Visual acuity, the degree of detail the eye can distinguish, is measured with a Snellen visual acuity chart. The chart should be attached to a wall or door so it can be viewed from a distance of exactly 20 feet.

Click here to view a Snellen Chart

The subject should perform the test initially with eyeglasses or contact lenses. The test can then be repeated with corrective eyewear. Standing 20 feet from the Snellen chart, the subject covers one eye with a slip of paper and reads the successively smaller lines of print until the letters can no longer be distinguished correctly. The last line that can be read without mistakes identifies the limit of visual acuity for that eye. The test is then repeated for the other eye.

If the smallest letters that can be read are on the 20-foot line (marked on the border of the Snellen chart), the subject has 20/20 vision in that eye. This is considered to be normal visual acuity. If the letters below the 100-foot line cannot be distinguished, the subject’s visual acuity is 20/100, considerably less than normal. On the other hand, visual acuity of 20/15 is better than average according to the Snellen test, but a farsighted (hyperopic) individual who achieves this score may not be able to focus well on closer objects.

Visual Acuity Test

1. Print off and hang the Snellen eye chart on wall at eye level.

2. Have your partner stand 20 feet in front of the chart, cover the left eye with a 3” x 5” card, and read the smallest set of letters possible.

3. Record the visual acuity value for that set of letters in the data sheet. (If the subject wears glasses, repeat the test with glasses on.

4. Repeat the procedure, using the left eye.

USE THIS DATA TABLE TO ENTER YOUR RESULTS

You may also try these interactive Visual acuity site

http://www.smbs.buffalo.edu/oph/ped/IVAC/IVAC.html

http://www.eyecareindia.com/subcontents.asp?subcontentid=37&sectionid=6

Astigmatism is blurring or distortion of the visual image by improper convergence of light rays as they enter the eye through imperfections in the cornea. Many people have varying degrees of astigmatism without being aware of the condition.

The test for astigmatism should be performed without corrective lenses because astigmatic compensation is usually included in eyeglass prescriptions.

The astigmatism test chart resembles a wheel with the spokes radiating outward from the center. When viewed from a distance of 8 to 10 feet, the lines in certain planes will seem exceptionally thick and dark to the astigmatic eye. The chart should be evenly lighted and viewed monocularly (one eye at a time) to test for astigmatism in each eye individually. Repeat the test with corrective lenses, if usually worn, to determine whether total astigmatic compensation is provided.

Click here to view an astigmatism chart

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. 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 astigmatism, some sets of lines appear sharply focused and dark while others are blurred and less dark.

Procedure

1. Hang the astigmatism chart on a wall at eye level.

2. Have your partner stand 20 feet in front of the chart, cover the left eye with a 3” x 5” card, focus on the spot in the center of the radiating lines and report which lines, if any, appear more sharply focused and darker.

3. Repeat the procedure using the left eye. (Repeat the test with glasses on if the subject wears glasses)

4. Record your results on the data sheets.

USE THIS DATA TABLE TO ENTER YOUR RESULTS

You may also try this interactive astigmatism test site

http://www.eyecareindia.com/subcontents.asp?subcontentid=39&sectionid=6

Accommodation is the changing of the shape of the lens that occurs when the normal eye is focused for close vision. It involves a reflex in which muscles of the ciliary body are stimulated to contract, releasing tension on the suspensory ligaments that are fastened to the lens capsule. This allows the capsule to rebound elastically, causing the surface of the lens to become more convex. The ability to accommodate is likely to decrease with age because the tissues involved tend to lose their elasticity.

Figures 6.5 Vision abnormalities

Nearsightedness, or myopia, is a condition in which incoming light rays come to a focus in front of the retina. The problem is corrected by placing a biconcave lens in front of the eye.

The focal point for incoming light rays is behind the retina in the condition known as farsightedness or hyperopia. A biconvex lens is used to correct this problem.

Astigmatism is a condition in which uneven focusing of the visual image results from distortion of the curvature of the lens or cornea. The problem is corrected with an optical lens that has the same degree of astigmatism, but at right angles to the plane of astigmatism in the eye. Procedure Hold the end of a meter stick against your partner’s chin so that the stick extends outward at a right angle to the plane of the face. Have your partner close the left eye. Hold a 3” x 5” card with a word typed in the center at the distal end of the meter stick. Slide the card along the stick toward your partner’s open eye, and locate the point closest to the eye where your partner can still see the letters of the word sharply focused. This distance is called the near point of accommodation, and it tends to increase with age. (See Table 6.1) Repeat the procedure with the right eye closed. Record the results in the laboratory data sheet USE THIS DATA TABLE TO ENTER YOUR RESULTS

Perform the following demonstrations with the help of a partner.

Rods are photoreceptors adapted to reception of dim light of a broad color range (that is, dim light – black-and-white vision). Cones are adapted for bright light – color vision because they require higher light intensity, but different cones can perceive different wavelengths (colors) of light. The cones are densest at the fovea and the rods in the peripheral portion of the visual field. You can demonstrate the monochrome nature of the peripheral field by means of this demonstration:

Procedure

1. While the subject stares forward, slowly bring a brightly colored piece of paper into the visual field from behind the subject’s head.
2. Stop when the subject indicates that the object has just entered the visual field.
3. Ask what color the object is. If you did the test properly and the subject isn’t cheating, the color is difficult or impossible to determine.

G) Near Point

Your near point is the minimum distance at which you can focus on an object.

Procedure

1. Place the beginning of a centimeter scale against your cheek, just below your right eye.
2. Have someone cover your left eye with an index card.
3. Hold a pin against the scale and move it along the scale until you find the minimum distance at which you can keep the pin point focused.
4. Record this as the near point for your right eye.
5. Repeat for the left eye.
6. If you wear glasses, determine your near point with glasses for each eye
7. Enter the data on the data sheet

USE THIS DATA TABLE TO ENTER YOUR RESULTS

Near Point of Accommodation vs Age

Table 6.1 Near Point of Accommodation

A small depression in the retina, the fovea centralis, has a high concentration of cones (color receptors). Your clearest vision results from stimulation of foveal cones, and whenever you look directly at an object, its image is focused on the fovea.

Click here to view a visual map chart

What is the size of the fovea?

1. Tape a Visual Map to the wall at eye level.
2. Cover the left eye with an index card (if you normally wear glasses for reading, leave them on).
3. Center your right eye in front of the dot in the small line of type.
4. Place one end of a ruler against the map and the other against your forehead to maintain a constant 300 mm distance. Stare intently at the dot in the center of the small line of type.
5. Without shifting your eye, notice which letters are clearly in focus on either side of the dot.
6. Draw around the dot a circle which encloses only those letters which were clearly focused.
7. Measure the diameter of the circle in mm and substitute into the Actual Diameter formula given below.

Actual foveal diameter = ______ mm

Near the fovea is a blind spot on the retina where the optic nerve attaches.

1. Position yourself in front of the Visual Map as before, with your left eye covered (if you normally wear glasses for reading, leave them on). Stare at the dot.
2. An observer moves a pencil, covered with white paper except for its tip, slowly across the map from left to right beginning at the dot. When the pencil point seems to disappear, a mark is made on the map.
3. When it reappears, another mark is made.
4. Repeat the procedure going vertically through the blind spot and twice diagonally through the blind spot.
5. Connect the marks with a curved line to complete the map.
6. Measure the approximate map diameter of the blind spot in mm and substitute into the Actual Diameter formula given below.

Actual blind spot diameter = ______ mm

USE THIS DATA TABLE TO ENTER YOUR RESULTS

Actual diameter = mm x 17 mm
                                  300 mm