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 5minutes

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 3rd at 24 ms, the 4th at 18 ms, the 5th at 12 ms, the 6th at 9 ms , the 7th 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 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 MAR Log MAR Decimal notation Meters Feet 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 6/9 20/30 1.5 0.2 0.66 6/6 20/20 0.0

The testing of near vision (N.V) This occurs at a distance of 30-40cm 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 J1 ,J2, J3, J4, J5…….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 J1 for the smallest line ,J2 follows it ,J3….J4. Each eye is tested separately.