Optics of ocular structure

It tells about optics of eyeball

1. OPTICS OF OCULAR STRUCTURE SALAL MOHAMMAD B.Optom,2 nd year 1

2. CONTENTS ::: 1.Cornea 2.Corneal curvature and thickness 3.Aqueous humor 4.Crystalline lens 5.Curvature of lens 6.Vitreous humor 7.Schematic and Reduced eye 8.Axial and Axis of the eye 2

3. CORNEA INTRODUCTION- -- Cornea is a transparent, avascular, watch-glass like structure. It forms 1/6 th of the outer fibrous coat of eyeball. Cornea is a anterior smaller sphere. Originally the cornea was considered of five distinct layers. Anterior to posterior these are --- 1. Epithelium 2.Bowmen’s membrane 3.Corneal stroma ( substantia propia ) 4.Descemet’s membrane 5.Endothelium 3

4. DIMENSION - Surfaces- 1. Anterior surfaces , is elliptical in shape. Horizontal diameter-- - 11.75 mm Vertical diameter------- 11.00 mm 2. Posterior surfaces , is circular in shape. Average diameter—11.5 mm Thickness--- 0.52 mm ( at centre ) 0.67 mm ( at peripheri ) Radius of curvature- Anterior curvature ---- 7.8 mm Posterior curvatire ----- 6.5 mm 4

5. Refractive power --- Anterior surfaces ------- +48 D Posterior surfaces----- -5 D Total power of cornea ---- +43 D Refractive index----- 1.376 Function- To act as a powerful refracting surface that transmits light into proper image formation. To help in refraction. To protect the intraocular content. Wound repair after anterior segment surgey or trauma. 5

6. Refraction through the cornea – Cornea cause convergence of light rays have convex surfaces. The cornea plays a major role in ocular refraction with power of about 40-45D ,because there is a huge difference in refractive indices of air (1.0) and cornea (1.376). In the eye ,cornea mainly works as single refracting surface. Cornea has homogenous material with very less thickness;hence works as single unit. 6

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8. AQUEOUS HUMOUR INTRODUCTION- -- The aqueous humour is a transparent watery fluid similar to the water. Maintanance of IOP and pathophysiology of glaucoma revolves around the aqueous humour dynamics. It is similar to plasma but containing low protein concentration. It is secreted from ciliary epithelium and a structure supporting the lens. It fills both anterior and posterior chamber of the eye. 8

9. The aqueous humour is involved with virtually all portions of the eye ,although the principal ocular structure concerned with it are : Ciliary body Posterior chamber Anterior chamber Angle of the anterior chamber Aqueous out-flow system 9

10. Physiochemical properties : Volume – About 0.31ml (0.25ml in ant. Chamber and 0.06ml in post. Chamber ) Refractive index -1.336 Density – its viscosity is 1.025 -1.040 Osmotic pressure – 3-5 mOsm /l pH - 7.2 Rate of formation – 2.3 ul /min. 10

11. Refraction through the Aqueous humour – A light rays after passing through the cornea ,light enters the Aqueous humour . Where it is refracted again towards the centre of the eye. This refraction is due to the difference in density between the cornea and aqueous humour . Aqueous flow from the post. Chamber to ant.chamber . The aqueous humour plays a major role in refraction,because there is a huge difference in refractive indices of air (1.0) and aqueous humour (1.336 ). 11

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13. CRYSTALLINE LENS INTRODUCTION— Lens is a transparent, biconvex, structure of eyeball. Lens placed between iris and vitreous in a saucer shaped depression ,the pateller fossa. Lens is cyrstalline structure , that is avascular and is devoid of nerves , and connective tissue. Lens is an asymmetric oblate spheroid structure. Lens is suspended by suspensory ligament or zonulus . 13

14. Lens consist of 3 layers--- Lens capsule Lens epithelium Lens fibre 14

15. Dimension – Equatorial diameter---- 6.5mm ,(at birth ) , increases 9—10 mm, (remain constant) Thickness---- 3.5mm (at birth ), 5mm (at extreme of age ). Weight----- 135 mg ( at 9 years), 255 mg (at 40-80 years ). Surfaces — it has two surfaces. Anterior surfaces –less convex than the posterior. Radius—10 mm Posterior surfaces---more curved than the anterior Radius----6 mm Refractive index ------ 1.39-1.42 an average Total dioptric power---- 16-17 D 15

16. Refraction through the Crystalline lens – Lens causes convergence of light rays ,have convex surfaces. Crystalline lens in the eye lies between aqueous and vitreous humour ,hence refractive power of lens is half compared to refractive power of cornea ,i.e. nearly 16-17D. Lens is thick and its material is also not homogenous in nature,hence lens simply does not work as two refractive surfaces (Ant. and Post.) rather it works as multiple refracting surfaces. Lens consist of several layers with different refractive indices ,and curvature is also different for different layers. 16

