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Retinal images
 

Retinal images

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    Retinal images Retinal images Presentation Transcript

    • Retinal Images Size/Shape & Relative Spectacle Magnification Gauri S Shrestha 1.2.2 Visual & optometric science
    • Nature of Retinal Image
      • Real
      • Inverted
      • Diminished in Size
      • Concave
        • r ~ -12 mm
    • Factors Affecting the Retinal Image
        • Diffraction
        • Aberrations
        • Defocus blur from uncorrected refractive error
        • Diffusive blur from light scatter
    • Factors Affecting the Retinal Image
      • The first three Factors depend upon
          • The wavelength of light
          • The pupil size.
      • The light scatter
          • Imperfections in the ocular media and from backward scatter from the retina and choroid.
    • Ocular application of Magnification
      • For the human eye,
        • retinal image for every eye is inverted and minified.
      • To express this variation in retinal image size
        • the concept of a relative magnification is introduced
      • Relative magnification
        • The ratio of the retinal image size after some change (refractive correction/ use of optical device) occurs to that before the change
    • Relative Magnification types
      • Relative distance magnification:
        • Bring given object closer to the eye.
      • 2. Relative size magnification:
        • Make object larger in size at same location.
      • 3. Projection magnification:
        • Due to the formation of a large image on a screen of an opaque or transparent object.
        • CCTV makes magnification possible at a convenient distance.
      • 4. Angular magnification:
        • Shape and/or power factor changes to achieve a larger retinal image.
        • Telescope
    • Relative distance Magnification
      • Relative distance magnification
      • All factors are kept constant except the location of the object
      • In general, moving a real object closer to the lens produces a greater lateral magnification
        • Closer is larger.
    • M r = relative retinal image size = retinal image size ( after ) retinal image size ( before )
    • F eye rp before retina y retb  b
    • F eye rp after retina y reta  a Rx
    • Formulas M rd = M ret a M ret b = tan  a tan  b
    • Relative size magnification
      • All factors are kept constant except the lateral size of the object.
        • the object size increase
      • Large print versions of magazines.
    • Projection magnification
      • Projected on the screen
      • LCD projector / OHP
      • Can considered as a special form of relative size magnification
    • Angular Magnification
      • AM, is defined as
    •                                                                                                          
    • Relative Spectacle Magnification (RSM)
      • Ratio of the retinal image size of the corrected eye to that of the schematic emmetropic eye
          • RSM = F A /Fs
          • Where,
            • Fs = the equivalent power of the schematic eye
            • F A = the equivalent power of the system (power of ametropic eye + power of spectacle lens)
      • F A = F + F E – x F F E
        • F = Equivalent power of spec. lens
        • F E = Equivalent power of eye,
        • x = distance between second principle planes of eye and spec. lens
      • RSM =
      Fs F+ FE – x F FE
    • Relative Spectacle Magnification
      • Refractive
        • Axial Vs Refractive
        • ? Suitable correction
          • Glasses Vs Contact lens
      • Purely Axial Ametropia
        • RSM = 1/1 + a F
        • Where, a = the distance between primary focal plane of the eye the secondary principle plane of the correcting lens
        • Schematic eye = Primary focal point = - 15.70 mm
    • Relative Spectacle Magnification
      • Spec is worn = 14 mm (vertex distance) from eye
      • RSM = 1 (max), when a = 0(almost zero)
      • For purely axial ametropia
        • 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
    • Relative Spectacle Magnification
      • Purely Refractive Ametropia
        • RSM = 1/ 1- x F
        • = 1 1
        • [1-(t/n)F1] [1-hFv]
        • 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)
        • RSM = 1 (max) , when , x = minimum (?almost zero)]
        • In purely refractive Ametropia
          • RSM is minimized by use of Contact lens
    • Thank you