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Optical aberrations


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Basics of Refractive Surgery

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Optical aberrations

  1. 1. Presenter : Dr Samuel Ponraj Moderator : Dr Shrinivas K Rao Optical Aberrations
  2. 2.  Optical aberration is an imperfection in the image formation of an optical system.  Aberrations fall into two classes:  monochromatic and  chromatic.
  3. 3. Ideal Optical System  Stigmatic imaging  Geometrical similarity
  4. 4.  No Field Curvature
  6. 6.  Monochromatic aberrations are caused by the geometry of the lens and occur both when light is reflected and when it is refracted. They appear even when using monochromatic light, hence the name.  Chromatic aberrations are caused by dispersion, the variation of a lens's refractive index with wavelength. They do not appear when monochromatic light is used.
  7. 7.  One needs to keep in mind these important points: unlike the standard eye model, an actual eye is:  An active optical system, with adjustable components and aberrations varying in time,  It is not strictly centered system,  It is not a rotationally symmetrical system, and  Final perception is the subject of neural processing.
  8. 8. WAVEFRONT ANALYSIS  Aberrations can be defined as the difference in optical path length (OPL) between any ray passing through a point in the pupillary plane and the chief ray passing through the pupil center.  This is called the optical path difference (OPD) and would be for a perfect optical system.
  9. 9.  Wavefront aberrometer shines a perfectly shaped wave of light into the eye and captures reflections distorted based on the eye’s surface contours.  Thus, it generates a map of the optical system of the eye, which can be used to prescribe a solution, correcting the patient’s specific vision problem.
  10. 10.  Another way of characterizing the wavefront is to measure the actual slope of light rays exiting the pupil plane at different points in the plane and compare these to the ideal; the direction of propagation of light rays will be perpendicular to the wavefront.  This is the basic principle behind the Hartman-Shack devices commonly used to measure the wavefront.  Wavefronts exiting the pupil plane are allowed to interact with a microlenslet array.
  11. 11.  If the wavefront is a perfect flat sheet, it will form a perfect lattice of point images corresponding to the optical axis of each lenslet.  If the wavefront is aberrated, the local slope of the wavefront will be different for each lenslet and result in a displaced spot on the grid as compared to the ideal.  The displacement in location from the actual spot versus the ideal represents a measure of the shape
  12. 12.  Wavefront maps are commonly displayed as 2- dimensional maps.  The color green indicates minimal wavefront distortion from the ideal.  While blue is characteristic of myopic wavefronts and red is characteristic of hyperopic wavefront errors.
  13. 13.  Once the wavefront image is captured, it can be analyzed.  One method of wavefront analysis and classification is to consider each wavefront map to be the weighted sum of fundamental shapes.  Zernike and Fourier transforms are polynomial equations that have been adapted for this purpose.  Zernike polynomials have proven especially useful since they contain radial components and the shape of the
  14. 14. Wave front technology
  15. 15.  Following the above division of the Zernike expansion adopted in ophthalmology, monochromatic eye aberrations are addressed as: (1) lower-order aberrations, with the Zernike radial order n<3, and (2) higher-order aberrations, with n≥3.
  16. 16.  The important optical aberrations that affect vision are:  2nd Order optical aberrations – currently measured in all eye exams providing sphere, cylinder and axis corrections  3rd and 4th Order optical aberrations – high order aberrations currently not measured in today’s eye exams but can account for up to 20% of the eye’s refractive error.
  17. 17.  5th and 6th Order optical aberrations –also high order aberrations not currently measured in today’s eye exam.  These aberrations are of less significance clinically, however they manifest in reduced vision for a small percentage of eyes.
  18. 18.  The lower-order aberrations are  Piston  Tilt  Defocus  Astigmatism  The 2nd order aberrations, defocus and primary astigmatism - are the most significant contributors to the overall magnitude of eye aberrations Lower-order aberrations
  19. 19.  Remaining lower-order forms, piston and tilt, or distortion, are usually ignored.  The former being not an aberration with a single imaging pupil, and  The latter being not a point-image quality aberration).
  20. 20. Higher order aberrations  Higher order aberrations are measured with wavefront aberrometers and expressed in terms that describe the shape and severity of the deviated light rays as they pass through the eye's optical system and strike the retina.  Coma, spherical aberration, and trefoil are the most common higher order aberrations .
  21. 21.  Coma causes light to be smeared like the tail of a comet in the night sky.  Double vision is a common symptom of coma.  Trefoil causes a point of light to smear in three directions, like a Mercedes-Benz symbol.  Spherical aberration is characterized by halos, starbursts, ghost images, and loss of contrast sensitivity (inability to see fine detail) in low light.
  22. 22.  Starbursts – Patterns of Small Lights Around Light Sources  Haloes – Circles of Light Around Light Sources  Ghosting – A Faint Duplicate of Each Object Similar to Double Vision  Glare – Intensification of Light Sources.  It's quite common for a patient to have an increase in all of these aberrations, resulting in distorted night vision when the pupil opens and allows light to enter through a larger area of the irregular corneal surface.
  23. 23. Coma  A comet-like tail or directional flare appearing in the retinal image, when a point source is viewed.  Because the eye is a somewhat nonaxial imaging device, and because the cornea and lens are not perfectly centered with respect to the pupil, coma generally is present in all human eyes.  A large amount of coma (0.3 μm of coma alone) may point to known corneal diseases,
