Biomedical Engineering Reference
In-Depth Information
Appendix B: Location
of Purkinje Image III
Purkinje image III is formed by reflection off the anterior surface of the crystalline
lens. Locating this image is a bit complicated because it is formed by rays that are
first refracted by the cornea, then reflected off the anterior surface of the crystal-
line lens, and then refracted once again by the cornea (Fig. B-1). We can simplify
the calculations by treating the combined refracting surface and mirror as a single
system that we call an
equivalent mirror
.
An equivalent mirror consists of the images of a reflecting surface and its cen-
ter of curvature as viewed through the
refracting
elements of the system. Purkinje
image III is formed by the equivalent mirror that is constituted of the anterior
surface of the crystalline lens and its center of curvature
as viewed through the cornea
.
To construct this equivalent mirror, we consider the anterior surface of the lens
and its center of curvature as objects and the cornea as a refracting element.
1
In
Figure B-2A, we've drawn the eye so that the lens is to the left of the cornea. This
allows us to use our linear sign convention.
The first step is to calculate the refracting power of the cornea
2
n
′
−
n
=
F
r
1.333
1.000
0.0078 m
−
=
F
= +42
F
.69 D
1. The cornea both refracts and reflects light. We are now treating it as a refracting element.
2. To determine the refracting power of the cornea, we use 7.80 mm. We do not use the radius of
curvature for the reduced eye because this would give us the total refracting power of the eye,
combining the powers of both the cornea and unaccommodated lens.
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