Biomedical Engineering Reference
In-Depth Information
(from
2.00 D) and the center thickness to 3.0 mm (from 1.5 mm). This solution
would be more cosmetically acceptable. We could also try to minimize the power
factor by making sure the vertex distance is as short as possible.
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SUMMARY
The retinal image size depends on the nature of the ametropia (axial or refractive)
and how it is corrected (contact lenses or spectacles). In uncorrected axial myopia,
the retinal image is larger than in emmetropia, while in uncorrected axial hypero-
pia, it is smaller than in emmetropia. Correction with a contact lens does not alter
this, whereas correction with a spectacle lens makes the retinal images in axial myo-
pia and hyperopia about the same size as in emmetropia. In comparison, the retinal
image sizes in uncorrected refractive myopia and hyperopia are both equal to that
in emmetropia. Again, correction with a contact lens does not alter this, whereas
a minus spectacle lens used to correct refractive myopia makes the image smaller
than in emmetropia, and a plus spectacle lens used to correct refractive hyperopia
makes the image larger than in emmetropia.
The correction of anisometropia may result in aniseikonia. In the case of axial
anisometropia, one would expect that correction with spectacles would be prefer-
able to contact lenses, but this often is not the case. When prescribing spectacles
for refractive anisometropia, it's sometimes necessary to minimize the difference in
spectacle magnification between the two eyes. This may require special lens designs
in which front surface power and lens thickness are selected for this purpose. Com-
monly, however, modest adjustments in front surface power and thickness, while
minimizing vertex distance, are sufficient.
KEY FORMULAE
Spectacle magnification:
=
(
)(
)
M
spect
M
power
M
shape
1
1
⎛
⎝
⎞
⎠
⎛
⎞
M
spect
=
⎟
t
⎟
1
−
dF
v
⎝
⎠
1
−
F
1
⎝
⎠
SELF-ASSESSMENT PROBLEMS
1. What is the spectacle magnification produced by a
−
6.50 DS polycarbonate
spectacle lens that has a front surface power of
3.00 D and a center thickness of
2.00 mm? (Assume a vertex distance of 12.0 mm.)
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