Multi focal lens design, history and optical principles


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Multi focal lens design, history and optical principles

  1. 1. Multi-focal lens design, history and optical principles
  2. 2. Optical principles of multifocal lens design <ul><li>Total refracting power at reading portion= power at distance + power of addition </li></ul>
  3. 3. Franklin’s Bifocal <ul><ul><li>Similar to that of One piece Executive bifocal </li></ul></ul><ul><ul><li>Integrated separate distant and near vision lenses </li></ul></ul><ul><li>Advantage: </li></ul><ul><ul><ul><li>Excellent optical properties: sharp image </li></ul></ul></ul><ul><ul><ul><li>Optical centres adjustment </li></ul></ul></ul><ul><ul><ul><li>Little chromatic aberration </li></ul></ul></ul><ul><li>Disadvantage </li></ul><ul><ul><li>Dividing line </li></ul></ul><ul><ul><ul><li>Annoying refraction </li></ul></ul></ul><ul><ul><ul><li>Dirty lens </li></ul></ul></ul><ul><ul><ul><li>Conspicuous and unsightly line </li></ul></ul></ul><ul><ul><li>Weak structure </li></ul></ul>
  4. 4. Solid Up curve Bifocal <ul><li>One piece bifocal </li></ul><ul><li>Upper flatter grinding </li></ul><ul><li>Advantages </li></ul><ul><ul><li>Invisibility and Strength </li></ul></ul><ul><ul><li>little chromatic aberration </li></ul></ul><ul><ul><li>Wide field of view for reading </li></ul></ul><ul><li>Disadvantages </li></ul><ul><ul><li>Limited surface power </li></ul></ul><ul><ul><li>Aberration for distance portion </li></ul></ul><ul><ul><li>Restricted field of vision for distance </li></ul></ul><ul><ul><li>Shifting of optic centre </li></ul></ul><ul><ul><li>Prismatic effect in distance portion </li></ul></ul>
  5. 5. Perfection Bifocal <ul><li>Modified franklin’s bifocal </li></ul><ul><li>More stable in frame </li></ul><ul><li>More distant vision portion </li></ul><ul><li>Advantages and disadvantages like franklin’s Bifocal </li></ul>
  6. 6. Cemented Kryptok Bifocal <ul><li>Cemented wafer: flint glass (n=1.67) </li></ul><ul><li>First cemented higher index material </li></ul><ul><li>Optically as well as cosmetically satisfactory </li></ul><ul><li>Difficulty manufacturing </li></ul>
  7. 7. <ul><li>Approximate power of distant portion= F1+F2 </li></ul><ul><li>Power of reading addition= F3+F4 </li></ul><ul><li>F3=-F2 </li></ul><ul><li>Power of reading addition= F4-F2 </li></ul>F2 F1 F3 F4
  8. 8. First commercial Kryptok Bifocal <ul><li>Fused bifocal under high temperature </li></ul><ul><li>Same curvature in front surface of lens </li></ul><ul><li>Spherical front surface </li></ul><ul><li>22mm segment </li></ul><ul><li>Barium crown glass in segment </li></ul><ul><li>Optic centre 11mm below segment top </li></ul>
  9. 9. <ul><li>Fd= n’-n/r1+n-n’/r2 </li></ul><ul><li>Fa= n’’-n/r1+n’-n’’/r3+n-n’/r2 </li></ul><ul><li>Or </li></ul><ul><li>Fa= n’’-n’/r1+n’-n’’/r3 </li></ul>
  10. 10. First commercial Kryptok Bifocal <ul><li>Advantages </li></ul><ul><ul><li>Clear glass </li></ul></ul><ul><ul><li>Firm segment attachment, no discoloration </li></ul></ul><ul><ul><li>Relatively invisible segment </li></ul></ul><ul><ul><li>Commercial production with low cost </li></ul></ul><ul><li>Disadvantages </li></ul><ul><ul><li>Difficult to fuse segments (600-700) </li></ul></ul><ul><ul><li>Unmatched curvature: traping of air bubble during fusion process </li></ul></ul><ul><ul><li>Alloy formation: hazy glassess </li></ul></ul><ul><ul><li>Transverse chromatic aberration </li></ul></ul>
  11. 11. <ul><li>Achromatic doublet: </li></ul><ul><ul><li>Dioptric power of two elements must be appositive in sign </li></ul></ul><ul><ul><ul><li>Positive refractive power convex crown glass+concave flint glass </li></ul></ul></ul><ul><ul><ul><li>Negative resultant power </li></ul></ul></ul><ul><ul><ul><ul><li>Distance portion negative but larger than near portion </li></ul></ul></ul></ul><ul><ul><li>Abbe value must be in the same ratio as the dioptric powers </li></ul></ul>F1/ ω 2=-f2/ ω 2
  12. 12. Straight top bifocal <ul><li>Fused bifocal </li></ul><ul><li>Compensation for vertical prismatic effect </li></ul><ul><li>Optic centre 5mm below segment top: D and 4/14mm below segment top for ribbon segment </li></ul>
  13. 13. Ultex lenses <ul><li>Power of reading portion be change in back surface curvature </li></ul><ul><li>19 to 20/38 to 40 </li></ul><ul><li>Optically consider two segments ; distant and near separate </li></ul>
  14. 14. Executive bifocals <ul><li>One piece bifocal with reading segment steeper curve </li></ul>Cu Cs Ru Rs
  15. 15. Double segment bifocals <ul><li>Close work above eye level eg electrician, painter, librarian </li></ul><ul><li>Fused as well as one piece available </li></ul><ul><li>Straight top and executive style </li></ul><ul><li>Upper segment add 2/3 of lower segment add </li></ul>
  16. 16. Trifocals Addition requiring more than +1.75DS 6-8mm segment height
  17. 17. Performance of multifocal lens
  18. 18. Optical performance of a bifocal lens can be considered in terms of <ul><li>Differential displacement at the (image jump) </li></ul><ul><li>Differential displacement at the reading level </li></ul><ul><li>Total displacement at the reading level </li></ul>2 bifocal lens may have three centers 1) dist optical center 2) seg optical center 3) resultant optical center