Biomedical Engineering Reference
In-Depth Information
Since this image is located 61.16 mm to the left of the spectacle lens, it is located
73.16 mm (61.16
73.16 mm) to the left of the cornea.
Next, we must determine the correction required in the plane of the cornea. For
a vertex distance of 12.0 mm, a
+
12.00 mm
=
−
7.50 D spectacle lens has an effective power of
−
6.88 D at the cornea. [We've already learned how to do this, but in case you need
a quick refresher: the focal length of corrective lens required in the plane of the cor-
nea is (1000/
−
7.5 D)
+
(
−
12 mm)
=
−
145.33 mm; the power is 1000/
−
145.33 mm
=
−
6.88 D.] This is the far-point vergence as measured at the cornea. Substituting into
the vergence relationship for accommodation, we have
+
F
A
F
FP
=
L
(1000)(1.000)
⎥
⎣
⎥
⎦
+
−
6.88 D
=
F
A
−
73.16 mm
F
A
= +
6.79 D
When wearing a spectacle lens, the eye must accommodate 6.79 D. This is less
than the 8.00 D of accommodation that would be required if the eye were corrected
with a contact lens.
CORRECTION OF PRESBYOPIA
In developed countries, presbyopia, with its symptoms of near blur and asthenopia,
probably prompts more visits to the eye doctor than any other single diagnosis.
Fortunately, it's usually straightforward to alleviate presbyopic symptoms by pre-
scribing plus power beyond that required for distance viewing. This can take the
form of single-vision lenses for near use or multifocal lenses that allow correction
of the patient's distance refractive error when viewing through the upper portion of
the lens and correction of presbyopia when looking through the lower portion. The
difference between the distance and near powers of a multifocal lens is referred to
as the add. If, for example, a 5.00 D myopic patient requires a
+
2.00 D add, the near
portion of the correction has a total power of
3.00 D.
How is the patient's add determined? It is based on the patient's amplitude of
accommodation, which is defined as the total amount the patient can accommodate.
In general, the goal is to prescribe an add that requires the patient to use one-half of
her accommodative amplitude at her habitual reading distance. So, if a 50-year patient
has an amplitude of accommodation of 2.50 D, the goal is to allow the patient to use
only half of this amplitude (
−
1.25 D) when reading. By doing so, the patient's range
of clear vision at near will be centered
dioptrically
at her reading distance. That is, she
can relax her accommodation 1.25 D to see objects further away than her reading
distance or accommodate another 1.25 D to see nearer objects.
Near-point testing to determine the patient's add is often performed in the pho-
ropter. The doctor selects a tentative add based on the patient's age and history,
and the patient is then instructed to view near-threshold letters located at her near
working distance (when looking through her combined distance correction and the
+
