Post on 15-Jul-2015
OBJECTIVES Image forming
mechanism Accommodation. Optical aberration of
the eye. Common defects in
image forming mechanism.
Saturday, February 21, 2015
Image forming mechanism CAMERA
Shutter. Focusing system. Diaphragm. Darkened interior.
EYE Eyelids. Cornea & crystalline lens. Iris. Choroid & pigmented
epithelium of retina.
Saturday, February 21, 2015
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.
Saturday, February 21, 2015
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 ListingReduced Eye
Simplified data by choosing single principal point & single nodal point.
Saturday, February 21, 2015
Axes of the Eye. 3 Principal axis
Optical axis. Visual axis. Fixation axis.
Saturday, February 21, 2015
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.
Saturday, February 21, 2015
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
Saturday, February 21, 2015
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.
Saturday, February 21, 2015
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.
Saturday, February 21, 2015
The Relaxation theory.
Saturday, February 21, 2015
Ciliary muscles contract Ciliary ring shortens. Choroid move forwards. Zonules relaxed. Lens capsule relaxed. Lens become more
spherical Increases Diopteric power
of lens
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.
Saturday, February 21, 2015
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.
Saturday, February 21, 2015
Optical aberration of the Eye. Spherical
Aberration. Spherical lens refracts
peripheral rays more strongly than paraxial rays.
Saturday, February 21, 2015
Optical Aberration of the Eye. Chromatic
Aberrations. Index of refraction of
any transparent medium varies with wavelength of the incident of light.
Saturday, February 21, 2015
Common defects Emmetropia Ametropia.
Hypermetropia. Myopia. Astigmatism.
Presbiopia.
Emmetropia (optically normal eye)
Saturday, February 21, 2015
Ametropia. Hypermetropia. (far sightedness) with accommodation at rest parallel rays of light
coming from infinity focused behind retina.
Saturday, February 21, 2015
Ametropia. Myopia(Near sightedness) with accommodation at rest parallel rays of light
coming from infinity focused infront retina.
Saturday, February 21, 2015
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.
Saturday, February 21, 2015
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