Biomedical Engineering Reference
In-Depth Information
The focal length of the spectacle lens:
1
f
′
F
=
(100)(1.00)
f
=
−
6.00 D
′
′
=
−
f
16.67 cm
Since the vertex distance is 1.5 cm, the secondary focal length must be
−
18.17 cm [i.e.,
−
16.67 cm
+
(
−
1.50 cm)
=
−
18.17 cm]. The correction
required in the corneal plane is:
1
f
′
F
=
(100)(1.00)
−
=
F
18.17 cm
=
−
F
5.50 D
The accommodation is calculated as follows
:
+
F
FP
=
L
F
A
(100)(1.00)
⎥
⎦
+
−
5.50 D
=
F
A
−
10.27 cm
⎣
F
A
= +
4.24 D
When corrected with spectacles, the eye must accommodate 4.24 D to
image an object at 20.00 cm upon the retina.
(b) In LASIK, the correction occurs in the plane of the cornea. Consequently,
the required accommodation is the same as if the eyes were corrected
with a contact lens. Since the stimulus to accommodation is
−
5.00 D, the
required accommodation is 5.00 D.
(c) Subsequent to LASIK, the former spectacle-wearing myopic patient must
accommodate 18% more in order to see the near object [(5.00 - 4.24 D)/4.24
D
0.18]. This must be taken into account when counseling the patient
prior to surgery and when determining the amount of correction. Because
of the patient's age (45 years old), correction with LASIK may precipitate (or
make more apparent) presbyopic symptoms. It may be decided to slightly
undercorrect one of the patient's eyes so that this eye can be used for viewing
near objects. This is referred to as
monovision.
Many, but not all, patients
adapt to a monovision correction.
=
