Biomedical Engineering Reference
In-Depth Information
For the left lens, the power factor is
1
Power factor
=
=
0.966
×
2.50 D)
When both lenses have a front surface curvature of
1
−
(0.014 mm)(
−
+
2.00 D and a center thick-
ness of 1.5 mm, the shape factor for each lens is
1
⎛
⎞
Shape factor
=
⎟
t
⎟
⎛
⎞
⎠
1
−
F
1
⎝
⎝
⎠
1
⎛
⎞
⎟
Shape factor
=
=
1.00
×
⎟
0.0015
1.586
⎛
⎝
⎞
⎠
1
−
+
2.00 D
Since the total spectacle magnification is the product of the power and shape
factors, a front surface power of
+
2.00 D and a center thickness of 1.5 mm result in
magnification of 0.935
for the left eye.
We can compensate for this difference by changing the shape factor for the right
lens. By increasing its curvature and thickness, we can increase its magnification
relative to the left lens.
3
Let's select a front surface curvature of
×
for the right eye and 0.966
×
+
10.00 D and a
thickness of 5.0 mm for the right lens. The shape factor is
1
⎞
⎟
The total spectacle magnification for the right lens is
⎛
Shape factor
=
=
1.03
×
⎟
0.005
1.586
⎛
⎝
⎞
⎠
1
−
+
10.00 D
M
spect
=
(
M
power
)(
M
shape
)
×
With this design, the spectacle magnification of the right lens (0.963
M
spect
=
(0.935)(1.03)
=
0.963
) is very
close to that produced by the flatter, thinner left lens, thereby all but eliminating
the aniseikonia. The right lens, however, would look much different than the left
lens, and this may not be cosmetically acceptable to the patient. When the shape of
a lens is intentionally manipulated to affect retinal image size, the lens is sometimes
referred to as an
iseikonic lens
.
×
In many cases it's not necessary to design an iseiknonic lens that fully compen-
sates for the difference in retinal image sizes. Modestly adjusting front surface
power and thickness may be sufficient to relieve symptoms. For example, in the
current case we could try increasing the right lens front surface power to
+
5.00 D
3. While changing the thickness and front surface power can significantly affect the shape factor,
changing the index of refraction has little effect.
