In constructing an ophthalmic lens, one of the lens curves of one surface becomes the basis from which the others are determined. This beginning curve, on which the net lens power is based, is called the
base curve. In single vision prescription ophthalmic lenses, the base curve is always found on the front surface.
In the case of spherical lenses, the front sphere is the base curve.
If the lens is in plus cylinder form, there are two curves on the front. The base curve is the weaker, or flatter, of the two curves. The other curve becomes the
cross curve. The back surface is quite naturally referred to as the
sphere curve.
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