2. OBJECTIVES
Image forming
mechanism
Accommodation.
Optical aberration of
the eye.
Common defects in
image forming
mechanism.
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4. Optics of Eye.
Focusing system of eye with refracting
structures. (Refractive Indices)
Cornea.(1.37)
Aqueous humour.(1.33)
Crystalline lens. (1.42)
Vitreous humour. (1.33)
So total Diopteric power of eye +60D
+44D by cornea
+16D by crystalline lens.
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5. Cardinal points of eye.
By Listing & Gauss
Schematic Eye.
For homocentric lens
system of eye there are 3
cardinal points
Principal foci (2)
Principal points (2)
Nodal points (2)
By Listing
Reduced Eye
Simplified data by
choosing single principal
point & single nodal point.
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7. Axes of the Eye.
3 Principal axis
Optical axis.
Visual axis.
Fixation axis.
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8. Accommodation.
Def – Mechanism to focus the diverging rays
coming from near objects on the retina to see
it clearly.
There ids increase in power of the crystalline
lens.
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9. Accommodation
Near point / Punctum proximum (P)– nearest
point at which objects can be seen clearly.
Far point / Punctum remotum (R) – distant point
at which objects can be seen clearly.
Range of accommodation – far point – near point.
Amplitude of accommodation -- Difference in
dioptic power P-R
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10. Mechanism of Accommodation.
Fish – retract the lens
Snakes & frogs – move the lens forwards
Horses -- move the head & tilt the retina
Human Being – change the shape of the lens.
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11. The Relaxation theory.
Thomas Young &
Helmholtz (1885)
At rest lens
compressed in
capsule by tension
of zonules d/t
relaxation of fibres
of ciliary muscle.
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12. The Relaxation theory.
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Ciliary muscles contract
Ciliary ring shortens.
Choroid move forwards.
Zonules relaxed.
Lens capsule relaxed.
Lens become more
spherical
Increases Diopteric power
of lens
13. Optical changes.
Slackening of the zonules.
Changes in curvature of lens surface.
Anterior pole move forwards.
Axial thickness increases.
Tension of lens capsule decreases.
Lens sinks down.
Lens substances change internally.
Pupillary constriction & convergence of eyes.
Choroid & Ora serrata move forwards.
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14. Optical aberration of the eye.
Def – lapses from
perfections.
Diffraction of light.
Caused by edge of an
aperture or rim of the
lens.
Actual pattern of
diffracted image –
alternate dark & bright
concentric rings.
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15. Optical aberration of the Eye.
Spherical
Aberration.
Spherical lens refracts
peripheral rays more
strongly than paraxial
rays.
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16. Optical Aberration of the Eye.
Chromatic
Aberrations.
Index of refraction of
any transparent
medium varies with
wavelength of the
incident of light.
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17. Common defects
Emmetropia
Ametropia.
Hypermetropia.
Myopia.
Astigmatism.
Presbiopia.
Emmetropia (optically
normal eye)
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18. Ametropia.
Hypermetropia. (far sightedness)
with accommodation at rest parallel rays of light
coming from infinity focused behind retina.
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19. Ametropia.
Myopia(Near sightedness)
with accommodation at rest parallel rays of light
coming from infinity focused infront retina.
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21. Astigmatism.
Refraction varies in
different meridia.
Regular.
Refractive power
changes uniformly
from one meridia to
other.
Irregular
Refractive power
changes irregularly
from one meridia to
other.
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22. Presbiopia.
Eyesight of old age.
Due to physiological
insufficiency of
accommodation.
Near vision fails
initially.
After 40 , near point
recedes beyond the
normal reading range
(25 cm) .
Saturday, February 21, 2015