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136 Ophthalmic Lenses

PROGRESSIVE ADDITION LENS MARKINGS
All the progressive addition lenses contain important markings, which are
used to identify lenses and to assist in their fitting and verification. The
important markings are explained below (Fig. 11.13):

Fig. 11.13: Left eye progressive lens
A and A1: They are two hidden circles, which are permanently etched
on the lens at 34 mm apart. When the ink marking is removed,
they are made visible by fogging.
B: This point is the distance optical centre (DOC) of the lens and
is also known as Prism Reference Point.
C: Hidden addition power situated at the temporal side and is
made visible by fogging.
D: 0-180° axis line passing through the DOC.
E: Fitting cross lies above the DOC.
F: This is the Distance Power (DP) circle to check the exact distance
power with the help of lensometer.
G: Hidden logo situated nasally and is made visible by fogging
when the ink marking is removed.
H: 7mm to 9 mm circle is the centre of the near vision area and is
inset by 2.5 mm.

PROGRESSIVE ADDITION LENS OPTICAL DESIGN

Every progressive lens design requires a globally smooth surface that
provides a gradual transition in curvature from the distance portion down
into the near portion. This gradual blending of curvature means that the
addition power is gradually changing across a large area of the lens.
Unfortunately, the superior optics and line free nature of progressive
addition lens does have a bit of a price to pay, i.e., the change in curvature