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146 Ophthalmic Lenses

Rate of change plots are the three dimensional graphical representation of
the variation in the optical characteristics of progressive addition lenses,
which plots vertically the value of a given optical characteristics at each
point of the lens in relation to a reference plane. It may be used to show:
a. Distribution of power
b. Astigmatism
c. Gradients of power variation
Rate of change plot is a better way to visualize the softness or hardness of
a lens design. They represent the rate of change in the sphere value between
two given points on the lens surface. The lower the difference in the sphere
power between two points, the lower the plotted altitudes. Thus, three-
dimensional plots are more demonstrative of lens characteristics than
contour plots. Flatter plots indicate softer design. The lens designer can
manage the softness in one of the two ways.
1. By lengthening the progression. Or
2. By carefully controlling the rate of change of the optical characteristics
between all points on the lens surface.
Since the length of the power progression is not easily demonstrated
on the rate of change plots, they give incomplete picture of the overall lens
design. Otherwise this is a more demonstrative procedure.
The graphical representations are useful tools to communicate the
geometrical features of PAL design to a trained observer. However, they
do not really correlate with wearer’s acceptance, as this is dependent on
the sum total of many different factors - astigmatic error, the power rise,
prismatic effects, distortions, the required dioptric power, cent ration, the
frame fit, vertex distance and the subjective impression of the wearer. So
the success of a progressive addition lens design should be judged on
extensive wearing trials rather than studying as these plots.

DESIGNING PROGRESSIVE ADDITION LENS
A progressive addition lens is designed not only to provide a presbyope –
an ability to see clearly at all distances but also to respect all the physiological
visual functions like foveal vision, extra foveal vision, binocular vision,
etc. The foveal area of the retina permits sharp vision at any distance within
a small field which follows the eyes rotation which is usually within 30°
angle.
Within this end the lens areas used for foveal vision must provide for
perfect retinal images. The wearers natural body and head positions
determine the vertical rotation of the eye for near and distance vision, and
therefore the optimal length of the lens power progression. The co-
ordination of the body, head and eye movements in relation to the objects