Biomedical Engineering Reference
In-Depth Information
0.00 D
-256.41 D
F
′
C
-256.41 D
+3.90 mm
+7.80 mm
Figure 14-14.
The vergence relationship can be used to calculate the location of
Purkinje image I by treating the anterior cornea surface as a mirror.
Since the light source is at infinity, it has a vergence of zero. As illustrated in
Figure 14-14, we can use the vergence relationship to locate the
reflected
image.
L
′
=
L
+
F
L
′
=
0
−
256.41 D
256.41 D
The reflected image is virtual, and its distance from the corneal surface is
L
′
= −
−
1
′
=
L
l
−
1
′
l
=
−
256.41 D
′
= +
3.90 mm
Purkinje image I is located 3.90 mm to the right of the corneal surface.
There is another way to approach this problem. Since the object is at infinity,
the image must be located at the secondary focal point of the mirror. The second-
ary focal length can be determined from the radius of curvature as follows:
r
l
0.0039 m or
+
=
2f
′
+
7.80 mm
=
2f
′
f
′
= +
3.90 mm
