Perform the following visual tests using your laboratory partner as a test subject. If your partner usually wears glasses, test each eye with and without the glasses.
1. Visual acuity test. Visual acuity (sharpness of vision) can be measured by using a Snellen eye chart (fig. 36.1). This chart consists of several sets of letters in different sizes printed on a white card. The letters near the top of the chart are relatively large, and those in each lower set become smaller. At one end of each set of letters is an acuity value in the form of a fraction. One of the sets near the bottom of the chart, for example, is marked 20/20. The normal eye can clearly see these letters from the standard distance of 20 feet and thus is said to have 20/20 vision. The letter at the top of the chart is marked 20/200. The normal eye can read letters of this size from a distance of 200 feet. Thus, an eye that is only able to read the top letter of the chart from a distance of 20 feet is said to have 20/200 vision. This person has less than normal vision. A line of letters near the bottom of the chart is marked 20/15. The normal eye can read letters of this size from a distance of 15 feet, but a person might be able to read it from 20 feet. This person has better than normal vision.
To conduct the visual acuity test, follow these steps:
a. Hang the Snellen eye chart on a well-illuminated wall at eye level.
b. Have your partner stand 20 feet in front of the chart, gently cover the left eye with a 3" x 5" card, and read the smallest set of letters possible.
c. Record the visual acuity value for that set of letters in Part A of Laboratory Report 36.
d. Repeat the procedure using the left eye.
2. Astigmatism test. Astigmatism is a condition that results from a defect in the curvature of the cornea or lens. As a consequence, some portions of the image projected on the retina are sharply focused, and other portions are blurred. Astigmatism can be evaluated by using an astigmatism chart (fig. 36.2). This chart consists of sets of black lines radiating from a central spot like the spokes of a wheel. To a normal eye, these lines appear sharply focused and equally dark; however, if the eye has an astigmatism some sets of lines appear sharply focused and dark while others are blurred and less dark.
To conduct the astigmatism test, follow these steps:
Figure 36.1 The Snellen eye chart looks similar to this but is somewhat larger.
20 15 20 13
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