Aberrations of Lenses


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Aberrations of Lenses - Dr James Beatty

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Aberrations of Lenses

  1. 1. Aberrations of Lenses Optics 2005 J.Beatty
  2. 2. Introduction  In practice refracting surfaces or systems fall short of theoretical perfection due to aberrations.  The eye converges waves onto retina. The deviation of the converging wavefronts from perfect spheres are called aberations.
  3. 3.  We will discuss and describe these aberrations in optical systems, and in the eye, and see how they may be eliminated or reduced.
  4. 4. Classification  Chromatic  Monochromatic  For a SPECIFIC wavelength of visible light:  Spherical refractive error(defocus)  Cylindrical (astigmatism)  Diffraction  Spherical aberration  Oblique astigmatism  Coma  Distortion  Curvature of field  Higher order
  5. 5. Chromatic Aberration  White light is dispersed into its component wavelengths or colours at an optical surface.  Shorter wavelengths (blue) are deviated more.  Dependent on the optical properties of the material, not the the optical shape.
  6. 6.  Corrected in optical systems by:  Achromatic lenses: composed of elements of varying material which neutralizes this dispersion, because the dispersive power is independent of the refractive index.
  7. 7.  In the eye:  Chromatic aberration = total dispersion from red to blue of 2.00D.  Emmetropic eye focuses for yellow-green (555nm).  Duochrome test: myopic eye sees red clearly, hypermetropic eye sees green clearly. Very NB clinically as myopes experience eye strain if they are overcorrected. ie. Rather leave them slightly myopic than hypermetropic.  NB: unable to be corrected by refractive surgery (not about shape)
  8. 8. Diffraction of Light Interference of light waves with each other at the edge of a wave front.  The result = edges of an image are never sharp.  This limits the amount of magnification that can be attained without losing sharpness.  Clinically: small pupil leads to increased diffraction.
  9. 9. Spherical Aberration  Caused by the prismatic effect of the lens => rays passing through the periphery are deviated more.  P = F x D P=prismatic power(D), F = lens power (D), D : decentration (cm).  Marginal focus: point where the peripheral rays converge.  Paraxial focus: point where the the central rays focus.  Circle of least confusion: ¼ the distance b/w the marginal and paraxial focuses.
  10. 10.  Corrected in optical systems by:  Occlusion of periphery using stops.  Adjustment of lens form (aplanatic/aspheric).  Use of doublets (principle lens + a weaker opposite power lens of different refractive index).
  11. 11.  Corrected in the eye by:  Anterior corneal surface is flatter peripherally.  Iris acts as a stop (optimum pupil 2- 2.5mm).  Lens nucleus has higher refractive index than cortex.  Cones are more sensitive to paraxial than to oblique light (Stiles-Crawford effect = light striking the photoreceptor obliquely is less effective and appears like a shorter wavelength).
  12. 12. Astigmatism of Oblique Incidence  Obliquely incident light rays passing through the lens (or looking through the edge of the lens) => toric effect with interval of Sturm, Sturm’s conoid, and circle of least confusion.  Worse with higher power lenses.  Less with meniscus lenses.  NB size of pupil makes no difference.
  13. 13.  Corrected in the eye by:  Aplanatic curve of the cornea.  Retina is curved and not flat, therefore the circle of least confusion falls on the retina.  Astigmatic image falls on the peripheral retina, therefore its appreciation is limited.  When prescribing glasses, NB pantoscopic tilt!
  14. 14. Coma  Spherical aberration of light from points not on the principle axis.  Rays passing through the periphery are deviated more and have unequal magnification => coma shaped image.  Avoid by limiting rays to the axial area.  Different from spherical aberration, in that the focus is laterally displaced, as apposed to spherical which is longitudinally displaced.  Ocular coma is unimportant in the eye (pupil).
  15. 15. Image Distortion  Prismatic effects of lens periphery => uneven magnification.  Concave lens => barrel distortion.  Convex lens => pin-cushion distortion.  Clinically a problem for pts. with high power specs. ie. aphakics.  Pts. can adapt to small amounts of distortion.
  16. 16. Curvature of Field  Also know as Petzval surface.  Due to curvature of lens surface and refractive index of lens.  Plane object => curved image.  Cannot be totally corrected under practical conditions.  Compensated for in the eye by curvature of the retina and accommodation.
  17. 17. Higher Order Aberrations WAVEFRONT TECHNOLOGY allows us to quantify aberrations.  With this new technology we can identify and quantify third, fourth and fifth order aberrations.  Pattern of aberrations are reproducable.
  18. 18. Spherical aberration (Sombrero)
  19. 19. Coma
  20. 20. Higher order aberrations
  21. 21. Post Lasik! (Sombrero hat) (Flipped over sombrero)
  22. 22.  PRK and Lasik increase high order aberrations.  The ↑ in aberrations is directly related to ↓ quality of vision, especially under scotopic conditions, low-contrast and glare.  Higher order aberrations cannot be corrected with spherocylinder or std. refractive surgery.