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# Determine d.v magnification

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### Determine d.v magnification

1. 1. 119/08/2011 PAKISTAN INTITUTE OF OPTHALMOLOGY
2. 2. Determing the distance magnification for low vision patients Presented by : Muhammad Yasin 19/08/2011 2PAKISTAN INTITUTE OF OPTHALMOLOGY
3. 3. Predicting the magnification required We can get the magnification using: Required magnification= denominator of present Visual acuity to denominator of desired visual acuity. For example :if actual visual acuity=10/60 And desired visual acuity=10/20 Then 60/20=3x magnification. 19/08/2011 3PAKISTAN INTITUTE OF OPTHALMOLOGY
4. 4. Magnification required = required VA present VA In Snellen notation to improve from 6/60 to 6/6 Magnification required = 6 x 60 6 x 6 = 10 x Watching TV 6/18 Reading Bus Numbers 6/6 Predicting the magnification required 19/08/2011 4PAKISTAN INTITUTE OF OPTHALMOLOGY
5. 5. Magnification used = achieved VA present VA So achieved VA = magnification x present VA For example a Px with a VA = 6/36, using a telescope with a Magnification = 4x would achieve a VA of: 4 x 6/36 = 6/9 Predicting the magnification required 19/08/2011 5PAKISTAN INTITUTE OF OPTHALMOLOGY
6. 6. If VA is measured in a LogMAR notation: Magnification = (1.25)n Where n = number of steps If the present acuity = 0.5 and the required acuity = 0.1 Then Magnification = (1.25)4 = 2.44x Predicting the magnification required 19/08/2011 6PAKISTAN INTITUTE OF OPTHALMOLOGY
7. 7. TRAIL WITH DISTANCE- VISION AIDS 19/08/2011 7PAKISTAN INTITUTE OF OPTHALMOLOGY
8. 8. Trial with Distance-Vision Aids  Distance vision aids for low-vision patients are usually afocal telescopes.  An afocal telescope is designed in such a way that if parallel rays of light enters the objective, parallel rays will emerge from the eyepiece.  However, if an afocal telescope is used at a distance closer than infinity, the divergence of the rays of light entering the objective will be greatly increased when the rays emerge from the eyepiece. 19/08/2011 8PAKISTAN INTITUTE OF OPTHALMOLOGY
9. 9. Trial with Distance-Vision Aids  Vergence amplification effect- increase in the divergence of light that occurs when an afocal telescope is used to view an object at a finite distance as referred by Bailley. (1978)  The value of vergence amplification effect- square of the magnification brought about by the telescope in question.  For a 3x telescope, the vergence amplification effect is equal to 32 or 9. 19/08/2011 9PAKISTAN INTITUTE OF OPTHALMOLOGY
10. 10. Trial with Distance-Vision Aids  When afocal telescopes are used as trial aids with low-vision patients, the vergence amplification effect can be avoided by holding in front of the afocal telescope a lens of the power indicated by the object distance.  Jose et al have suggested that only two telescopes are needed to asses a low-vision patient’s prognosis for successful telescope use:  A 2.5x monocular telescope  A combination of 6x/8x monocular telescope 19/08/2011 10PAKISTAN INTITUTE OF OPTHALMOLOGY
11. 11. Trial with Distance-Vision Aids Best CorrectedBest Corrected Distance AcuityDistance Acuity MagnificationMagnification 20/100 2.5x 20/100-20/300 6.0x 20/300-20/600 8.0x 19/08/2011 11PAKISTAN INTITUTE OF OPTHALMOLOGY
12. 12. PROCEDURES FOR THE USE OF TRAIL TELESCOPE 19/08/2011 12PAKISTAN INTITUTE OF OPTHALMOLOGY
13. 13. Procedures for the use of trial telescopes  If the best corrected distance acuity is poorer than 20/600, Jose et al. state that the prognosis is poor for the use of a telescope without extend training. They suggested procedures for the use of trial telescopes:  1. Doctor should focus the telescope initially for a 13-foot viewing distance.  2. The patient first should be asked to locate a large object at the end of the examination room. 19/08/2011 13PAKISTAN INTITUTE OF OPTHALMOLOGY
14. 14. Procedures for the use of trial telescopes  3. The test should be discontinued if patient cannot find the chart within a few minutes.  4. For the 6x telescope, the successful patient is the one who quickly finds the chart at an acuity level six times that of the best corrected distance acuity (e.g. from 20/300 to 20/50). 19/08/2011 14PAKISTAN INTITUTE OF OPTHALMOLOGY
15. 15. Points to Remember!  Magnification by telescope provide only large image rather than the clear image.  Patient having cloud media(i.e corneal opacity) will still have hazy vision even through optical devices.  Optical devices are recommended according to required magnification and patient ADL. 19/08/2011 15PAKISTAN INTITUTE OF OPTHALMOLOGY
16. 16. Points to Remember!  Utility of optical devices should be demonstrated before prescribing to patient.  Emphasize and instruct the key role of illumination to patient.  Patient will be on follow up as recommended by low vision practionar. 19/08/2011 16PAKISTAN INTITUTE OF OPTHALMOLOGY
17. 17. TYPES OF AFOCAL TELESCOPES 19/08/2011 17PAKISTAN INTITUTE OF OPTHALMOLOGY
