|Year : 1963 | Volume
| Issue : 2 | Page : 34-37
On visual charts
|Date of Web Publication||28-Jan-2008|
S N Cooper
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Cooper S N. On visual charts. Indian J Ophthalmol 1963;11:34-7
As Indian ophthalmologists we have had the opportunity and the compulsion to use test-charts in different languages and one cannot help noticing that the ease in reading different language charts varies considerably. It is common knowledge that even in the Roman script we find certain letters more easily readable than others.
Again it is a well-known fact that Landolt's rings are more difficult to be made out than Roman letters on Snellen's chart. In my younger days I was dissatisfied with the Gujrati and Devnagri script for visual testing because they are usually prepared by artists who make the letters thick and thin as they are ordinarily written. So I prepared Gujrati and Devnagri test-charts of my own with uniform thickness to conform exactly to specifications for Snellen's charts. Even with them, in the ability to read a 6/6 line in Roman Script would generally correspond to a 6/9 one on Gujrati script and a 6/9 one on Devnagri script.
In my clinical experience with standard Snellcn's chart at 6 m. distance there appeared to he not much difficulty for subjects to differentiate between different letters from 6/12 upwards. Whereas for the 6/6 line the letters L T U V were the easiest letters to read, the most difficult being B C O R S Y . The surprising thing was that in many cases between O and C the difficulty was not of distinguishing between the two but to distinguish them at all, the subjects not being able to make out the central white parts of O and C, saying that they look like black dots. With such confusion can we accept Snellen's standard for visual testing without a murmur ?
Not being satisfied I have tried to analyse this confusion. According to Snellen's specifications the test letter must subtend an angle of 5' and the details an angle of I'. When we prepare a letter for the required distance, we find out the tangent of 5' which is .00145 and multiply it by the distance. Thus for 6/12 line we multiply .00145 by 12 m. which is equal to 17.4 mm. i.e. a letter 17.4 mm big placed 12 m. away will subtend an angle of 5' at the macula of a reduced eye. We draw a square of 17.4 mm. and try and fit in letters with the details separated by 1/5 of 17.4 i.e. 3.5 mm. Would that he correct ? The letter X fitted in square of this description is nearly I-I/6 times as big as when it is drawn in a circle of 17.4 mm. diameter, as it should be. Otherwise the whole letter subtends an angle of 8' or so and not 5'. My contention is that if we use Snellen's chart, then the principle of preparing the letter for test-charts should be to draw a circle of the size calculated for the letter and not a square and then fit in the letter with details equal to 115th the diameter of the circle. As a matter of fact certain letters simply cannot fit into this pattern; for example if we try to cramp E in a circle of this description we cannot get a distance of 1/5th diameter for the details and so letters like BEMW and most of the vernacular letters cannot and should not he utilised. If we stick to this rule you will notice that the chances of reading the whole line and not only part of it will be greater. [Figure - 2].
Corning to letters in general they fall under 3 groups, - linear, curved and curvilinear, and it is our ability to distinguish between curves and lines in different meridians that constitutes a visual test. Which letters shall we select ? When we analyse Roman letters there are either horizontal lines, verticle lines, cross lines and circles to be distinguished. So in each line there should be a letter bearing horizontal, vertical and cross-lines and a cicrular letter. As such it is not necessary to have more than 3 letters in a line. Take for example the letters HXO, in a line. It contains the horizontal and vertical lines in H, the cross-lines in X and the circle in O. If a subject can read that, he can read any letter. It would be possible to make other combinations similarly and the chart I am presenting here is a typical one. The letters have been prepared in circles not in squares as I have stated. For that purpose numerals are the best figures to adopt, but there are very few linear stimuli in numerals. [Figure - 2].
However in actual testing one comes across an annoying experience. The subject fails to realise that vision testing is a test for the different tenses and not an examination in reading, because such a one begins to read, every time a lens is changed, from the top of the board and giving a detailed description of how a letter looks like how it does not. It causes annoyance and a waste of time.
