Biomedical Engineering Reference
In-Depth Information
- 3.00 D
+ 3.00 D
0.00 D
- 33.33 cm
Figure 12-10.
A lens cap is placed over the objective to create a telemicroscope.
The working distance in this example is 33.33 cm.
3.00 D lens cap that would be
used to view an object at a distance of 33.33 cm. The telemicroscope is sometimes
mounted in a spectacle frame so that the patient has use of his hands.
While tele-
microscopes provide patients with a greater working distance than plus lens
magnifiers, their field of view is more limited.
Figure 12-10 shows a telemicroscope with a
+
A fully corrected myopic patient with age-related macular degeneration can
barely see 5 M print at a distance of 40.0 cm when looking through his bifocal
add. He would like to read 1 M print and can do so with his
12.5 D magnify-
ing glass, but must hold the material too close to the lens to allow him to make
notes. If we wish to fit this patient with a
+
Galilean telescope, what power
lens cap should we prescribe? How far should the reading material be held from
the telemicroscope? How does this working distance compare with that obtained
using the magnifying glass?
+
2.5
×
The telescope provides 2.5
×
magnification. Since the patient needs a total of 5.0
×
magnification (5 M/1 M
) is
required. This can be obtained by halving the distance from 40 to 20 cm. For the
light rays entering the telescope objective to be parallel, the lens cap must be
=
5
×
), an additional 2.0
×
magnification (5.0/2.5
=
2.0
×
+
5.00 D
(100/20 cm
5 D). The working distance of 20 cm with this telemicroscope is greater
than the 8 cm (100/12.5 D) required when using a
=
12.5 D magnifying glass (and
would be more suitable for allowing the patient to make notes).
16
Another approach to determine the lens cap power is to treat it as an add and
use the add equation. This formula works whether the magnification is lateral
(as with the virtual image formed by a stand magnifier) or angular, as is the case
with telescopes. For the current problem, the equivalent viewing distance is 8.00 cm
+
16. In clinical practice, the patient's working distance and corresponding lens cap power are often
determined first, and the magnification of the telescope is based upon these.
