Optical system of eye gauri s

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Optical system of eye gauri s

  1. 1. Gauri S Shrestha, M.Optom, FIACLE OPTICAL SYSTEM OF EYE
  2. 2. INTRODUCTION <ul><li>Eye is a compound optical system. </li></ul><ul><li>It is an adaptive optical system. </li></ul><ul><li>Various elements in the path of light. </li></ul>
  3. 3. The eye and the camera <ul><li>Compact </li></ul><ul><li>Wide field of view </li></ul><ul><li>Operates on various luminance level </li></ul><ul><li>No error of centration </li></ul>
  4. 4. <ul><li>Sign convention </li></ul><ul><ul><li>Distance measured in the same direction as incident light travel is regarded as positive in sign </li></ul></ul><ul><ul><li>l and l’, f, r are measured from the lens </li></ul></ul><ul><ul><li>Vertical distance above optical axis is taken as positive </li></ul></ul><ul><ul><li>Light always travels from left to right </li></ul></ul><ul><ul><li>Angles in anticlockwise direction is positive </li></ul></ul><ul><ul><li>The angle b/w a ray to axis is measured from the ray to the axis </li></ul></ul>Laws of optical image formation
  5. 5. Symbols <ul><li>Refractive index – n </li></ul><ul><li>Object distance – l </li></ul><ul><li>Image distance – l’ </li></ul><ul><li>First focal length – f </li></ul><ul><li>Second focal length – f’ </li></ul><ul><li>Radius of curvature – r </li></ul><ul><li>Object height - h </li></ul><ul><li>Image height – h’ </li></ul><ul><li>Reciprocal of distance –> L=1/l, R= 1/r </li></ul><ul><li>Quantity represented by italics print ( F=power of lens ) </li></ul><ul><li>Geometrical point - in Roman capital (F= the first principal focus) </li></ul>
  6. 6. Real and virtual <ul><li>Real object is one from which incident rays diverge </li></ul><ul><li>A virtual object is one towards which incident rays are converging </li></ul><ul><li>A real image is one towards which rays converge and is can be received in screen </li></ul><ul><li>A virtual image is one from which refracted or reflected rays appear to emanate </li></ul>
  7. 7. Refraction at the spherical surface <ul><li>Power and vergence </li></ul><ul><ul><li>F=n’/f’= -n/f=n’-n/r </li></ul></ul><ul><ul><li>L’= L+F </li></ul></ul><ul><li>Thin lens </li></ul><ul><ul><li>F=1/f’ =-1/f </li></ul></ul>n’ n n’-n l’ l r - =
  8. 8. N N’ n 1 n k+1
  9. 9. Unequifocal system <ul><li>One of the example – eye </li></ul><ul><ul><li>First and last media has different refractive index. </li></ul></ul><ul><li>This systems have six cardinal points- </li></ul><ul><ul><li>F and F’, the 1 st and 2 nd principle foci. </li></ul></ul><ul><ul><li>P and P’, the 1 st and 2 nd principal points. </li></ul></ul><ul><ul><li>N and N’, the 1 st and 2 nd nodal points. </li></ul></ul><ul><li>Cardinal points are always symmetrically positioned; PP’=NN’ and FP=N’F’ </li></ul><ul><li>Equivalent power of the system, </li></ul><ul><ul><li>F = n k+1 /f’ =-n 1 /f where, f’= P’F’ , f = PF </li></ul></ul><ul><ul><li>n 1 =ref.index of 1 st medium n k+1 =ref. index of last medium </li></ul></ul>
  10. 10. Unequifocal system <ul><li>A ray from object point Q directed </li></ul><ul><li>towards P, making an angle u with </li></ul><ul><li>the optical axis, emerges from P’ </li></ul><ul><li>making an angle u’ with the optical </li></ul><ul><li>axis </li></ul><ul><li>n k+1 u’=n 1 u </li></ul><ul><li>Another ray directed to N, emerge </li></ul><ul><li>out from N’ without undergoing a </li></ul><ul><li>change of direction </li></ul><ul><li>These two pairs of rays can be </li></ul><ul><li>used to construct the image B’Q’ </li></ul><ul><li>of an object BQ. </li></ul>Transverse magnification, m= h’/h = L/L’