Secondly what the refractionists fail to realise is, it is possible or even easier to do subjective testing with larger letters than smaller letters. The usual practice is to get down as low down on the chart with spherical correction and try further testing on these smaller lines. To avoid these two difficulties it is better and easier to present for purposes of testing, a geometrical figure to which the patient's only response would be, whether it appears better or worse and nothing more. To my mind the pattern of a wheel with eight spokes answers this purpose admirably. It provides a single symmetrical stimulus for the macular and paramacular region spread evenly over the area. The eight straight lines are the linear stimuli in the different meridians, and the two circles provide the circular stimuli. With a little imagination one can visualise in it the patterns of all the letters of the alphabet. The pattern should be big enough to be seen with a vision of 6/18 or better and can be used for illiterates as well. Incidentally it serves the purpose of an astigmatic fan test.
It can be seen therefore that visual charts are required for two purposes -- (i) for subjective testing and (2) to register visual acuity accurately. It is best to keep the two purposes apart. For subjective testing, any design, any letter in any language can do, but for determining absolute visual acuity there should he a uniform standard.
| Registration of Visual Acuity|| |
When we travel through different countries and see the visual charts in different hospitals and consulting rooms, standards for recording as they exist are so different. In America the testing distance is 20 feet, in England 6 m. in Germany 5 in. In some instances vision is recorded in reduced fractions, some in decimals, some in degrees of visual angle. In a world with at least a pretence towards unification and mutual understanding in peaceful sciences this is a most unsatisfactory state of affairs and with unification of standards for distance and weights let there be unification for registration of visual acuity.
For reasons I shall say in a minute, I prefer to have a working distance of 5 m. and nomenclature of acuity in degrees of visual angle which the lest-design or letter subtends at 5 m.
I present to you for comparison the conventional methods. You will see at a glance, that, when tested at 5 in. the 5/50 letter subtends an angle of 50', a 5/40 one 40' and so on.
You cannot find such convenient similarity in the other arrangements. Does it matter if our test distance is 5 m. instead of 6 m. I have done this experiment on myself several times, and there is absolutely no difference between the results for 5 m. and 6 m. Besides in modern days, with Liliput size rooms it will be easier to conform to the requirements if the testing distance is kept at 5 m.
Coming to the testing distance it has to be rigidly standardized for accuracy in registering vision but it may not be so for determining refraction.
For determining refraction my practice is to use a chart first at 1 m. distance. It is not realised that one can determine astigmatic and spherical errors more easily at i m. than at 5 or 6 m. Then it is a matter of adding -0.75 to -1D to the result to get the subject to read the chart 5 or 6 m. away.
Here is a test-chart I have prepared for the purpose and which I have been using for over five years. [Figure - 4]
My usual practice is after doing a quick retinoscopy, to place adequate spheres to make the wheel as clear as possible at i m, Then ask the patient whether he sees lines in any particular meridian more clearly than others. Minus cylinders are placed in the corresponding meridian to make the wheel appear equally clear. Cross cylinders are now used to check the axis and degree of astigmatism. The gaze is now transferred above to the letters and the spherical correction is corrected till the letters appear clearmost. To this result I now add -0.75 and do a duochrome test at 5 m, or simply make him read the chart at 5 m. Very little correction now is required to make the patient see clearmost at 5 m.
There is one additional advantage in this method of testing. We come across a case where after determining refraction at 1 m. he does not take the necessary -0.75 to -1.0D extra for the 5 m. distance. He may not take more than -0.25 extra or sometimes not even that to see the 5 or 6 m. distance chart or rarely he takes more than -1, i.e. -1.25 or -1.5 extra to see the same chart. Sometimes the additional figure required may be different for the two eyes. This indicates an imbalance between the sympathetic and the parasympathetic to which I had drawn attention in a paper I had read before the International at Brussels. It is one form of accommodation anomaly which is not clearly known but which I think is one cause of asthenopia where none other is found. That however is a different subject altogether. Our attention thus can be easily drawn to such a anomaly.
| Summary|| |
Visual charts are required for two purposes - (1) for determining subjective testing and (2) for accurately registering visual acuity.
For (1) a geometrical figure in the form of a wheel is preferred. It is preferable to do subjective testing principally at 1 m. distance and then check and register the acuity at 5 m. by adding -0.75 or -1D to the 1 m. subjective test result.
For (2), a distance of 5 m. is preferred, because the registration can be done either as the visual angle that the letter subtends in minute at 5 m. from the eye or in the usual fraction form 5/50, 5/40 etc. to 5/5, the visual angle in minutes being numerically the same as the distance in the denominator.
For the second purpose the distance and charts have got to be standard all over the country, and I propose that that should be undertaken by our Society.
[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4]
[Table - 1]
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