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4 Ophthalmic Lenses

CURVATURE OF SURFACES
Most of the optical lenses have curved surfaces; the surface may be described
as convex, if it bulges out of the material and concave if it is depressed into
the material. Figure 1.4 illustrates a cross- sectional view of different curved
forms. The measure of the shape of a curved surface is known as curvature.
Spherical surface has uniform curvature in all its zones, whereas, surfaces
other than spherical, the curvature varies in different zones and from one
meridian to another.

Fig. 1.4: Forms of curvature

The curvature of a spherical surface can be read with a simple instrument
called “spherometer”, which measures the height of the vertex of the curve
above a fixed chord (Fig. 1.5).

Fig. 1.5: Spherometer