  11. 11. Effectivity <ul><li>lx= lo-d and </li></ul><ul><li>Lx= n/lx= n/lo-d= n/(n/Lo)-d </li></ul><ul><li>= Lo/1-(d/n) Lo </li></ul>( ( d l 0 lx n n ) ) d lo lx
  12. 12. Components of the eye’s optical system <ul><li>The cornea </li></ul><ul><li>The anterior chamber </li></ul><ul><li>The iris and pupil </li></ul><ul><li>The crystalline details </li></ul><ul><li>The retina </li></ul>
  13. 13. Cornea <ul><li>Cornea- a highly transparent structure </li></ul><ul><li>Account 2/3 of the eye’s refractive power </li></ul><ul><li>Dimension of cornea </li></ul><ul><ul><li>Shape is elliptical </li></ul></ul><ul><ul><li>HVID – 11.7 , VVID – 10.6 mm </li></ul></ul><ul><ul><li>Area 1.3cm 2 or 1/14 of the total area of globe </li></ul></ul><ul><ul><li>Bi-spherical globe ; cornea smaller, r 2 = 6.5 mm </li></ul></ul><ul><ul><li>Thickness 0.52 in center, 0.67 mm at limbus </li></ul></ul><ul><ul><li>Sagittal depth = 2.6 mm </li></ul></ul><ul><li>The cornea is not symmetrical and corneal curvature flattens towards the periphery </li></ul>
  14. 14. Corneal shape <ul><li>Meniscus lens </li></ul><ul><li>Not a solid of rotation about any axis </li></ul><ul><li>Front apical radius 7.7 mm K= 48.83 D </li></ul><ul><li>Back apical radius 6.8 mm K=-5.88 D </li></ul><ul><li>Actual refractive index, cornea= 1.376 </li></ul><ul><li>Power of cornea ~ +43D (2/3 of total power of eye) </li></ul><ul><li>Not optically homogenous </li></ul><ul><li>n(ground substance)=1.354, n(collagen)=1.47 </li></ul>
  15. 15. Anterior chamber <ul><li>Cavity between cornea and, iris and lens </li></ul><ul><li>Filled with aqueous humor, </li></ul><ul><li>water contents 98% </li></ul><ul><li>Depth of AC – about 3.00 mm excluding corneal thickness </li></ul><ul><li>Change in AC depth change the total power. 1mm forward shift of lens- increase about 1.4D in power </li></ul><ul><li>Refractive index of aqueous humour= 1.336 </li></ul>
  16. 16. The iris and pupil <ul><li>Amount of light regulated by the iris and pupil. </li></ul><ul><li>Pupil reactions in normal conditions: </li></ul><ul><ul><li>Direct reflex, Consensual reflex and Near reflex </li></ul></ul><ul><li>Failure of these reflexes indicates underlying disorder. </li></ul><ul><li>Size can be affected by external or secondary agencies – drugs, emotions etc </li></ul><ul><li>Effects on quality of images. </li></ul>
  17. 17. Pupil size and its effect <ul><li>Large pupil size – large blur circle on retina, for ametropic eye </li></ul><ul><li>Pinhole aperture if placed, size of blur circle reduce. </li></ul><ul><ul><li>Uncorrected myopic individual squint to obtain a pin hole effect. </li></ul></ul><ul><li>Smaller the pupil size- more is the eye’s depth of focus, objects focus in near point (NPA) </li></ul><ul><li>Pinhole improvement - </li></ul><ul><ul><li>pinhole improvement- possibility of corneal / lenticular light scattering, irregular astigmatism </li></ul></ul><ul><ul><li>If VA worse by pinhole- macular disease considered </li></ul></ul>
  18. 18. LENS ANATOMY
  19. 20. Lens details <ul><li>Both anatomically and optically, </li></ul><ul><ul><li>highly complex structure </li></ul></ul><ul><ul><li>composed of radial fibers </li></ul></ul><ul><li>Serves two purposes </li></ul><ul><ul><li>Balance the eyes refractive power </li></ul></ul><ul><ul><li>Provide mechanism for focus </li></ul></ul><ul><li>A Unique structure grows throughout life. </li></ul>
  20. 21. Lens details <ul><li>With ageing various changes </li></ul><ul><ul><li>Impairs flexibility and transparency </li></ul></ul><ul><ul><li>Center thickness increase </li></ul></ul><ul><ul><li>Radii of curvature may become longer </li></ul></ul><ul><li>Weight </li></ul><ul><ul><li>at birth 65 mg </li></ul></ul><ul><ul><li>At 90 yrs 260 mg </li></ul></ul><ul><li>Equatorial diameter </li></ul><ul><ul><li>At birth 5 mm </li></ul></ul><ul><ul><li>At 20 yrs 9-10 mm </li></ul></ul>
