Biomedical Engineering Reference
In-Depth Information
Let's summarize what we've discussed so far. To read the medicine label, the patient
requires angular magnification of 4
. One possibility is to use relative distance
magnification by moving the label four times closer, from the reference distance
of 20 to 5 cm (the equivalent viewing distance). This is not a practical solution,
however, because the patient can't accommodate the 20 D required to view an
object located at a reading distance of 5 cm. We can overcome this problem by
placing the reading material at the focal point of a lens with a focal length equal
to the equivalent viewing distance (5 cm). This lens power has a power of
×
20 D,
which is called the equivalent viewing power. Since the rays emerging from the
magnifying glass are parallel, the patient does not need to accommodate if she
looks through the magnifying lens with her distance prescription. Furthermore, as
long as the object is at the lens's focal point, the angular magnification does not
change as the distance from the lens to eye changes.
+
We've said that angular magnification is the same as long as the reading material
remains at the focal point of the magnifying lens. In fact, as the distance between the
plus lens and eye increases (with the object remaining at the focal point of the lens),
it may
appear
that the image seen through the lens is becoming larger. This illusion
occurs because the angle subtended by reading material
not
seen through the magnify-
ing lens becomes smaller as the distance from the eye to the material increases. Con-
sequently, the image seen through the lens
appears
to be growing larger relative to the
shrinking background material to which it is being compared. If you were to measure
the retinal image size of the material viewed through the lens, however, you would
find that it remains constant as the lens moves farther from the eye. There will be no
change in the patient's ability to resolve detail in the image seen through the lens.
USE OF PLUS LENSES FOR LOW VISION
The most commonly utilized near magnification devices are plus lenses. These can
be mounted in handheld magnifiers, spectacles (high-plus single vision lenses or
high-plus bifocal adds), loupes mounted to and positioned in front of the patient's
spectacles or stand magnifiers.
Consider a 25-year-old patient who is having difficulty reading personal corre-
spondence. The smallest print this patient can read at 40.0 cm is 2 M. The patient
wishes to read print that is half as large—1 M print. What advice should we give
to this patient?
. This can be obtained when
the patient holds the printed material at one-half the reference distance. Since the
reference distance is 40.0 cm, the patient will be able to read his correspondence
when it is held 20.0 cm from his eyes. (When 1 M print is at a distance of 20.0 cm,
it subtends the same angle as does 2 M print held at a distance of 40.00 cm.)
The patient requires an angular magnification of 2
×
