Biomedical Engineering Reference
In-Depth Information
12
Magnification and
Low Vision Devices
Because of disease, developmental abnormalities or trauma, certain patients
do not obtain satisfactory vision even when their ametropia is fully corrected.
These patients are said to have
low vision
. A patient with age-related macu-
lar degeneration, for instance, may have best corrected acuity
1
of 20/100. This
suboptimal visual acuity may limit the patient's ability to participate in various
daily activities that others take for granted, such as reading the newspaper and
correspondence.
As the population ages, the prevalence of low vision will increase. To obtain
satisfactory vision, these patients may use magnifying devices such as plus lenses,
electronic magnifiers, or telescopes. In this chapter, we introduce the optics of
these important clinical tools with an emphasis on devices used for near vision.
This chapter is not intended to be a comprehensive discussion of low-vision
devices.
Students and residents often find magnification confusing. This may stem from
the various forms of magnification that can be specified. For example, in this topic
we refer to lateral, relative distance, size, angular, effective, and spectacle magni-
fication. It's important to keep these terms straight. Table 12-1 summarizes the
application of these terms in low vision and clinical practice. While these terms
might not make much sense at this point, you may find it helpful to refer back to
this table as you read through the next sections.
1. Best corrected acuity
refers to the visual acuity obtained when the patient's ametropia is fully
corrected.
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