  21. 22. Lens detail <ul><li>Thickness </li></ul><ul><ul><li>at birth 3.5 – 4 mm </li></ul></ul><ul><ul><li>Adult life 4.75 – 5 mm </li></ul></ul><ul><li>Radius of Curvature </li></ul><ul><ul><li>Ant surface 10 mm </li></ul></ul><ul><ul><li>Post surface 6 mm </li></ul></ul><ul><li>Refractive index of lens </li></ul><ul><ul><li>nucleus 1.41 </li></ul></ul><ul><ul><li>Pole 1.385 </li></ul></ul><ul><ul><li>Equator 1.375 </li></ul></ul><ul><li>Total power 15 -18 D. </li></ul>
  22. 23. LENS OPTICS <ul><li>Scattering of incident light </li></ul><ul><ul><li>Young lens 20% </li></ul></ul><ul><ul><li>60 years 60% </li></ul></ul><ul><li>Absorption of blue light </li></ul><ul><ul><li>Young lens 30% </li></ul></ul><ul><ul><li>60 years 60% </li></ul></ul><ul><li>Absorption of UV </li></ul><ul><ul><li>UVB (295-315 nm) </li></ul></ul><ul><ul><li>UVA (315-400nm) </li></ul></ul><ul><li>Transmission of visible light </li></ul><ul><ul><li>Young lens transmits approx 90% </li></ul></ul><ul><ul><li>Decreases with increasing age </li></ul></ul>
  23. 24. TRANSPARENCY <ul><li>During early stages– Opaque </li></ul><ul><li>Later on becomes transparent </li></ul><ul><li>Avascular </li></ul><ul><li>Absence of chromophores </li></ul><ul><li>Presence of highly organized structure </li></ul><ul><li>Perfect packing of crystalline cause minimal light scatter. </li></ul>
  24. 25. TRANSPARENCY <ul><li>Combined refractive index of epithelial layer and the capsule same as aqueous. </li></ul><ul><li>Newly formed cortical fibers do not scatter light </li></ul><ul><li>Regular arrangement of lens fibers with minimal intracellular space </li></ul>
  25. 26. ACCOMODATION <ul><li>Lens accounts for about one third of the refraction of the eye. </li></ul><ul><li>Provides a mechanism of focusing at different distances – accommodation </li></ul><ul><li>Accommodative power </li></ul><ul><li>at birth- 14-16 D </li></ul><ul><li>at 25yrs- 7-8D </li></ul><ul><li>at 50yrs- 1-2D </li></ul>
  26. 27. Chromatic aberration <ul><li>When white light is refracted at an optical interface, it is dispersed into its wavelengths or colours. </li></ul>
  27. 28. Spherical aberration <ul><li>Rays passing through the periphery of the lens are deviated more than those passing through the paraxial zone of the lens </li></ul>
  28. 29. Solution for aberrations in eye <ul><li>Nucleus has a higher refractive index than the cortex </li></ul><ul><li>Iris acts as a stop to reduce spherical aberration. </li></ul><ul><li>Retinal cones much more sensitive to light entering the eye paraxially. </li></ul>
  29. 30. OPTICAL CHANGES IN CATARACTOUS LENS <ul><li>Visual Acuity reduction. </li></ul><ul><li>Contrast sensitivity reduction. </li></ul><ul><ul><ul><li>Posterior subcapsular opacities. </li></ul></ul></ul><ul><li>Myopic shift. </li></ul><ul><li>Monocular diplopia. </li></ul><ul><li>Glare. </li></ul><ul><li>Color shift. </li></ul><ul><li>Visual field loss. </li></ul>
  30. 31. The retina <ul><li>Anatomically an outgrowth of the brain </li></ul><ul><li>Thin but intricate structures </li></ul><ul><li>Lines the posterior portion of the globe </li></ul><ul><li>Extend functionally up to the ora serrata </li></ul><ul><li>Nerve fibres from retinal receptors travel across its surface and exit via optic nerve. </li></ul><ul><li>Also supplied with blood vessels </li></ul><ul><ul><li>Visible by ophthalmoscope </li></ul></ul><ul><ul><li>Seen entoptically when cast shadows. </li></ul></ul>
