Retinal images

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Retinal images

  1. 1. Retinal Images Size/Shape & Relative Spectacle Magnification Gauri S Shrestha 1.2.2 Visual & optometric science
  2. 2. Nature of Retinal Image <ul><li>Real </li></ul><ul><li>Inverted </li></ul><ul><li>Diminished in Size </li></ul><ul><li>Concave </li></ul><ul><ul><li>r ~ -12 mm </li></ul></ul>
  3. 3. Factors Affecting the Retinal Image <ul><ul><li>Diffraction </li></ul></ul><ul><ul><li>Aberrations </li></ul></ul><ul><ul><li>Defocus blur from uncorrected refractive error </li></ul></ul><ul><ul><li>Diffusive blur from light scatter </li></ul></ul>
  4. 4. Factors Affecting the Retinal Image <ul><li>The first three Factors depend upon </li></ul><ul><ul><ul><li>The wavelength of light </li></ul></ul></ul><ul><ul><ul><li>The pupil size. </li></ul></ul></ul><ul><li>The light scatter </li></ul><ul><ul><ul><li>Imperfections in the ocular media and from backward scatter from the retina and choroid. </li></ul></ul></ul>
  5. 5. Ocular application of Magnification <ul><li>For the human eye, </li></ul><ul><ul><li>retinal image for every eye is inverted and minified. </li></ul></ul><ul><li>To express this variation in retinal image size </li></ul><ul><ul><li>the concept of a relative magnification is introduced </li></ul></ul><ul><li>Relative magnification </li></ul><ul><ul><li>The ratio of the retinal image size after some change (refractive correction/ use of optical device) occurs to that before the change </li></ul></ul>
  6. 6. Relative Magnification types <ul><li>Relative distance magnification: </li></ul><ul><ul><li>Bring given object closer to the eye. </li></ul></ul><ul><li>2. Relative size magnification: </li></ul><ul><ul><li>Make object larger in size at same location. </li></ul></ul><ul><li>3. Projection magnification: </li></ul><ul><ul><li>Due to the formation of a large image on a screen of an opaque or transparent object. </li></ul></ul><ul><ul><li>CCTV makes magnification possible at a convenient distance. </li></ul></ul><ul><li>4. Angular magnification: </li></ul><ul><ul><li>Shape and/or power factor changes to achieve a larger retinal image. </li></ul></ul><ul><ul><li>Telescope </li></ul></ul>
  7. 7. Relative distance Magnification <ul><li>Relative distance magnification </li></ul><ul><li>All factors are kept constant except the location of the object </li></ul><ul><li>In general, moving a real object closer to the lens produces a greater lateral magnification </li></ul><ul><ul><li>Closer is larger. </li></ul></ul>
  8. 8. M r = relative retinal image size = retinal image size ( after ) retinal image size ( before )
  9. 9. F eye rp before retina y retb  b
  10. 10. F eye rp after retina y reta  a Rx
  11. 11. Formulas M rd = M ret a M ret b = tan  a tan  b
  12. 12. Relative size magnification <ul><li>All factors are kept constant except the lateral size of the object. </li></ul><ul><ul><li>the object size increase </li></ul></ul><ul><li>Large print versions of magazines. </li></ul>
  13. 13. Projection magnification <ul><li>Projected on the screen </li></ul><ul><li>LCD projector / OHP </li></ul><ul><li>Can considered as a special form of relative size magnification </li></ul>
  14. 14. Angular Magnification <ul><li>AM, is defined as </li></ul>
  15. 15.                                                                                                          
  16. 16. Relative Spectacle Magnification (RSM) <ul><li>Ratio of the retinal image size of the corrected eye to that of the schematic emmetropic eye </li></ul><ul><ul><ul><li>RSM = F A /Fs </li></ul></ul></ul><ul><ul><ul><li>Where, </li></ul></ul></ul><ul><ul><ul><ul><li>Fs = the equivalent power of the schematic eye </li></ul></ul></ul></ul><ul><ul><ul><ul><li>F A = the equivalent power of the system (power of ametropic eye + power of spectacle lens) </li></ul></ul></ul></ul><ul><li>F A = F + F E – x F F E </li></ul><ul><ul><li>F = Equivalent power of spec. lens </li></ul></ul><ul><ul><li>F E = Equivalent power of eye, </li></ul></ul><ul><ul><li>x = distance between second principle planes of eye and spec. lens </li></ul></ul><ul><li>RSM = </li></ul>Fs F+ FE – x F FE
  17. 17. Relative Spectacle Magnification <ul><li>Refractive </li></ul><ul><ul><li>Axial Vs Refractive </li></ul></ul><ul><ul><li>? Suitable correction </li></ul></ul><ul><ul><ul><li>Glasses Vs Contact lens </li></ul></ul></ul><ul><li>Purely Axial Ametropia </li></ul><ul><ul><li>RSM = 1/1 + a F </li></ul></ul><ul><ul><li>Where, a = the distance between primary focal plane of the eye the secondary principle plane of the correcting lens </li></ul></ul><ul><ul><li>Schematic eye = Primary focal point = - 15.70 mm </li></ul></ul>
  18. 18. Relative Spectacle Magnification <ul><li>Spec is worn = 14 mm (vertex distance) from eye </li></ul><ul><li>RSM = 1 (max), when a = 0(almost zero) </li></ul><ul><li>For purely axial ametropia </li></ul><ul><ul><li>If the secondary principle plane of the correcting lens is placed at the primary focal plane of eye the retinal image size is that of schematic eye = Knapp’s Law </li></ul></ul>
  19. 19. Relative Spectacle Magnification <ul><li>Purely Refractive Ametropia </li></ul><ul><ul><li>RSM = 1/ 1- x F </li></ul></ul><ul><ul><li>= 1 1 </li></ul></ul><ul><ul><li>[1-(t/n)F1] [1-hFv] </li></ul></ul><ul><ul><li>Where, x = the distance from the secondary principle plane of spec. lens to the entrance pupil of the eye (3 mm from ant. Surface of cornea) </li></ul></ul><ul><ul><li>RSM = 1 (max) , when , x = minimum (?almost zero)] </li></ul></ul><ul><ul><li>In purely refractive Ametropia </li></ul></ul><ul><ul><ul><li>RSM is minimized by use of Contact lens </li></ul></ul></ul>
  20. 20. Thank you

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