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Recumbent prisms and fresnel prisms


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About the history, optics, and availability of Fresnel prisms and recumbent prism

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Recumbent prisms and fresnel prisms

  1. 1. Prepared by Loknath Goswami B.Sc Optom 2nd year
  2. 2. Prism  A prism is an object made up of a transparent material like glass or plastic that has at least two flat surfaces that form an acute angle (less than 90 degrees)  A traditional prism has two flat, nonparallel surfaces  Parallel light entering the prism is bent towards the base of the prism and leaves the back surface at an angle  A prism is thicker at the base than at the apex. The larger the prism, the thicker the base of the prism will be
  4. 4. Properties of prism  Dispersion  Deviation occurs towards the base  Total internal reflection occurs in some specific prisms.  Prism never change the vergence of light rays. A prism does not magnify or minify the image.
  5. 5. Recumbent prisms  Prisms which deviate the light by 90˚  Right angle prisms and prisms used in recumbent spectacles
  6. 6. History  In 1935, Mr. Andrew McKie Reid designed the ‘recumbent spectacles’, as a visual aid for patients suffering from spinal lesions and other diseases who were confined to the bed in a supine position for weeks or months
  7. 7. History  The spectacle was made by Messrs Hamblin of Wigmore Street, London and was shown for the first time at the Oxford Ophthalmological Congress in July 1935
  8. 8. Recumbent spectacles  Recumbent spectacles are equipped with reflecting prism that enable a person lying flat on his or her back to see straight ahead while looking straight up  The spectacle consists of a pair of prisms mounted in a frame which could either be worn over the patient’s own glasses or the refractive prescription could be incorporated in the frame  These can be useful for those wishing to read while being required to remain on their backs for long periods of time  The prisms must be perfectly aligned, or diplopia occurs, making wear impossible
  9. 9. Recumbent spectacles
  10. 10. Principle  The prisms had an apical angle of 35 degrees and basal angles of 70 and 75 degrees  These angles were calculated such that a book held in the normal position on the chest could be read by a person in the supine position even though apparently looking at the ceiling  So, with the base of the prism facing the patient, the line of vision through refraction and double internal reflection within the prisms was bent almost 90 degrees
  11. 11. Optics a 2 = 52˚
  12. 12. Optics
  13. 13. Recumbent spectacles Advantage Disadvantage  Help people to read in supine position  It is heavy and not much comfortable
  14. 14. History  In 1970, the optical sciences group of San Rafael, California, developed a series of Fresnel Press-on prisms – thin plastic membrane prisms ranging in powers from 0.5 to 30 prism diopters
  15. 15. History  These prisms are an adaptation of the hand- ground lenses designed by the French engineer and physicist Augustine Fresnel in 1821  Fresnel’s lenses were originally used for lighthouse beacons.
  16. 16. Fresnel lens and Fresnel prism
  17. 17. Fresnel lens and Fresnel prism
  18. 18. Fresnel prism  A Fresnel prism attempts to avoid thickness by building a tower of small prism  A Fresnel prism is only 1 mm thick  The refraction depends on the apical angle  The angle between the two surfaces remains constant across the prism.  E.g. 3M Press-on
  19. 19. 3M Press-On  Manufacturer: 3M  Location: USA  Material: PVC (Polyvinyl chloride)  Thickness: 1mm  Usable area: 65mm diameter  Base is noted in one corner of the prism, if it is cut then base may be found by running finger nail along prism side
  20. 20. Press-on prism availability 1∆, 2∆, 3∆, 4∆, 5∆, 6∆, 7∆, 8∆, 9∆, 10∆, 12∆, 15∆, 20∆, 25∆, 30∆, 35∆, 40∆
  21. 21. Optics
  22. 22. Optics
  23. 23. Fresnel prism trial sets  Fresnel prisms trial optics are similar to traditional prism trial optics(ring) found in trial ring sets.  Thin and light weight (particularly in the higher diopter powers)  Raised edges around the Fresnel prism trial optic prevents scratching of prism features when placed on flat surfaces  Allows for accurate testing of large oculomotor imbalances and allows patient opportunity to experience a Fresnel prism
  24. 24.  Recommend for prism cover test  Available in two, 7 ring sets: TS4000 & TS5000  TS4000: 2∆, 3∆, 4∆, 5∆, 6∆, 8∆, 10∆  TS5000: 12∆, 15∆, 20∆, 25∆, 30∆, 35∆, 40∆ TS4000 TS5000
  25. 25. A coin has two faces
  26. 26. Advantages  It is very thin and extremely lightweight  It is flexible and can be applied to an existing spectacle lens  It can be cut to any shape with scissors or a razor blade. This means that it can be cut and applied to one sector of a lens  It can be applied or removed at anytime
