1.-SAHITHI GANESHULA

2. Optics of human eye Eye as a camera Components Schematic eye and reduced eyes Axes and visual angles Optical aberrations 3. Eye as a camera Eyelids- shutter Cornea- focusing system Lens- focusing system Iris- diaphragm Choroid- dark chamber Retina-light sensitive film 4. Components The cornea The anterior chamber The iris and pupil The crystalline lens The retina 5. Cornea Reasons of refraction: Curvature. Significant difference in refractive indices of air and cornea. 6. Vertical diameter slightly less than horizontal Front apical radius 7.7 mm K= 48.83 D Back apical radius 6.8 mm K=-5.88 D Actual refractive index cornea= 1.376 Power of cornea +43D (2/3 of total eye power) Not optically homogenous (ground substance)=1.354, n(collagen)=1.47 7. The anterior chamber Cavity between cornea and iris Filled with aqueous humor. Depth of AC about 2.5-4.0 mm Change in AC depth change the total power. 1mm forward shift of lens- increase about 1.4D in power Refractive index of aqueous humor= 1.336 8. Iris and Pupil 2-4mm Average size: depth of focus increases Concept used as pin hole test in refractionSmall pupil Retinal image quality improves Size of blue circle increasesLarge pupil Regulate amount of light entering the eye At 2.4mm pupil size, best retinal image obtained, as aberration and diffraction are balanced. 9. The crystalline lens Birth 3.5 4 mm Adult life 4.75 5 mm Thickness Ant surface 10 mm Post surface 6 mm Radius of curvature Nucleus 1.41 Pole 1.385 Equator 1.375 Refractive index of lens 15 -18 d.Total power At birth- 14-16 D At 25yrs- 7-8D At 50yrs- 1-2D Accommodative power 10. Lens accounts for about one third of the refraction of the eye. ACCOMODATION Provides a mechanism of focusing at different distances. OPTICAL CHANGES IN CATARACTOUS LENS Visual Acuity reduction. Myopic shift. Monocular diplopia. Glare. Color shift. 11. Vitreous Refractive index same as aqueous. 12. Retina Maximum resolving power at fovea. A concave spherical surface with r =-12 mm. Advantages of curvature of retina over plane image forming surfaces of cameras and optical instruments: The curved images formed by the optical system is brought in the right order. A much wider field of view is covered by the steeply curved retina 13. Schematic eye and reduced eyes Two principal foci Two principal points Two nodal points Gullstrands schematic eye: Two principal foci Single principal Single nodal point Listings reduced eye: 14. Schematic and reduced eyes It is a theoretical optical specification of an idealized eye, retaining average dimensions but omitting the complications. Useful for understanding ophthalmologic problem and conceptualizing the optical properties of the human eye. To calculate cardinal points, the radii of curvature and distances separating the refracting surfaces must be known. 15. Axes and visual angles 16. OPTICAL AXIS: line passing through centre of cornea, lens and meets retina on nasal side of fovea VISUAL AXIS: line joining fixation point, nodal point and fovea FIXATION AXIS: line joining fixation point and centre of rotation 17. Visual angles ANGLE ALFA:angle formed between visual axis and optical axis at the nodal point ANGLE GAMMA:angle between optical axis and fixation axis at the centre of rotation ANGLE KAPPA:angle between visual axis and pupillary line. Point on centre of cornea is considered as centre of pupil. 18. Optical aberrations Diffraction of light Spherical aberrations Chromatic aberrations Decentering Oblique aberrations Coma 19. Diffraction Bending of light caused by edge of the aperture or rim of the lens. 20. Spherical aberrations A convex spherical lens refracts peripheral rays more strongly than paraxial rays, so peripheral rays are focused closer. 21. Why less in human eyes??? Human cornea is periphery is flatter than centre. In human lens, central portions have greater density and greater curvature. Iris blocks the peripheral rays, only paraxial rays enter the system. 