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18. VITREOUS HUMOUR INTRODUCTION— The vitreous is an inert ,transparent , colourless,jelly like that provides a clear optical medium as well as a pathway of nutrient utilized by the lens , ciliary body ,and the retina. The vitreous is clear avascular filling the space bounded by the lens /retina and optic disc. It occupy approx. 80% of the volume of the globe. It serves the optical fuinction and also acts as important supporting structure for the eyeball. 18

19. 19 Vitreous gel consist of delicate meshwork of type 2 of collagen fibrils interpersed . It may be divided into 3 parts. 1. Hyaloid layer or membrane 2.Cortical vitreous 3.Medullary vitreous

20. Physiochemical properties : Volume – 4 cc(ml) Weight – 4g pH – 7.2 Water – 99% Refractive index - 1.336 Osmotic pressure – 5-8 mOsm /l Density – its viscosity is 1.030 -1.045 Function – One of the refractive media of the eye. Support of intraocular structure (lens and retina). Maintanance of spherical shape of eye. 20

21. Refraction through the vitreous humour – A light rays after passing through the lens ,light enters the Vitreous humour . Where it is refracted again towards the central part of the retina. This refraction is due to the difference in density between the lens and vitreous humour . The vitreous humour plays a major role in refraction,because there is a difference in refractive indices of lens ( 1.39 – 1.41) and vitreous humour (1.336). 21

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23. Physiological optical defect of eye – Diffraction of light Spherical Aberrations Chromatic Aberration Oblique Aberration Coma Decentring Distortion 23

24. Pathological optical defect (Refractive error ) of eye – Myopia (short sightedness) Hypermetropia (long sightedness) Astigmatism 1. 24

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26. Absence of optical element of the eye – A condition where crystalline lens is absent, its termed as Aphakia . Common cause of aphakic are congenital disorders or post-surgical removal of crystalline lens. Aphakia causes a refractive error in terms of high degree hypermetropia . 26

27. SCHEMETIC AND REDUCED EYE SCHEMETIC EYE INTRODUCTION – Scientist Gauss put forward the three pair of cardinal points,all lying on the principal axis of the eye. Tscherning and Helmholtz contributed a lot to understand the optics of human eye. Gullstrand who presented the most authentic model of schemetic eye for better understanding of optics of the eye. 27

28. Anterior surface of cornea is used as refrence point for calculation of distance. Two principal foci : Anterior (F1),and Posterior (F2),are situated 15.7mm in front of and 24.4mm behind the anterior surface of cornea,repectively . Two principal points : Anterior (P1),and Posterior (P2),are situated at 1.35mm and 1.60mm away from the anterior surface of cornea,respectively . Two nodal points : First (N1),and Second (N2),are located 7.08mm and 7.33mm behind the anterior surface of cornea,respectively . 28

29. Refractive indices of various refracting surfaces when accommodation is relaxed are- Cornea – 1.376 Aqueous and Vitreous humour – 1.336 Crystalline lens cortex – 1.386 Crystalline lens nucleus – 1.406 Dioptric power or refracting power of the eye while accommodation is minimum and maximum is – Complete eye – 58.64 D and 70.57 D,respectively . Corneal system – 43.05 D and 43.05 D,respectively . Lens system – 19.11 D and 33.06 D,respectively . 29

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31. REDUCED EYE INTRODUCTION- Listing describes the reduced eye . In reduced eye also,all the distances of various cardinal points are calculated from anterior surface of cornea as reference point. Anterior focal point (F1),and posterior focal point (F2), are situated at 15.7mm in front and 24.4mm behind the anterior surface of cornea ,respectively. Principal point (P) is located 1.5mm behind the reference plane (anterior surface of cornea ). 31

32. Nodal points (N) is placed 7.2mm and posterior focal length is 22.9mm . Uniform refractive index of ocular structure is 1.336 . Total dioptric power of the eye is 58.2 D. 32

33. AXES OF THE EYEBALL 1.Optical axis- line passing through the centre of the cornea(p), centre of the lens(n),and meet the retina(r) , on the nasal side of the fovea 2.Visual axis— Line joining the fixation point(o),nodal point (n),and the fovea(f). 3.Fixation axis --- line joining the fixation point(o),and the centre of rotation(c).. 33

34. VISUAL ANGLE 34

35. 1.Angle alpha - It is angle(ONA), formed between the optical axis (AR), and visual axis (OF), at the nodal point (N). 2.Angle gamma - it is angle (OCA) between the optical axis (AR),and fixation axis (OC),at the centre of rotation of the eyeball(C) 3.Angle kappa - it is angle (OPA) ,between the visual axis(OF) and pupillary line (AP), 35

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