig. 2 : Design principles of single vision, bifocal and progressive lenses Fig. 2c : Progressive. 6 Copyright © 2006 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.