18. 18. Types of Afocal Telescopes:  Types of Afocal Telescopes:  Galilean or Keplerian telescopes. GalileanGalilean KeplerianKeplerian *Convex objective and a*Convex objective and a concave ocularconcave ocular *No Erecting prism*No Erecting prism * Magnification 1x to 3x,4x* Magnification 1x to 3x,4x *head-borne telescope of*head-borne telescope of low powerlow power *Convex objective and*Convex objective and convex ocularconvex ocular *Requires erecting prism to*Requires erecting prism to erect inverted image.erect inverted image. * Powers ranging 3x to 15x* Powers ranging 3x to 15x * Bulky and hand-held* Bulky and hand-held telescopes for specific taskstelescopes for specific tasks 19/08/2011 18PAKISTAN INTITUTE OF OPTHALMOLOGY
19. 19. Afocal telescopes Keplerian telescopes Galilean telescopes Monocular BinocularMonocular Binocular 19/08/2011 19PAKISTAN INTITUTE OF OPTHALMOLOGY
20. 20. Factors determined the use of distance magnification system. Ease of use(e.g field of view,spotting,focussing,scanning etc.) Requirement for free hand magnification. Requirement for mobility. Contrast or image brightness. Weight,cost,cosmesis etc. 19/08/2011 20PAKISTAN INTITUTE OF OPTHALMOLOGY
21. 21. EXAMPLES OF TELESCOPES ON THE UTILITY BASIS 19/08/2011 21PAKISTAN INTITUTE OF OPTHALMOLOGY
22. 22. EXAMPLES OF TELESCOPES ON THE UTILITY BASIS  On the basis of utility factors telescopes may classified as  hand held,  spectacle mounted,  bioptic,  Max T.V  ocutech etc. 19/08/2011 22PAKISTAN INTITUTE OF OPTHALMOLOGY
23. 23. HAND HELD TELESCOPES  Hand held telescopes are most appropriate for short term viewing or sporting activities. For example : Reading bus numbers or street signs , or viewing a chalk board in the classroom. 19/08/2011 23PAKISTAN INTITUTE OF OPTHALMOLOGY
24. 24. HAND HELD TELESCOPES 19/08/2011 24PAKISTAN INTITUTE OF OPTHALMOLOGY
25. 25. SPECTACLE MOUNTED TELESCOPES Spectacle mounted telescopes are for extended viewing(e.g watching T.V program) Or hands free use(e.g Driving) 19/08/2011 25PAKISTAN INTITUTE OF OPTHALMOLOGY
26. 26. SPECTACLE MOUNTED TELESCOPES 19/08/2011 26PAKISTAN INTITUTE OF OPTHALMOLOGY
27. 27. Max T.V  max tv are hands-free, head-mounted glasses provide 2.1x magnification AND can be adjusted so that each eye lens can be focused separately [±3 diopters], making the system adaptable to most patients!  The glasses will focus on objects from distances of 3m [10 feet] to infinity, and are perfect for watching TV, sporting events, movies, theatre, bird watching, or any other distance viewing activity in which a magnified image would be helpful.  With flexible temples, MAXTV telescopic glasses are the most comfortable television viewing device ever. 19/08/2011 27PAKISTAN INTITUTE OF OPTHALMOLOGY
28. 28. MaxTV 19/08/2011 28PAKISTAN INTITUTE OF OPTHALMOLOGY
29. 29. Bioptic Telescope  Bioptics, also known as a bioptic in the singular, and sometimes more formally termed a bioptic telescope, is a term forA bioptic is a special pair of glasses with a telescope permanently mounted in the glasses' lens. While looking straight ahead, a bioptic user sees a normal, unmagnified image through the glasses. Then by dipping 19/08/2011 29PAKISTAN INTITUTE OF OPTHALMOLOGY
30. 30. Bioptic Telescope one's head slightly, the bioptic user instantly sees a magnified image through the telescope. This "bi-optical" system allows the user to rapidly switch between a normal view and a magnified view without ever using his or her hands.It is used to improve distance vision for those with severely impaired eyesight, especially those with albinism. 19/08/2011 30PAKISTAN INTITUTE OF OPTHALMOLOGY
31. 31. Bioptic Telescope 19/08/2011 31PAKISTAN INTITUTE OF OPTHALMOLOGY
32. 32. Selsi Sportglasses  Selsi Sport Glasses are made for distant viewing. They are like binoculars where you turn the knobs on the lenses to adjust the focus. Each eye focused individually. These glasses are worn like a pair of eyeglasses. Good quality sports binocular are with coated achromatic lenses.(Magnification 2.5x,8x,6x,4x) 19/08/2011 32PAKISTAN INTITUTE OF OPTHALMOLOGY
33. 33. Selsi Sportglasses 19/08/2011 33PAKISTAN INTITUTE OF OPTHALMOLOGY
34. 34. Ocutech VES Telescope  Ocutech is the low vision telescopic aids, including the AutoFocus, which focuses automatically and provides natural, hands-free magnification.  Ocutech is a leading developer of optical low vision solutions for patients suffering from macular degeneration and other low vision impairments. 19/08/2011 34PAKISTAN INTITUTE OF OPTHALMOLOGY
35. 35. Ocutech VES Telescope 19/08/2011 35PAKISTAN INTITUTE OF OPTHALMOLOGY
36. 36. REFERENCES  Bennett and Rabbetts' clinical visual optics  A guide to low vision practitioners (American Academy)  Slide show.net  Wikipedia  Theory and practice of optics and refraction by A.K.Khurana  NOAH-low vision aids 19/08/2011 36PAKISTAN INTITUTE OF OPTHALMOLOGY
37. 37. THANK YOU 19/08/2011 37PAKISTAN INTITUTE OF OPTHALMOLOGY