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162 Ophthalmic Lenses

When the progressive lenses were, first introduced, the recommended
minimum fitting height was 22 mm. That was fine when the eye sizes of
the spectacle frames were large and had deep ‘B’ measurements. However,
as the smaller and narrower frame shapes become popular, a need for
progressive with shorter corridor was crucial to their continued success.
Smaller frames proportions differ from large frames in that they have a
higher horizontal to vertical ratio. In other words the lens has to be not
only shorter vertically but also relatively wider. The second challenge is to
provide distortion free distance vision in the lens periphery. Finally the
shorter corridor lens must account for the fact that the wearer’s head and
eye movement are influenced by frame size. Wearer of small frames tend
to move their head more and eyes less when transitioning from distance to
near viewing than that of large frame wearers. When looking down a short
corridor lens, the eyes simply run out of the lens, so the head has to move
to maintain focus on the items of regard.
To make the progressive addition lens work in small frames, many lens
manufacturers came out with shorter corridor progressives. In 1999,
American Optical introduced A.O. Compact – the first short corridor
progressive lens. A.O. Compact has a 13 mm corridor that permits full
reading function with a minimum fitting height of 17 mm. Soon other
companies followed the suit and shorter corridor progressive becomes
popular: Nikon Presio, Essilor Ellipse, Shamir Piccolo, Hoya Hoyalux
Summit CD, Kodak Concise, Pentex Mini AF etc., became popular.
Internal Progressive

Internal progressive addition lens is a big step forward in overcoming many
of the limitations of progressive lenses widely available today. Seiko is
proud to have designed and patented the world’s first internal progressive
lenses. In an internal progressive lens, the curves producing changing power
are positioned on the back surface of the lens, and front surface is spherical.
The backside is a free form surface with progressive surface and cylinder
surface on the back. Therefore, it is possible to have a spherical front curve.
The design provides a field of vision that is 30% wider than a front side
progressive lens. This is due to the fact that the progressive surface rests
closer to the eyes. Besides Seiko, Rodenstock ILT is also being test marketed
in the USA. The new internal design offers following additional advantages:
1. Expansion of the visual fields
2. Magnification differences between various areas of the lens are
reduced.

Customized Progressives
Customized or personalized or individualized progressive surface design
is the most unique and the latest development in the progressive addition