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How to use Snellen Eye Chart and Interpret Results
I show how to use a Snellen Eye chart and interpret the results.
Decoding Snellen’s Chart
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Recording Visual Acuity
Visual acuity using the Snellen’s chart is recorded as a fraction, separately for each letter. The numerator denotes the distance at which the eye being tested perceives the letter, while the denominator denotes the distance at which a normal eye can perceive the letter.
For example, a vision of 6/60 means that a normal eye can read the letter on the 6/60 even at 60 metres, but the eye that is being tested can read the letter only at 6 metres.
Optotypes of the Snellen’s Chart
Each letter on the Snellen’s chart is called an optotype and each optotype is made of a squaretype font. Not all the letter of the alphabet can be printed in such a way as to meet the requirements of the chart, so only a few select letters are used in the common chart: E, C, F, D, O, P, L, N, Z, T. A few other letters are also occasionally found: H, K, R, U, V.
A complex mix of mathematics and optics go into the successful construction of an efficacious Snellen’s chart. For the purposes of this article, we will use trigonometry formulas to try and recreate the optotypes of the Snellen’s chart. The statement that governs the whole of Optics reads as follows. ‘The smallest letter on the chart seen from a particular distance will subtend an angle of 5´ at the nodal point.’
We will treat this statement as absolute fact, and use it as the basis of recreating each line of the chart, starting from the largest letter on the chart.
minimum angle of resolution of the eye = 1 arc minute
1 arc minute = 1/60th of a degree
1 degree = 60 arc minutes
According to the statement, at 60 metres, the angle subtended by the 6/60 line would be 5´ of arc, with each limb of the letter subtending an angle of 1´ of arc.
Therefore, to construct the 6/60 line, we need the dimensions of the letter in usable physical measurements.
Applying the laws of trigonometry, tan? = opposite/adjacent;
tan (5/60) = a /60
a = 60 x tan (5/60)
a = 60 x 0.0014544421
a = 0.08726 metres = 8.726 cm
Therefore, the largest letter has to have a height of 8.726cm
Some of My optometry Related videos are given below:
1. Paediatric Vision Charts
https://youtu.be/UDPM0yUItNk
2. Diplopia Charting and Interpretation
https://youtu.be/qjZAn4y8vAA
3. Retinoscopy
https://youtu.be/AdKxOOzlx4Q
4. 4 Prism base out test
https://youtu.be/ZtO5NmVQQSk
5. Bagolini striated glass test
https://youtu.be/aOMbp3Lr2h4
6. Jackson cross cylinder
https://youtu.be/hMusaIvTLVo
7. Duochrome Test
https://youtu.be/_W4RygGxXxg
8. Worth’s four dot test
https://youtu.be/YAqo25e1YEc
9. Amplitude of Accommodation
https://youtu.be/aaDDxgsjOPQ
10. Brock string
https://youtu.be/VjSkFlnoCU
11. worth four dot test
https://youtu.be/3P7cS2o_94c
12. worth four dot test in hindi
https://youtu.be/yNumkEbd4Sk
13. Subjective refraction
https://youtu.be/eiXQiZ1GjaU
14. Chsme ka number kaise nikale
https://youtu.be/6Vwx6XWkpKU
15. How to keep your eyes healthy
https://youtu.be/oKSkj7hnFt0
16. Corona and eye
https://youtu.be/25S4vdLkvfI
17. Covid 19 and conjunctivitis
https://youtu.be/exanGLlgygs
18. Corona virus ke laksan aur bachaw ke upay
https://youtu.be/mftOMio9bVM
19. Role of social distancing
https://youtu.be/HhnEhJGL_kk
20. Hirschberg test | Corneal reflex test | CLR
https://youtu.be/zWwFktmNVW4
21. Cross Retinoscopy
https://youtu.be/MfLzuytORs
22. 15 Eye exercise to relax and strengthen your eye muscles
https://youtu.be/Y3sJLyonKWc
23. Maddox Rod test
https://youtu.be/9W4EFS2VvNE
24. Duochrome Test
https://youtu.be/kyRteBAxIyE
25. Aankho ke liye best hai ye exercise
https://youtu.be/S2mbz4bOkEw
26. Cover Test
https://youtu.be/8psmE5k65mQ
Chapters
00:00 Start
00:15 History
01:03 Recording Visual Acuity
01:42 Optotype ofthe Snellen’s Chart
05:14 Using the chart in a clinic
07:42 Interpretation of the chart
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#24, រៀនរាប់លេខពី ១ ដល់ ១០០ Learning count number from 1 100 | SChBeginner
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In this video will teach about How to count number in English,
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