Biomedical Engineering Reference
In-Depth Information
Lens optical axis
Lens optical axis
32.5 mm
32.5 mm
Visual
axis
Visual
axis
2.5 mm
2.5 mm
−
4.00 DS
−
4.00 DS
30.0 mm
30.0 mm
Temporal
Temporal
Midline
f
f
Figure 10-11.
A total of 2.0
Δ
base-in prism will be included in a spectacle prescription
of
−
4.00 DS OU if each eye looks through a point 2.5 mm nasal to the optical axis.
The visual axis intersects the fovea,
f
. (You are looking down upon the patient's head.)
If the lens is decentered 2.5 mm temporal to the patient's pupil, she experiences
1.0
Δ
base-in. To obtain a total prismatic effect of 2.0
Δ
, the optical center of each
lens must be decentered 2.5 mm temporally. As can be seen in Figure 10-11, this
will occur if the optical centers of the two lenses are separated by a distance of
65 mm rather than the patient's IPD of 60 mm.
Prisms can be used to assist patients who manifest visual field defects. Because
they are thin, light, and easily changed,
Fresnel prisms
are often used for this
purpose. They come in a thin (on the order of 1 mm thick) flexible plastic sheet
that can be cut to a customized size and applied to the patient's lenses. If the initial
choice of prism power needs to be changed, the Fresnel prism can be removed eas-
ily and replaced by a prism of a more appropriate power. Figure 10-12 shows that
the prismatic power is due to a series of small prism apexes contained in the sheet.
Consider a patient with right homonymous hemianopia.
2
Objects to the right of
his fixation point are not visible unless he turns his eyes/head to the right. To see
things far to the right, he would need to turn his eyes/head far in that direction.
By prescribing a prism with its base pointed to the patient's right, objects located
in the blind area are imaged to the left allowing the patient to see them with less
movement of his eyes/head. As can be seen in Figure 10-13, Fresnel prisms can be
2. A patient with this condition is blind in the right visual fields of both eyes.
