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12 Ophthalmic Lenses

this is the only lens which is best for rimless frame. Abbe value is 45 and
the material has 100% UV protection property. It can be easily tinted and
coated with anti-reflection coating.

PROPERTIES OF OPHTHALMIC LENSES

Fig. 2.2: Incident light on an ophthalmic lens

When a beam of light falls on a lens surface, some of the light passes through
the lens, some of the light is reflected from the lens surface and some of the
light is absorbed by the lens material (Fig. 2.2). The measure of the
proportion of light reflected from the surface is called reflectance. The
measure of proportion of light absorbed is the absorption and the measure
of the proportion of light transmitted is the transmittance. Each one is
expressed as a fraction of the total quantity of light in the beam. If the
intensity of the beam of light is represented by the number 1, reflectance
by R, absorption by A and transmittance by T, the intensity may be
expressed as follows:

R + A + T = 1
The aim of the ophthalmic lenses is to restore clear vision to an ametropic
eye by placing a corrective lens between the viewed object and the eye and
the dioptric function of the lens depends on the characteristics of the lens
material. Extensive knowledge of the properties of these materials is,
therefore, needed in the following areas:
1. Optical properties
2. Mechanical properties