Biomedical Engineering Reference
In-Depth Information
distance—are not sufficiently strong. By moving the lenses away from his eyes,
the hyperopic patient is moving the secondary focal points of the spectacle lenses
closer to his far points (Fig. 7-11). Rather than using a stronger lens, the same
effect can be obtained by moving a lens away from the cornea, thereby increasing
its effectivity.
Up to now, we've discussed examples that involve replacing a contact lens with a
spectacle lens. The same principles apply to replacing a spectacle lens with a contact
lens. As a practical matter, refractions are almost always performed at the spectacle
plane, about 15.00 mm in front of the cornea. Therefore it is more common to cal-
culate the contact lens power from the spectacle power than vice versa.
Let's look at an example.
Your refraction at a vertex distance of 15.00 mm results
in a spectacle prescription of
9.00 D in each eye. If the patient wishes to be fit
with contact lenses, what power should they be?
−
A
Position 1
n
= 1.000
n
′
= 1.333
F
′
of lens
FP of eye
B
Position 2
n
= 1.000
n
′
= 1.333
FP of eye and
F
′
of lens
Figure 7-11.
The effective power of a plus spectacle lens that is used to correct hyper-
opia is increased by moving the lens away from the eye [from position 1 in (
A
) to
position 2 in (
B
)].
