Biomedical Engineering Reference
In-Depth Information
FIgUre 10.10
Types of grating stimuli used to measure acuity and contrast sensitivity, including a high-contrast (from the left
to the right) square-wave grating, a high-contrast, sine-wave grating, and a low-contrast, sine-wave grating. The
graph to the right of each grating illustrates its luminance profile.
FIgUre 10.11
Optotype symbol examples. From left top: Snellen, Green, Danielle, Monoyer, Dennett, digital numbers, by
resolution, variation of Landolt C, and targeting generic (DIN). On the right: Types of grating stimuli used to
measure acuity and contrast sensitivity, including a high-contrast, square-wave grating (from the left to the
right), a high-contrast, sine-wave grating, and a low-contrast, sine-wave grating. The graph to the right of each
grating illustrates its luminance profile.
Various instruments are clinically used to assess visual acuity, usually consisting of
tables with symbols, letters, or numbers with a definite dimension and for which single
elements are separated by a specific angle at the examined distance (Figure 10.11).
With noncooperative patients, black/white gratings are used, presenting precise angles
of separation at the distance examined (Figure 10.12).
The parameters used to measure the visual acuity are usually expressed in the form of
a fraction, the numerator of which indicates the distance at which the subject must be kept
so that he/she can properly identify the distance, and the denominator of which indicates
