The principles of snellens charts In order to

The principles of snellen’s charts - In order to discriminate the form of an object its several parts must be differentiated. - Each 2 separate cones in the macula are stimulated (ON) while the one between them remains unstimulated (OFF). - The average diameter of the macular cone is 0. 004 mm (4 microns), this forms the smallest distance stimulated cones.

- The normal eye should be able to appreciate a retinal image of this size. - It was found that in order to produce an image of minimal size (0. 004 mm ) the object must subtend an angle of one minute at the macula and this is taken as standard of normal visual acuity. - These principles were included in Snellen's test types (vision charts); these types consist of letters of gradually decreasing sizes. MAR=minimum angle resolution

- Each letter is of such a shape that can be closed in a square the size of which is 5 times the thickness of the line composing the letter.

- The size of the squares consisting the breadth of the lines is such that their edges subtend visual angle one minute on the macula when they are at a specified distance a way. - Each entire letter subtend an angle of 5 minutes at the same distance. A A A 5 minutes

- The first line of the type is constructed that this angle is formed at distance of 60 ms, the second letter at 36 ms , the 3 rd at 24 ms, the 4 th at 18 ms, the 5 th at 12 ms, the 6 th at 9 ms , the 7 th at 6 ms. and so. - In some charts additional lines are inserted which subtend one minute angle at 5 and 4 meters respectively. - If a person is placed at certain distance which is usually taken at 6 meters , if he has normal V. A he must read easily down to line with size 9 and the 6 size line should just be distinct. - If he can not reach this limit his vision is defective ( D. V ), but if he can exceed this limit , his visual acuity is above the standard ( hyper acute). - The result of the test is expressed by a fraction the numerator of which denotes the distance while the denominator denotes , the size of the letter in the seen line.

• Example V. A = distance / numerator = 6 size/denominator = 24 -If the person can read the letter of size 6 from 6 meters his visual acuity is (6/6); -If he can see the size 9 from 6 meters his V. A. (6/9); -If he can see the size 12 from 6 meters his V. A. (6/12); -If he can see the size 18 from 6 meters his V. A. (6/18); -If he can see the size 24 from 6 meters his V. A. (6/24).

- If he can see the letter of size 5 or 4 from 6 meters his V. A. is (6/5) or (6/4) respectively, his vision is hyper acute but if he can not read letter size 60 he is low visioned.

Measurement notation Snellen ‘s fraction Meters Feet MAR Log MAR Decimal notation 6/120 20/400 20 1. 3 0. 05 6/60 20/200 10. 0 1. 0 0. 1 6/45 20/150 8. 0 0. 9 0. 13 6/36 20/120 6. 0 0. 8 0. 16 6/24 20/80 4. 0 0. 6 0. 25 6/18 20/60 3. 0 0. 5 0. 33 6/12 20/40 2. 0 0. 3 0. 5 6/9 20/30 1. 5 0. 2 0. 66 6/6 20/20 1. 0 0. 0 1. 0

The testing of near vision (N. V) - This occurs at a distance of 30 -40 cm that called the reading distance (working distance). - The first test of this kind was constructed by Jaegar in 1867 , it consists of the ordinary print fonts (complete set of type {Printing}) of varying size as use at that time. - Recently a new test card which, approximate Jaegar original choice are used. - These are traditionally called J 1 , J 2, J 3, J 4, J 5……. they are sufficient for accurate practical purpose. - In testing N. V. the patient remains , seated on the chair, with a good light thrown over the left shoulder and he is asked to read from up down at the known reading distance. - The N. V. is recorded as J 1 for the smallest line , J 2 follows it , J 3…. J 4. - Each eye is tested separately.