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32 Ophthalmic Lenses

power of the unknown lens must equal that of the trial lens of opposite
sign, e.g, a + 2.00 D lens neutralizes a – 2.00 D lens. To measure this
accurately, the neutralizing lens must be placed in contact with the back
surface of the spectacle lens. However, with highly curved lenses, this is
not possible and an air space intervenes. It is, therefore, better to place the
neutralizing lens against the front surface of the spectacle lens.
Neutralization is, thus, somewhat inaccurate for curved lenses of more than
about 2.00D. An error of up to 0.50D may be possible with powerful lenses.
Nevertheless for relatively low power lenses, neutralization is still a very
useful technique.
The Geneva lens measure can be used to find the surface powers of a
lens by measuring the surface curvature (Fig. 3.9). The total power of a
thin lens equals the sum of its surface powers. However, the instrument is
calibrated for lenses made of crown glass and a correction factor must be
applied in the case of lenses made of materials of different refractive indices.
Focimeter is used to measure the vertex power of the lens. The image of
the target is seen as a ring of dots when a spherical lens is tested (Fig. 3.10).

Fig. 3.9: Geneva lens measure watch

Fig. 3.10: Target as seen in Focimeter