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Aspheric Lenses 97

Fig. 9.7: Aspheric surfaces cannot be accurately
measured with conventional instruments

whether the correct design has been supplied. The most common qualitative
test to detect whether or not a lens is spherical is to use a lens measure in a
sagittal section across the front surface. Spherical or toroidal surface will
give a constant reading, whereas aspheric surface will vary in power from
the centre to periphery. However, in low power the change in surface
curvature towards the edge of the lens is so subtle that it becomes difficult
to verify with the instrument.

PRISM IN ASPHERIC LENSES

The geometric centre of the aspheric lens is to be aligned with the optical
centre of the patient’s eyes. This accurate centering of the aspheric lens,
throws up an issue with the prescribed prism. Some optical dispensers like
to achieve prismatic effect by decentring the lens. Clearly this will move
the geometric centre of the lens away from the optical axis of the eye which
will cause unwanted optical problems for the patient. In order to maintain
the alignment of the aspheric geometrical centre with the pupil and also
have prism in the lens, we need to order the lens with surfaced prism.
Typically the lens will have aspheric front side and the rear surface with
prism and cylinder surfaced in it. However, the true benefit of aspheric
lens is achieved only when the optical centre of the eye is in alignment
with geometric centre of the aspheric lens, which is also the pole of the
aspheric surface. In a true aspheric lens, asphericity begins right near the
centre of the lens which also prevents shifting of centre to create prism.
This suggests grinding prism to decentre the lens is also not the right practice
in aspheric lens.