Biomedical Engineering Reference
In-Depth Information
TABLE 12-4.
AMETROPIA, TUBE LENGTH, AND MAGNIFICATION
Myopia
Hyperopia
Galilean
Keplerian
Galilean
Keplerian
Tube length
Must shorten
Must shorten
Must lengthen
Must lengthen
Magnification
Decreased
Increased
Increased
Decreased
Telemicroscopes
Suppose a telescope that is focused for infinity is used to view an object that is
at, say, 25.00 cm from the telescope. This situation is illustrated in Figure 12-9,
which shows a patient viewing an object with a Galilean telescope that has a
+
10.00 D objective and a -30.00 D eyepiece. Without the telescope, the object
has a vergence of less than - 4.00 D at the eye; with the telescope, the vergence
is about
20.00 D. The telescope amplifies the object vergence, and this ampli-
fication makes it impractical to use a telescope that is focused for infinity to view
near objects (because too much accommodation is required to focus the image on
the retina).
−
Lens caps.
A telescope can be adapted for near use by placing a plus lens over the objective
and positioning the object at the primary focal point of this lens. The plus lens
images the object at infinity; therefore, the rays that enter the telescope have zero
vergence. A plus lens used in this manner is called a
lens cap
, and a telescope that
is fitted with a lens cap is sometimes called a
telemicroscope
.
- 4.00 D
+ 10.00 D
+ 10.00 D
- 30.00 D
+ 6.00 D
- 20.00 D
6.67 cm
- 25.00 cm
Figure 12-9.
When a Galilean telescope (that is focused for distance) is used to
view a near object, the vergence is amplified. Without the telescope the vergence
at the eye would be less than -4.00 D, but with the telescope it is about -20.00 D.
The separation of the objective and eyepiece—the tube length of the telescope—is
6.67 cm.
