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Ophthalmic lens reflections & ARC


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Dispensing Optics

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Ophthalmic lens reflections & ARC

  2. 2. REFLECTIONS Troublesome to the wearers because2) Produce ghost image3) Falsification of image position4) Haze and loss of contrast5) Produce veiling glare
  3. 3. Reflected image is troublesome if2) It’s located near a fixated object3) It’s sufficiently intense to stand out from its background4) It is in focus
  4. 4. TYPES OF THE SURFACE REFLECTION(1) • Most intense • 4.3 % of the total reflection
  5. 5. (2) Intensity of the image is 3.94%
  6. 6. (3) Most troublesome though intensity is only 0.17 %
  7. 7. (4) Intensity of the image is 0.09 %
  8. 8. (5) Intensity is 0.08 % of the incident light
  9. 9. METHODS TO CONTROL1) Base curve selection2) Alteration of pantoscopic tilt3) Changing the vertex distance4) Selection of small lens sizes5) Patient education and counseling6) ANTIREFLECTIVE COATINGS
  11. 11. Optical principle of ARC • Works on the principle of INTERFERENCE • Conditions should be satisfied for good ARC are 1) Amplitude condition 2) Path condition
  12. 12. How does it work? A B M N Air (Na) Cancellation P O X X’Coating (nc) ¼ wavelength Reinforcement K L Y Y’ Glass (Ng) Q R
  13. 13. PRODUCTION OF ARC GLASS  Clean the lens thoroughly  Place in the vacuum chamber and remove all the air  Heat to about 300 C for hardness  MgF2 pellets are heated to about 2500 C for vaporization which deposits on the lens  Thickness can be confirmed by 1) Observing the color of the light reflected from lens surface 2) Photoelectric cell
  14. 14. PLASTIC CR-39 lenses are heated to less than 100 C ( Excessive heating may lead to loss of stability and optical characteristic ) lenses are boiled for 2 hours to remove water vapor and volatile organic compounds Process carried out with electron beamevaporation sources and micro processorcontrolled monitoring technique Thin coat of NaO2 ( Quartz ) is applied for hardness on which the ARC is given
  15. 15. Hydrophobic coat • hydrophobic top coat • any number of anti- reflection vacuum layers (metal oxides) • hard coating layer • adhesion layer • lens substrate less than 1/300th the thickness of a human hair
  16. 16. Advantages of ARC
  17. 17. • Better cosmesis• Better vision –increased transmission –reduced secondary reflections (night driving) –reduced side glare –reduced eyeball reflections
  18. 18. DRAWBACK OF ARC • Historical reasons, education • “Durability” • “Difficult to clean”
  19. 19. .........are ARC lenses