Multi focal lens performance 2

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Multi focal lens performance 2

  1. 1. Performance of multifocal spectacle
  2. 2. Review of previous lecture
  3. 3. Learning objective <ul><li>Estimate segment inset </li></ul><ul><li>know optical performance of some commonly prescribed bifocals </li></ul><ul><li>Derive mathematical derivation of optical performance </li></ul><ul><li>Judge the type of bifocal on basis of refractive error and addition </li></ul>
  4. 4. TERMINOLOGY Segment Inset = (IPD-NCD) /2 Total Inset = (GCD-NCD) /2
  5. 5. Lateral placement of optical centre of the segment <ul><li>Inward placement of segment OC from distance fitting point: Segment inset </li></ul><ul><ul><li>Field of view for individual segment </li></ul></ul><ul><ul><li>Horizontal prismatic effect </li></ul></ul><ul><li>The correct amount of segment inset depends on </li></ul><ul><ul><li>IPD </li></ul></ul><ul><ul><li>Vertex distance0 </li></ul></ul><ul><ul><li>Fixation distance </li></ul></ul><ul><ul><li>Power of distance correction in horizontal meridian </li></ul></ul>
  6. 7. Qs. <ul><li>Given the distant prescription -2.00DS with +2.00Ds add OU. The frame size is 50mm with DBL of 18mm, and the patient’s PD is 64mm for distance vision and 60mm for near vision. What is the segment inset? Total inset? </li></ul><ul><li>Given distance prescription +2.00Ds combined with 1 pd base in, with +2.00ds add, OU. The frame size is 46mm, with DBL 18mm. The patient’s PD is 70/66. what is the segment inset? The total inset? </li></ul>
  7. 8. Vertical placement of optical centre of the segment <ul><li>Optical axis pass through the centre of rotation of the eye </li></ul><ul><li>1mm lower distance optical centre for 2 degree of pantoscopic tilt </li></ul><ul><li>Minimuc 6 degree of pantoscpic tilt </li></ul>
  8. 9. Optical location of various segments of Bifocals
  9. 10. Differential displacement (Image jump) <ul><li>Differential displacement at the segment top (image jump) </li></ul><ul><li>Differential displacement at the reading level </li></ul><ul><li>Total displacement at the reading level </li></ul><ul><li>Chromatic aberration </li></ul>
  10. 11. Differential displacement at the segment top (image jump) <ul><li>Three optical centres </li></ul><ul><li>Prentice rule </li></ul><ul><ul><ul><li>d=distance from segment top to segment pole </li></ul></ul></ul><ul><ul><ul><li>Fa= power of add </li></ul></ul></ul><ul><li>Independent of distance power </li></ul>Image jump= d Fa
  11. 12. Qs <ul><li>A patient requires the distance prescription +3.00ds with a +2.00ds add. What is the impage jump 1. for a 22mm round fused bifocal segment; 2, for a 22mm wide fused straingt top segment; 3. 38mm round ultex segment 4. for executive style segment? </li></ul>
  12. 13. Differential displacement at the segment top (image jump) <ul><li>Size of scotoma </li></ul><ul><ul><ul><li>Linear height of apparent scotoma (h)= reading distance (cm)/100cm per pd </li></ul></ul></ul><ul><ul><ul><li>Size of the scotoma= image jump X h </li></ul></ul></ul><ul><ul><li>Calculate size of scotoma for above example? </li></ul></ul><ul><li>Zone of confusion (Blind area) </li></ul><ul><ul><ul><li>Size of pupil </li></ul></ul></ul><ul><ul><ul><li>Vertex distance </li></ul></ul></ul><ul><ul><ul><li>add </li></ul></ul></ul>
  13. 14. Differential displacement at the reading level <ul><li>Base down prismatic effect </li></ul><ul><li>No prismatic effect </li></ul><ul><li>Base up prismatic effect </li></ul><ul><ul><li>Differential displacement = Da Fa </li></ul></ul><ul><ul><ul><li>Da= vertical distance between segment optical centre and reading level </li></ul></ul></ul><ul><ul><ul><li>Fa= power of add </li></ul></ul></ul>
  14. 15. Qs <ul><li>Distance Rx +3.00Ds with add +2.00ds, what is the differential displacement at reading level, if the reading level is 8mm below the distance optical centre of the lens </li></ul><ul><li>1. 22mm kriptok bifocal whose segment top is 2mm below the distance optical centre </li></ul><ul><li>2. d segment bifocal whose segment top is 3mm below the distance centre </li></ul><ul><li>3. 38mm Ultex bifocal whose segment top is 2mm below the distant centre </li></ul><ul><li>4. executive segment bifocal whose distance and near optical centre both lie at segment top with a reading level 5mm below the segment top </li></ul>
  15. 16. Total displacement at the reading level <ul><li>Total displacement= d D F D +d A F A </li></ul><ul><li>Which type of bifocal segments have got minimal total image displacement for hyperopia and myopia? </li></ul>
  16. 17. Qs <ul><li>Distance Rx +3.00Ds with add +2.00ds, what is the Total displacement at reading level, if the reading level is 8mm below the distance optical centre of the lens </li></ul><ul><li>1. 22mm kriptok bifocal whose segment top is 2mm below the distance optical centre </li></ul><ul><li>2. d segment bifocal whose segment top is 3mm below the distance centre </li></ul><ul><li>3. 38mm Ultex bifocal whose segment top is 2mm below the distant centre </li></ul><ul><li>4. executive segment bifocal whose distance and near optical centre both lie at segment top with a reading level 5mm below the segment top </li></ul>
  17. 18. Qs <ul><li>Distance Rx -3.00Ds with add +2.00ds, what is the Total displacement at reading level, if the reading level is 8mm below the distance optical centre of the lens </li></ul><ul><li>1. 22mm kriptok bifocal whose segment top is 2mm below the distance optical centre </li></ul><ul><li>2. d segment bifocal whose segment top is 3mm below the distance centre </li></ul><ul><li>3. 38mm Ultex bifocal whose segment top is 2mm below the distant centre </li></ul><ul><li>4. executive segment bifocal whose distance and near optical centre both lie at segment top with a reading level 5mm below the segment top </li></ul>
  18. 19. Chromatic aberration <ul><li>Color fringe </li></ul><ul><li>Longitudinal chromatic aberration </li></ul><ul><li>Transverse chromatic aberration= dF d /V </li></ul><ul><li>Transverse chromatic aberration of eye at reading level= d D F D /V D + d A F A /V A </li></ul>
  19. 21. Qs <ul><li>Distance Rx -3.00Ds with add +2.00ds, what is the transverse chromatic aberration at reading level, if the reading level is 8mm below the distance optical centre of the lens </li></ul><ul><li>1. 22mm kriptok bifocal whose segment top is 2mm below the distance optical centre </li></ul><ul><li>2. d segment bifocal whose segment top is 3mm below the distance centre </li></ul><ul><li>3. 38mm Ultex bifocal whose segment top is 2mm below the distant centre </li></ul><ul><li>4. executive segment bifocal whose distance and near optical centre both lie at segment top with a reading level 5mm below the segment top </li></ul>
  20. 22. Image jump in different designs <ul><li>Image jump is ZERO in executive design </li></ul><ul><li>Image jump is least in D-bifocal </li></ul><ul><li>Kryptok </li></ul><ul><li>Image jump is HIGHEST in Ultex design </li></ul><ul><li>The lenses with the seg optical center located at or near to the seg top are referred to as “No jump bifocals” </li></ul>8
  21. 23. THANK YOU

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