OPTICAL ABBERATIONS in ophthalmology l.pptx

Optical aberration , chromatic abberations

1. OPTICAL ABBERATIONS MODERATOR : Dr.JOHNSON RAJ KUMAR Presented by : D r.Vijay 1 st Year pg resident

2. OPTICAL ABBERATIONS DefINTION : A n aberration is an imperfection in the image formation of an optical system. The Human eye is by no means optically perfect. The various types of abberations are: 1) Monochromatic 2) Chromatic

3. TYPES: 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 . Types : 1) Diffraction of light 2) Spherical Abberations 3) Chromatic Abberations 4) Decentring 5) Oblique Astigmatism 6) Coma 7) Curvature of Field 8) Distortion

4. Natural mechanisms to decrease aberration in human eye: 1 ) Cutting of peripheral rays by iris. 2) High refractive index of the core of nucleus of lens. 3) Low sensitivity of peripheral retina. 4) More sensitivity of retina to perpendicular rays (Stiles-Crawford effect)

5. Diffraction of Light : Bending of light rays caused by the edge of an aperture or the rim of a lens . • The actual pattern of image produced by lens is by the pupil is a series of concentric bright and dark bands with a centre bright spot . • This pattern is termed as airy disc . In eye with pupil diameter of 2mm , the spot of concentric rings is 0.01mm Diffraction blur increases with small size of pupil Thus, optimal visual acuity occurs at pupil diameter of 3-4mm.

6. Spherical Abberations : Spherical lens refract peripheral rays more than the para-axial rays. • Which in the case of the convex lens brings the more peripheral rays to focus closer to the lens 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.

7. Correction of SA: SA can be reduced by occluding the periphery of lens by use of STOPS so that only paraxial zone is used. Spherical aberration in the human eye is reduced by the aspheric shape of the lens and the cornea APLANTIC SURFACE 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 .

8. Correction of SA 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.

9. Ocular SA: 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. ( 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 .)

10. (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.

11. Chromatic Abberation : The index of refraction of any transparent medium varies with the wave length of the incident light . The shorter the wavelength of the light, the more it is deviated on refraction . In human eye, the blue light is focused slightly in front of the red. In reality , this effect is minimized by narrow spectral sensitivity bands of long and mid wavelength cones + Fovea is largely lacking blue cones. Rays of greatest intensity (Yellow) form a sharply defined image. Colours of longer and shorter focus form circles of relatively low intensity and therefore neglected. Emmetropic eye is hypermetropic for red rays and myopic for blue and green rays This forms the basis for BICHROME test used in subjective refraction.

12. OCULAR CA: 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 . 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.

13. DUCHROME TEST: Duochrome test helps us to determine position of focal point with respect to the fovea Useful to avoid over minusing patient. Myopic eyes : see red letters more clearly Hyperopic eyes : see green letters more clearly Colour blindness doesn’t invalidate the test bcoz it depends on the position of the image with respect to retina. A coloured blind should be asked whether the left or right side of letter appears clearly.

14. Correction of CA: 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 . 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 Combining Convex lens of high refractive power and low dispersive power with concave lens of low refractive power and high dispersive power , aberration can be neutralised. The earliest achromatic lenses were made by combining elements of flint and crown glass

15. Decentring: The Cornea and Lens surfaces alter the direction of incident light rays, causing them to focus on the Retina. These surfaces are not centered on a common axis. The crystalline lens is usually slightly decentred and tipped with respect to the cornea and visual axis of eye. The centre of curvature of cornea is situated abut 0.25mm below the axis of the lens. Deviation is so small –they are functionally neglected.

16. Oblique Astigmatism: Objects in the peripheral field are seen by virtue of oblique incident narrow pencil of rays which are limited by the pupil. Because of this the refracted pencil shows oblique astigmatism . The peripheral portion of lens forms Strums conoid and therefore in any peripheral oblique axis, two images (FH) are formed. OA is much more evident when Bi concave or Bi convex lenses are used. This aberration primarily influences the image quality of spherical lenses. When the wearer looks at an angle through the lens, there is a deviation which he perceives as blur. The higher the dioptric power of the lens, the more pronounced this error becomes . A dot is no longer imaged as a dot, but as two image lines

17. OCULAR OA: Factors that reduce OCULAR OA: 1) The aplantic curvature of the cornea reduces OA and also SA 2) The retina being a spherical surface , the circle of least confusion of the strums conoid formed by OA falls on the retina. 3) The astigmatic image falls on peripheral retina which has relatively poor resolving power compared with retina at macula. The visual appreciation of the astigmatic image is therefore limited.

18. Correction of OA: Mitigated by deviating from the spherical shape Aspheric Surface lens or BEST FORM LENSES usually in meniscus form are used

19. Coma: Different areas of the lens form foci in planes other than the chief focus. This produces in the image plane a coma effect from a Point source of light. This results in unequal magnification of image formed by different zones of the lens. The composite image is not circular but elongated like a coma or a comet.

20. Correction of Coma: Coma can be avoided by limiting rays of the axial area of lens and using the principal axis of lens only. complete correction can be achieved by using a combination of lenses symmetric about a central stop.

21. Curvature of Field: Causes an planar object to project a curved ( nonplanar ) image. It can be thought of as arising from a "power error" for rays at a large angle. Those rays treat the lens as having an effectively smaller diameter and an effectively higher power, forming the image of the off axis points closer to the lens .

22. A lens aberration that causes a flat object surface to be imaged onto a curved surface rather than a plane . The surface of the image formed by the eye is also curved, fortunately, the retina is also curved ! For lens systems, using best form lenses with non-spherical shapes can help.

23. Image distortion: Not about sharpness, but faithful reproduction of the shape of the object. It occurs when magnification varies with the distance of the object from the optic axis. Problem only for high powers Tends to falsify the positions of objects and cause vertical lines to wave Aphakes ! Minimized by very steep back base curves Convex lens = Pin cushion distortion Concave lens = Barrel distortion.

24. SUMMARY: The various types of abberations are: 1) Monochromatic 2) Chromatic--- Types : 1 ) Diffraction of light--------------------------------------------------------------- 2 ) Spherical Abberations ------------ 3 ) Chromatic Abberations 4) Decentring 5) Oblique Astigmatism 6) Coma 7) Curvature of Field 8) Distortion

25. Decentring:

26. Oblique astigmatism: Aspheric Surface lens or BEST FORM LENSES usually in meniscus form are used

27. COMA:

28. Curvature of field:

29. Image distortion:

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