  24. 24. Coma  Spherical aberration applied to light coming from points NOT lying on the principal axis.  Rays passing through the periphery of the lens are deviated more than central rays & come to a focus nearer the principal axis.  Results in unequal magnification of the image formed by different zones of the lens.  Differs from spherical aberration in that the image formed is laterally displaced.
  25. 25. Ocular application • May be avoided by limiting to the axial area of the lens. • Not of clinical significance due to the same reasons for oblique astigmatism… which are: 1. Aplanatic surface of the cornea 2. Retina is a spherical surface 3. Coma image falls on peripheral retina which has poor resolving power compared to the macula; visual appreciation of astigmatic image is limited
  26. 26. Spherical Aberration  Fortunately, spherical aberration is relatively easy to understand.  For a normal photopic eye, spherical aberration may vary from approximately 0.25 D to almost 2 D.  Light rays entering the central area of a lens are bent less and come to a sharp focus at the focal point of a lens system.  However, peripheral light rays tend to be bent more by the edge of a given lens system so that in a plus lens, the light rays are focused in front of the normal focal point of the lens and
  27. 27.  This is why many lens systems incorporate an aspheric grind, so that the periphery of the lens system gradually tapers and refracts or bends light to a lesser degree than if this optical adaptation was not included.  The variation in index of refraction of the crystalline lens (higher index in the nucleus, lower index in the cortex) is responsible for neutralization of a
  28. 28.  It was seen that the prismatic effect of a spherical lens is least in the paraxial zone and increases towards the periphery of the lens.  Thus, rays passing through the periphery of the lens are deviated more than those passing through the paraxial zone of the lens
  29. 29.  In other words, the parallel light rays of incoming light do not converge at the same point after passing through the lens. Because of this, Spherical Aberration can affect resolution and clarity, making it hard to obtain sharp images.  Results in out-of-focus image.
  30. 30. Spherical lens: Peripheral rays have shorter focal length than paraxial rays.
  31. 31. Correction of Spherical Aberration  Spherical aberration may be reduced by occluding the periphery of the lens by the use of 'stops' so that only the paraxial zone is used.
  32. 32.  To achieve the best results, spherical surfaces must be abandoned and the lenses ground with aplanatic surfaces; that is, the peripheral curvature is less than the central curvature .  Aspherical lenses are lenses with complex curved surfaces, such as where the radius of curvature changes according to distance from the optical axis.
  33. 33. aspheric doublet lens  Another technique of reducing spherical aberration is to employ a doublet. This consists of a principal lens and a somewhat weaker lens of different refractive index cemented together .  The weaker lens must be of opposite power, and because it too has spherical aberration, it will reduce the power of the periphery of the principal lens more than the central zone.  Usually, such doublets are designed to be both aspheric and achromatic.
  34. 34. aspheric doublet lens.
  35. 35. Ocular application  The effect of spherical aberration in the human eye is reduced by several factors:  (1) The anterior corneal surface is flatter peripherally than at its centre, and therefore acts as an aplanatic surface.  (2) The nucleus of the lens of the eye has a higher refractive index than the lens cortex… Thus the axial zone of the lens has greater refractive power than the periphery.
  36. 36.  (3) The iris acts as a stop to reduce spherical aberration. The impairment of visual acuity that occurs when the pupil is dilated is almost entirely due to spherical aberration (Optimum pupil size is 2–2.5 mm.)  (4) Retinal cones are much more sensitive to light which enters the eye paraxially than to light which enters obliquely through the peripheral cornea (Stiles–Crawford effect).  This directional sensitivity of the cone photoreceptors limits the visual effects of the residual spherical aberration in the eye.
  37. 37. Oblique astigmatism  Occurs when rays of light traverse a spherical lens obliquely… a toric effect is introduced forming a Sturm’s conoid
  38. 38. Chromatic aberration  Because the index of refraction of the ocular components of the eye varies with wavelength, colored objects located at the same distance from the eye are imaged at different distances with respect to the retina.  This phenomenon is called axial chromatic aberration. In the human eye the magnitude of chromatic aberration is approximately 3 D.
  39. 39. Chromatic aberration  However, significant colored fringes around objects generally are not seen because of the preferential spectral sensitivity of human photoreceptors.  Studies have shown that humans are many times more sensitive to yellow–green light with a central wavelength at 560 nm than to red or blue light.
  40. 40.  When white light is refracted at an optical interface, it is dispersed into its component wavelengths or colors .  The shorter the wavelength of the light, the more it is deviated on refraction.  Thus a series of colored images are formed when white light is incident upon a spherical lens
  41. 41. Correction of Chromatic Aberration  The dispersive power of a material is independent of its refractive index. Thus, there are materials of high dispersive power but low refractive index, and vice versa.
  42. 42. Achromatic Lens  Special optics design of two mated lens – concave and convex – which more precisely focus the wavelengths of light onto the same plane.  Achromatic lens systems are composed of elements (lenses) of varying material combined so that the dispersion is neutralized while the overall refractive power is preserved
  43. 43.  The earliest achromatic lenses were made by combining elements of flint and crown glass.