Biomedical Engineering Reference
In-Depth Information
Figure 14-17.
In keratometry, luminous objects (called mires) are reflected off the
cornea. Since the dimensions of the mires are known, the radii of curvature for the
corneal principal meridians can be calculated.
SUMMARY
Similar to spherical refractive surfaces and lenses, mirrors can change the vergence
of light rays that are incident upon them. When using the vergence relationship for
mirrors, it is necessary to take into account that the direction of light is reversed
after reflection.
A refracting surface, such as a surface of a spectacle lens, can reflect light. Bother-
some reflections can be reduced through the use of antireflection coatings.
Reflections off the eye's four primary refracting surfaces are referred to as Pur-
kinje images. Purkinje image I, formed by the anterior surface of the cornea, has
numerous clinical applications, including corneal topography, keratometry, and the
Hirschberg test.
KEY FORMULAE
Relationship between a mirror's radius of curvature and its power:
−
2
F
=
r
Relationship between a mirror's focal length and its power:
−
1
F
=
f
′
Relationship between a mirror's focal length and radius of curvature:
r
′
Image location for a mirror:
−
2
f
=
1
′
=
L
′
l
