|Year : 1996 | Volume
| Issue : 3 | Page : 161-164
New standardized visual acuity charts in hindi and gujarati
Bakulesh M Khamar1, Usha H Vyas2, Tejas M Desai2
1 S.C.L. General Hospital, Ahmedabad, India
2 CH. Nagari Eye Hospital, Ahmedabad, India
Bakulesh M Khamar
S.C.L. General Hospital, Ahmedabad
Source of Support: None, Conflict of Interest: None
Conventional Snellen visual acuity chart has unequal difficulty score and irregular progression in letter size causing jumping effect at different visual acuity levels. There is also increase in number of letters from above downwards. Consequently one or two mistakes per line has different meaning of visual acuity at different levels. We designed a new visual acuity chart of fourteen lines in Hindi and Gujarati to facilitate standardization in visual acuity measurement. These charts are designed for use at six meter distance, and the illumination is provided from front. These charts provide a standardized way of measuring visual acuity using local languages.
Keywords: Visual Acuity - Clinical Research - Snellen Chart - Vision Testing - Standardization.
|How to cite this article:|
Khamar BM, Vyas UH, Desai TM. New standardized visual acuity charts in hindi and gujarati. Indian J Ophthalmol 1996;44:161-4
|How to cite this URL:|
Khamar BM, Vyas UH, Desai TM. New standardized visual acuity charts in hindi and gujarati. Indian J Ophthalmol [serial online] 1996 [cited 2019 Aug 24];44:161-4. Available from: http://www.ijo.in/text.asp?1996/44/3/161/24577
Visual acuity is the most important parameter of all visual function. This parameter of visual function is the centre of all ophthalmic examination. The change in visual acuity is a major outcome parameter used to define morbidity and judge the efficacy of majority of interventions in ophthalmology. Recording of visual acuity by different people and at different locations requires standardization. In clinical practice, visual acuity is measured by Snellen visual acuity chart at 6 meters distance. Visual acuity is measured in the form of number of lines read. The change in visual acuity is judged by the number of lines gained or lost. Routinely one or two mistakes per line are allowed.
There have been advances in the understanding of physiology of vision. Attempts have been made to incorporate this information in designing visual acuity charts in English. However, this information has not been put to use in research or clinical practice in India. To the best of our knowledge an attempt has not been made to incorporate this information in designing visual acuity charts in Indian languages.
| Deficiencies of Snellen Visual Acuity Chart|| |
In conventional Snellen chart, the number of letters increases from above downwards and so one or two mistakes per line has a different meaning at different levels of visual acuity [Figure - 1]. For example missing one letter at visual acuity of 6/60 is interpreted as vision to be less than 6/60, but the identical mistake at 6/9 level is of no great significance and vision is recorded as 6/9.
Physiology of the eye is such that some letters are easy to read while others are difficult to read when other parameters are same. For example, in Gujarati chart "S" is the letter used for 6/60 visual acuity. This is one of the most difficult letters to identify [Table - 1]. This results in a situation where a person cannot read a line of 6/60 but can read a line of 6/36.
Visual acuity measurement measures minimum angle of resolution (MAR). This is done by changing the size of letters from one line to the next line. In commonly used charts, change in size of letters does not reflect regular change in MAR. This irregular progression makes it difficult to interpret the change in visual acuity measured as number of lines gained or lost. In conventional charts, change in the amount of MAR from one line to the next is not identical, and so loss of visual acuity by a line at different levels of visual acuity does not imply same amount of change in MAR. For example, a drop of two lines from 6/60 to 6/24 results in 0.4 log MAR unit change while from 6/24 to 6/12 results in 0.3 log MAR unit change.
To overcome these problems of commonly used visual acuity charts, Bailey and Lovie made the first attempt to design a new visual acuity chart. However, the chart had different number of letters in each line. This deficiency was overcome in the subsequent design of a visual acuity chart by Ferris et al for the Early Treatment Diabetic Retinopathy Study (ETDRS)., This chart in English was designed for 4 meters.
| Standardized Visual Acuity Chart in Local Languages|| |
We designed new visual acuity charts for Hindi and Gujarati to overcome the deficiencies of the conventional visual acuity charts. These charts are designed for 6 meters distance. To achieve this we calculated the size of letters in each line of the chart, determined the difficulty score for letters in Hindi and Gujarati, designed the lines for different visual acuity level with identical visual difficulty score and eliminate guess work. We also modified the ETDRS chart in English by changing the letter size to be suitable for 6 meters distance. A housing was also designed so that these charts have uniform level of illumination and can be used easily in clinical practice.
| Calculation of Difficulty Score for Letters|| |
For calculating difficulty score, 100 patients were selected randomly from our out patient department for this study and visual acuity was noted at threshold.