22. Chromatic aberrations Different color rays have different focal length, as refractive indices of media varies with wavelength of incident light. 23. Why less in human eyes??? Due to narrow spectral sensitivity bands of long and mid wavelength cones. Foveal lack of blue cones. 24. Decentering Lens is usually slightly decentred. Corneal centre of curvature is situated about 0.25mm below the axis of lens. Clinically insignificant. 25. Oblique aberrations When the object is in peripheral visual field, a thin incident narrow pencil of rays enter limited by the pupil. Peripheral portion of lens forms Sturms conoid, so two line foci are formed. Minimum in human lens due to curvature of retina. Significant in biconvex/biconcave lens. 26. Oblique aberrations 27. Coma Different areas of lens form foci in planes other than chief focus, producing a coma effect in image plane, from a point source of light. 28. Emmetropia 24-25mm 43 diopters 18 diopters Accomodation at rest 29. REFRACTIVE ERRORS Ametropia: a refractive error is present Myopia: Near sightedness Hyperopia(Hypermetropia): Far sightedness Presbyopia: Loss of accommodative ability of the lens resulting in difficulties with near tasks Astigmatism: the curvature of the cornea and/or lens is not spherical and therefore causes image blur on the retina 30. REFRACTIVE ERRORS Anisometropia: a refractive power difference between the 2 eyes (> 2D) Aniseikonia: a difference of image size between the 2 eyes as perceived by the patient Aphakia: (Phakos=lens), aphakia is no lens Pseudophakia: artificial lens in the eye 31. Myopia A form of refractive error in which parallel rays of light entering the eye are focused in front of retina with accommodation being at rest. 32. Etiological types Axial(MC)-increased AP length of eyeball Curvatural-increased curvature of cornea, lens or both Index-increased refractive index of lens with nuclear sclerosis Positional-anterior placement of lens Myopia due to excessive accommodation 33. Clinical types of myopia Congenital Simple or developmental Degenerative or pathological Acquired 34. Congenital myopia Common in premature babies or with birth defects Stationary(8-10D) Associated with Increase in axial length Esotropia Other congenital anomalies of eye Early and full correction under cycloplegia Poor prognosis in unilateral cases with severe myopia and anisometropia 35. Simple myopia Physiological/school myopia Commonest type Results due to normal biological variations in development of eye Age of onset- 7-10yrs Moderate severity-6D 37. Pathophysiology Genetic factors More growth of retina Stretching of sclera Increased axial length Degeneration of choroid Degeneration of retina Degeneration of vitreous General growth process Degenerative changes in sclera Decrease d vision 38. Myopic Crescent 39. Lacquer Cracks Breaks in Bruchs membrane 40. Sub retinal neovascularization 41. Sub retinal hemorrhage 42. Foster Fuchs's spots 43. Posterior staphyloma 44. Complications Macular hemorrhage Retinal tears, detachment Vitreous hemorrhage Choroidal hemorrhage Complicated cataract Nuclear sclerosis Primary open angle glaucoma 45. Clinical features - Symptoms Distant blurred vision Half shutting of eyes Asthenopic symptoms Muscae volitantes Night blindness Divergent squint 46. Signs Prominent eyeballs Large cornea Anterior chamber is deep Large & sluggishly reacting pupil Fundus examination-changes seen only in pathological myopia 47. Optical treatment Concave lenses Children Adults Contact lenses 48. Optical treatment Minimum acceptance providing maximum vision Low myopia(reading is possible at - cm-->difficultly reading fine print , headache , visual fatigue 77. Increasing Near Point of Accommodation with Age Age (years) Distance (cm) 10 7 20 10 30 14 40 20 50 40 78. SYMPTOMS The need to hold reading material at arm's length. Blurred near vision Headache Fatigue Symptoms worse in dim light. 