Biomedical Engineering Reference
In-Depth Information
10
Prisms
Prisms are useful clinical tools that refract light, changing the direction of incident
light rays. Since their surfaces are flat, prisms do not affect the vergence of light.
Another way of saying this is that prisms have no dioptric power—they do not affect
light vergence and do not have focal points.
Figure 10-1 shows the refraction that occurs at the two surfaces of a prism.
Refraction at each surface is predictable from Snell's law. For this prism (and all
ophthalmic prisms),
light rays are deviated toward the base of the prism
. When
an observer views an object through the prism, he or she sees an image of the object.
The image is displaced toward the apex of the prism
. Images formed by prisms
are virtual.
Those readers who have used a lensometer to neutralize prism know that the
lensometer target is displaced toward the prism base, not the apex as you'd expect
from our discussion so far. What's happening here? A Keplerian telescope, which
produces an inverted image (Chapter 12), is incorporated into the lensometer
design, resulting in the target being displaced toward the prism base.
THICK AND THIN PRISMS
The refractive properties of a prism are dependent on its index of refraction and
apical angle,
. As the refractive index or apical angle increases, the angle of devi-
ation,
d
, also increases. The apical angle and angle of deviation are labeled in
Figure 10-1.
Based on the size of their apical angle, prisms may be divided into thick and
thin prisms, with thin prisms having an apical angle less than 10 degrees. For thick
prisms, the angle of deviation is dependent on the angle that light rays strike the
prism (i.e., the angle of incidence). This can be seen in Figure 10-2, which shows
the prism deviation angle for a thick prism as a function of the angle of incidence.
Note that the prism has a minimum angle of deviation given by the trough in the
curve.
α
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