Difficulty score of each letter was calculated on the basis of how often that letter was read correctly at threshold by a group of patients. For example if "N" is presented for 50 times but missed for 20 times, the difficulty score of "N" is 60%. When this is measured for best corrected visual acuity line it becomes difficulty score at threshold.
For the Gujarati chart we have selected 11 letters out of which "/ill/" is the easiest letter and "/ill/" is the most difficult letter [Table - 1]. Similarly, 9 letters were selected for the Hindi chart, where "/ill/" is the easiest letter and "/ill/" the most difficult letter. The difficulty score for various letters in Gujarati and Hindi is given in [Table - 1].
| Formation of Lines with Identical Difficulty Score|| |
According to the difficulty score of each letter, we have divided letters into 3 groups of high, intermediate, and low groups. We designed a line of 5 letters from these 3 groups. A line difficulty score is a sum of difficulty score of each letter. We designed 14 lines for Hindi and Gujarati visual acuity charts such that the line difficulty score are similar [Table - 2].
We have not used same letters for all lines to eliminate the possibility to predict a letter in subsequent lines.
| Spacing and Size of Letters and Lines|| |
Each line has 5 letters. The space between two letters is one letter wide and the space between two lines is equal in height to the letter of the lower line [Figure - 2], [Figure - 3]. The letter size ranges from 4.37 mm to 87.24 mm from below upwards [Table - 3]. The progress of letter size from one line to another is 1.26 times the height of the letter in the lower line, which is a doubling of 0.1 log unit [Table - 3].
| Physical Characteristics of a Chart|| |
The chart size is 63.50 x 60.40 cm. Letters are printed on a white canvas in black colour. The chart is rolled on a wooden stick and fixed into a box. The size of the box is 105 x 37 x 19cm. The chart is illuminated by fluorescent lamps in front [Figure - 4]. The lamps are shielded so that the direct light coming from light source does not enter the patient's eye. This gives an average illumination of 2237 lux on the surface of the chart. The illumination provided is uniform all over the chart with no area getting illumination more than or less than 3.5% of average.
| Testing Distance|| |
The visual acuity charts are designed for 6 meters distance.
| Discussion|| |
These new visual acuity charts follow the design of a visual acuity chart for ETDRS. [2,3] The difficulty score of each letter was determined for Gujarati and Hindi. Because of different shapes of letters in different languages it was not possible to find letters with identical difficulty scores for each language. In English the lowest difficulty score for a letter was 70.6% as determined for ETDRS. For Gujarati and Hindi it was 53% as determined in this study and used in design of the new visual acuity charts. Because of large variation in difficulty scores of individual letters in Gujarati and Hindi they were grouped. Selection of letters from groups allowed us to have lines with almost equal difficulty scores. The maximum difference in line difficulty score between any two lines in Hindi was 2.53% while it was 2.3% in Gujarati. This is lower than the ETDRS chart, which is 4.8%.
As it is customary to check vision at 6 meter distance, we have designed the chart for this distance.
Visual acuity is also dependent on luminance. For this reason it is necessary to provide uniform illumination over the entire chart. It should not change from one examination to the next. Fluorescent lights used in our charts serve this purpose better than the tungsten lamps used in conventional charts. The visual acuity as a function of illumination remains constant in photopic range. The range of illumination prescribed for DRS/ETDRS charts is 807 to 1345 lux for the testing distance of 4 meters. This when converted for 6 meters distance becomes 1815.75 to 2926.25 lux. The 2237 lux provided in our charts is within this range.
These new standardized visual acuity charts in Indian languages could have an advantage in clinical research studies as well as clinical practice over the conventionally used Snellen charts.
| Acknowledgement:|| |
This study was supported by Nagri Eye Research Foundation.
| References|| |
Bailey I, and Lovie J: New design principles for visual acuity letter charts. Am J Optom Physiol Opt. 53:740-745, 1976.
Ferris III FL Kassoff A, Bresnick GH, et al. New visual acuity charts for clinical research. Am J Ophthalmol 94:91-96, 1982.
Ferris F, Sperduto R: Standardized illumination for visual acuity testing in clinical research. Am J Ophthalmol 94:97-98, 1982.
Gerald Westheimer. Visual acuity. In Robert A. Moses, William M. Hart Jr., Alder's physiology of the eye, clinical application. 1st Indian edition J.P. Brothers, New Delhi 1989, chap. 17.
[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4]
[Table - 1], [Table - 2], [Table - 3]