  27. 27. Disadvantages  Fresnel prisms look different than conventional lenses. It gives striated appearance (to observers) of the grooves. The striated appearance can be improved to some extent by use of a lightly tinted or photochromic lens  Fresnel prisms have a number of small ledges, they are harder to clean than conventional lenses
  28. 28.  High-powered prisms will cause a slight decrease in visual acuity Visual acuity loss is due to scattering of light, caused by the grooves. The scattering of light causes both a loss of contrast and the presence of bothersome reflections. The minimal acuity decrease through Fresnel prisms is less than one line on a Snellen chart at a 90% contrast level compared with acuity through conventional prism.  The flexible static vinyl material that gives these lenses the ability to be easily cut to custom shapes, unfortunately, reduces the contrast of objects viewed through the lens
  29. 29. Chart photographed with and without a 40 diopter Fresnel press-on
  30. 30. When to prescibe  When a prismatic component is to be included in a patient’s prescription, the advantages and disadvantages of press-on prisms should be carefully weighed to determine the suitability of using press-on prism.
  31. 31. When are Fresnel prism used?  High amount of prism Fresnel prism is very thin comparatively to the conventional prism. So it is useful for high amount of prism Conventional prism Fresnel prism
  32. 32.  Use and reuse Fresnel prism lenses are easy to apply and remove. They may be used and reused. This is helpful when determining how a given prism amount will work long term or for use during visual training.
  33. 33.  Sectorial application A partially paralyzed extraocular muscle may result in a different amount of prism needed for different directions of gaze. A Fresnel prism can be cut to fit that particular lens area. Prism is present only where it is needed.
  34. 34.  Visual field defects  A person may have a visual field defect  For example, the case of bilateral hemianopia, the right half of the visual field is blind for both right and left eyes.
  35. 35.  Fresnel prism can be applied to the right side of the both lenses.  In this instance, prism base direction would be base right.  With prism in place, the wearer looks to the right but does not have to turn the eyes as far to see an object in the right- hand field of view
  36. 36.  Slowing of nystagmus  Nystagmus is a defined as regular and rhythmic to-and-fro involuntary oscillatory movements. Such movement is involuntary and reduces vision.  In some cases, nystagmus may slow when the person looks to one side or the other  For example, if the examiner sees that movement slows when the person looks to the right, equal amounts of prism may be applied to both lenses. The correct base direction would be base left. Because the eyes turn toward the apex, prism base left will keep the head pointed straight while the eyes turn to the right. Since the eyes are turned to the right, nystagmus slows.
  37. 37. How to apply a Fresnel prism to a spectacle lens  Firstly we have to determine correct base direction  After that we have to take the carrier lens out of the spectacle frame  We have to place the Fresnel prism on the back of the carrier lens with its smooth side against the carrier. Making sure base direction is properly oriented and then we have to mark the boundary  With a razor blade, trim the Fresnel prism along the marking
  38. 38. How to apply a Fresnel prism to a spectacle lens  Reinsert the carrier lens into the frame  Wash both carrier and Fresnel prism with a weak solution of lotion-free liquid detergent  In a bowl of warm water, or under a stream of warm water, apply the smooth side of the Fresnel to the carrier. Work out any air bubbles that may be trapped between the two surfaces  Then we can give the lenses to the wearer, but have to instruct the wearer to handle the lenses with care for 24 hours until drying is complete.
  39. 39. How to clean Fresnel prisms  The manufacturer’s recommended method of cleaning these lenses is to rinse under warm running water. If the lenses have dirt in grooves, use a soft brush. Blot dry with a soft, lint-free cloth. Hard contact lens cleaning solutions have also been used to clean Fresnel optics.
  40. 40. How to clean Fresnel prisms Finally, the right tool to clean your Fresnel Optics. The secret is the cloth, a lint-free micro fiber cloth that can reach into the small deep grooves of a Press-On Lens. Use with the alcohol free lens cleaner to remove finger prints, oils, make-up and the daily dirt and grime that can get onto your lens. Costs $5.00
  41. 41. REFERENCES  Troy E. Fannin, Clinical optics 1st edition, Pg No. 106-107  System For Ophthalmic Dispensing-2nd edition, Clifford W. Brooks , Irvin M. Borish, Pg No. 516-519, 536   definition-refraction-quiz.html   
  42. 42. For letting me speak here