79. CORRECTION 80. SPECTACLES Plus lens (or) Convex lens 81. Surgery Monovision LASIK Monovision & CK IntraCor Refractive lens exchange Corneal Inlays & Onlays 82. ASTIGMATISM A defect of an optical system causing light rays from a point source to fail to meet in a focal point resulting in a blurred and imperfect image. 83. Conus of Sturm:- Geometric configuration of light rays emanating from single point source & refracted by spherocylindrical lens 84. Focal interval of Sturm :- Distance between 2 focal lines Circle of least diffusion At the dioptric mean of focal lines the cross section of sturms conoid appears as circular patch of light rays best overall focus 85. Types Regular astigmatism change in refractive power is uniform from one meridian to another With-the-rule astigmatism Against-the-rule astigmatism Oblique astigmatism Bi-oblique astigmatism Irregular astigmatism Irregular change of refractive power in different meredia 86. Types of regular astigmatism Simple astigmatism Simple hyperopic astigmatism Simple myopic astigmatism Compound astigmatism Compound hyperopic astigmatism Compound myopic astigmatism Mixed astigmatism 87. Regular Astigmatism : Correctable by Spherocylindrical lenses Etiology : 1. Corneal - abnormalities of curvature [common] 2. Lenticular is rare. It may be: i. Curvatural - abnormalities of curvature of lens as seen in lenticonus. ii. Positional - tilting or oblique placement of lens , subluxation. 3. Retinal - oblique placement of macula [rare] 88. Symptoms : Blurring of vision Asthenopic symptoms Tilting of head Squinting [Half closure of eyelid] 89. Investigations: Retinoscopy Keratometry Computerized corneal Tomography Astigmatic fan test Jackson cross cylinder 90. Treatment Optical treatment Spectacles RGP contact lenses Toric contact lenses Surgical correction 91. Guidelines for optical treatment Small astigmatism- treatment is required In presence of asthenopic symptoms Decreased vision High astigmatism- full correction Better to avoid new astigmatic correction in adults because of intolerable distraction Bi-oblique,mixed,high astigmatism are better treated by contact lenses Correction of spherical component 92. Irregular Astigmatism Etiology : Corneal -[ Scars , Keratoconus , flap complications, marginal degenration ] Lenticular -[Cataract maturation] Retinal-[scarring of macula,tumours of retina,choroid] 93. Symptoms : Defective vision Distorsion of objects Polyopia Investigations: - Placido's disc test reveals distorted circles - Computerized corneal topography 94. Treatment : Optical treatment : - RGP contact lenses -Hybrid contact lenses -Scleral lenses - Piggyback lens Surgical treatment: - penetrating keratoplasty 95. Astigmatism correction requires prescription of convex cylindrical lenses at 180 +/- 20 deg or concave cylindrical lenses at 90 +/- 20 deg with the rule and vice versa 96. Contact lenses Toric contact lenses Soft lenses [SL] Rigid gas permeable lenses [RGP] RGP do not conform to the asymmetry of corneal surface but replaces it totally and also provides clarity of vision ,more durable. Soft lenses are more comfortable to wear ,easy to fit, adhere more tightly to cornea . 97. Refractive surgeries Astigmatic Keratotomy Photoastigmatic refractive Keratectomy [PRK] Relaxing incisions with compression sutures LASIK surgery 98. Residual astigmatism : The amount of astigmatism that still remains after correction of a refractive error. In the case of correction of corneal astigmatism using rigid contact lens ,lenticular residual astigmatism is exposed. 99. Anisometropia Difference in refractive power between eyes refractive correction often leads to different image sizes on the retinas( aniseikonia) aniseikonia depend on degree of refractive anomaly and type of correction closer to the site of refraction deficit the correction is made-->less retinal image changes in size 100. Anisometropia Glasses : magnified or minified 2% per 1 D Contact lens : change less than glasses Tolerate aniseikonia ~ 5-8% Symptoms : usually congenital and often asymptomatic Treatment anisometropia > D-->contact lens unilateral aphakia-->contact lens or intraocular lens