Vision Visual Acuity Visual acuity VA is the

Vision

Visual Acuity Visual acuity (VA) is the ability to see the details and contours of objects clearly. It is tested for both distant as well as near vision. This acuity of vision refers to the ability of the eye to recognize two point sources of light, or two parallel lines, as separate rather than one.

TEST FOR DISTANT VISION Snellen’s chart. The distant vision is clinically tested with this chart. It has a series of printed letters of varying sizes, black on a white background, arranged in eight lines. The top letter, the largest, is visible to the naked eye at a distance of 60 meters, and the subsequent letters at distances of 36, 24, 18, 12, 9, 6 and 5 meters respectively. The distances for each line are indicated above it (or below it in some charts).

• In the E Test chart, the letter E is printed in 8 lines, the “legs” of the letters pointing in different directions. A person (or a child) who cannot read, has to indicate the direction in which the legs of each letter are pointing.

Figure 1: Snellen’s test types

The test chart for testing eyes usually is placed 6 m. away from the tested person , and if the person can see the letters of the size that he or she should be able to see at 6 m. He or she is said to have 6/6 vision, that is normal vision.

Color Vision Color sense: is the ability of the eye to discriminate between colors excited by light of different wave length. There are three primary colors: blue, green, and red. To perceive these three primary colors, there are three different cone systems: 1 - blue cone system 2 - green cone system 3 - red cone system

Color Blindness Color blindness: is a defect of vision affecting the ability to distinguish colors , 8% of males and 0. 4 % of females have color blindness. Color blindness is a sex linked recessive characteristic diseases. It may also Occur due to injury or diseases of retina, or in other nerve portions of the eye.

Clinical Method for Stating Color Vision Color vision is tested by Ishhara’s chart. These consist of numerical constituted by dots of different colors. A normal person can read the numerical ; whereas a color blind can not. The subject should be asked to read each plate from distance 75 cm. within 30 second.

• A random pattern of gray level dots is first put together. • A digit pattern is then added which is defined by yellow/blue variation only. Since most people with red/green colorblindness can see yellow/blue they will be able to see the digit 5 in this test pattern. • Another digit pattern which is defined by red/green variation is added. Here is the pattern composed of the random brightness pattern and the red/green pattern. • Finally all three are added: People with red/green deficiency will not be able to see the red/green pattern and will see the 5. People with normal vision will see both the patterns.

Figure 2: Ishihara color plates for testing color vision What number do you see revealed in the pattern of dots below?

What number do you see revealed in the pattern of dots below?

What number do you see revealed in the pattern of dots below?

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