  31. 32. The retina <ul><li>At fovea eye attains its maximum resolving power. </li></ul><ul><li>Also regarded as part of a concave spherical surface with r =-12 mm. </li></ul><ul><li>Cameras and optical instruments, image form on plane surfaces but the curvature of retina has two advantages </li></ul><ul><ul><li>The curved images formed by the optical system is brought in the right order. </li></ul></ul><ul><ul><li>A much wider field of view is covered by the steeply curved retina </li></ul></ul>
  32. 33. THE SCHEMATIC AND REDUCED EYE <ul><li>It is a theoretical optical specification of an idealized eye, retaining average dimensions but omitting the complications </li></ul><ul><li>Useful for understanding ophthalmologic problem and conceptualizing the optical properties of the human eye. </li></ul><ul><li>To calculate cardinal points, the radii of curvature and distances. separating the refracting surfaces must be known. </li></ul>
  33. 35. Optical anatomy of reduced eye
  34. 36. Schematic Eye and reduced eye
  35. 37. Schematic eye <ul><li>First principal point 1.6mm </li></ul><ul><li>Second principal point 1.90mm </li></ul><ul><li>First nodal point 7.08 </li></ul><ul><li>Second nodal point 7.33 </li></ul><ul><li>First focal point -16.7mm </li></ul><ul><li>Second focal point +22.2mm </li></ul><ul><li>Refractive power +60.00 D </li></ul>
  36. 38. Entrance and exit pupil <ul><li>Incident pencil of rays directed towards and filing the entrance pupil would pass through the entire area of the real pupil after refraction by cornea </li></ul><ul><li>Vice versa for exit pupil </li></ul><ul><li>Entrance pupil </li></ul><ul><ul><li>3mm behind anterior surface of cornea </li></ul></ul><ul><ul><li>13% larger than real pupil </li></ul></ul><ul><li>Exit pupil 3% larger than real pupil </li></ul><ul><li>U’/u= 0.82 (schematic eye) =constant </li></ul>
  37. 39. Entrance and exit pupil E Eo E’
  38. 40. The visual axis <ul><li>Fovea is displaced temporally and downwards from the expected position </li></ul><ul><li>Visual axis is the imaginary line which passes through the nodal point and meets at fovea </li></ul><ul><li>Angle between visual axis and optical axis is called α </li></ul><ul><ul><li>Positive when visual axis in object space lies on the nasal side of the optical axis </li></ul></ul><ul><ul><li>5º </li></ul></ul>
  39. 41. Field of vision
  40. 42. Blind spot <ul><li>Optic nerve head </li></ul><ul><li>No retial receptors </li></ul><ul><li>Dimension 2mmX1.5mm </li></ul><ul><li>Substend an angle of 5-7 º at 2 nd nodal point </li></ul><ul><li>It is 15 º temporal side of the visual field and 2º below it </li></ul>
  41. 43. Test for blind spot X
  42. 44. Reduced eye <ul><li>Study of optical imagery of the eye on the basis of a simple analogue </li></ul><ul><li>Entrance pupil, exit pupil coincide with hypothetical pupil= P=centre of pupil </li></ul><ul><li>n= 4/3 </li></ul><ul><li>Power= 60D </li></ul><ul><li>fe= -1/Fe= -1/60= -16.67mm </li></ul><ul><li>f’e= n’/Fe= +22.22mm </li></ul><ul><li>r= (n’-1)/Fe= +5.56mm </li></ul>
  43. 45. Comparison of the Gulstrand_Emsley schematic eye with the simple reduced eye <ul><li>Assessment </li></ul>
  44. 46. The retinal image size <ul><li>Retinal image is inverted </li></ul><ul><li>An object 50mm high is situated on the optical axis of the standard emmetropic reduced eye at a distance of 250mm from its principal point. Find the position and size of the optical image </li></ul>
  45. 47. Object at infinity <ul><li>Retinal image size depends on the angular subtense of the object </li></ul><ul><li>n’ sin u’= n sin u </li></ul><ul><li>If n=1, u=0 </li></ul><ul><li>n’u’= nu= u </li></ul><ul><li>u’= u/n’ </li></ul><ul><li>u’= -h’/fe’ or </li></ul><ul><li>h’= -u’fe’= -(u/n’)fe’ = -uFe </li></ul>
  46. 48. <ul><li>Thank you </li></ul>

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