Biomedical Engineering Reference
In-Depth Information
uncorrected hyperope suffers a disadvantage. [When corrected with contact lenses,
all three patients will have the same NPA (50.00 cm) since each has a corneal far-
point vergence of 0.00 D.]
ACCOMMODATION WHEN AMETROPIA
IS CORRECTED WITH SPECTACLES
Ametropia can be corrected in the plane of the cornea (with, for example, contact
lenses, laser procedures, or orthokeratology) or in the spectacle plane. The amount
of accommodation that is required to image a near object on the retina depends on
whether the ametropic correction is in the plane of the cornea or in the spectacle
plane.
First consider the case of a myopic patient who is fully corrected with a
5.00 D
contact lens. How much accommodation is required to image an object located
10.00 cm anterior to the cornea onto the retina?
−
Recall that when an eye is corrected with a contact lens, the required accommo-
dation is equal (and opposite) to the stimulus to accommodation. In this example,
the required accommodation is 10.00 D (which is equal to the accommodation
required by an emmetropic eye for the same stimulus to accommodation).
The patient needs spectacles (vertex distance of 15 mm) to use for those times
when she is not wearing her contact lenses. What should be the power of the
spectacle lenses? When wearing the spectacles, how much accommodation is
required to see clearly an object that is located 10.00 cm anterior to the cornea?
In Chapter 7, we learned how to perform calculations involving lens power effec-
tivity. You should confirm that at the spectacle plane of 15 mm, the required spec-
tacle lens has a power of
185 mm.)
As shown in Figure 8-9, the object is located 85.00 mm in front of the
−
5.41 D. (The focal length is 200
−
15 mm
=
−
5.41 D
spectacle lens. The resulting image vergence is
′
=
+
F
L
L
(1000)(1.000)
⎥
⎦
′
=
L
+
(
−
5.41 D)
−
85.00 mm
′
= −
L
17.17 D
17.17 D, but because the lens is
15 mm anterior to the cornea, this is not the vergence at the eye. To calculate the
vergence at the eye, we must first locate the image formed by the spectacle lens.
The vergence leaving the spectacle lens is
−
