Biomedical Engineering Reference
In-Depth Information
TABLE 8-2.
ACCOMMODATION TO VIEW AN OBJECT AT 10.00 CM
Correction in Corneal Plane
Prescription Worn
Required Accommodation*
−
5.00 DS
−
5.00 DS contact lens
+
10.00 D
−
5.00 DS
−
5.41 DS spectacle lens
+
8.65 D
+
5.00 DS
+
5.00 DS contact lens
+
10.00 D
+
5.00 DS
+
4.65 DS spectacle lens
+
11.42 D
*As measured in the corneal plane.
When properly corrected with spectacles, this myope must accommodate
8.65 D to see an object located at a distance of 10.00 cm from the cornea. The
same myope must accommodate 10.00 D when wearing contact lenses (Table 8-2).
This has important clinical implications. Consider a 45-year-old patient who
wears spectacles and wishes to be fitted with contact lenses or to undergo a laser
procedure. More accommodation is required when the correction is moved to the
plane of cornea, and this may lead to asthenopia and/or near blur. It is important
for the clinician to be aware that the accommodative demand increases when a
spectacle-wearing myopic patient is corrected with contact lenses or a laser refrac-
tive procedure.
3
Now, let's consider a patient with hyperopia.
When corrected with contact
lenses, how much accommodation is required for a 5.00 D hyperopic patient to
see clearly an object located at a distance of 10.00 cm from the cornea?
As with a myopic eye corrected with contact lenses, or an emmetropic eye, the
required accommodation is equal in magnitude to the stimulus to accommodation,
10.00 D.
How much accommodation is required when this hyperopic patient wears her
spectacles? Assume a vertex distance of 15 mm.
Applying the principles of lens effectivity, you should verify that the spectacle
lens must have a power of
215 mm).
As can be seen in Figure 8-10, the object vergence at the spectacle lens plane is
−
+
4.65 D. (The focal length is 200
+
15 mm
=
7.11 D. The virtual image (labeled
I
SP
in Fig. 8-10), which serves as the object for the accommodation lens, is located
11.76 D, resulting in an image vergence of
−
3. To minimize near symptoms, one eye of a bilateral presbyopic myopic patient is sometimes
undercorrected (i.e., the eye remains slightly myopic after it has been fitted with a contact lens or
following a laser procedure). The undercorrected eye is used to view near objects, and the fellow
eye, which is typically fully corrected, is used to view distant objects. This type of correction is
referred to as
monovision
.
