Optical Conditions in the Eye. Marmor & Ravin, 1997, p.3. Eye ball.

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Presentation transcript:

Optical Conditions in the Eye

Marmor & Ravin, 1997, p.3. Eye ball

Unger, 1982 Eye Chart?

Human Eye: Two lenses CorneaLens Strong (in air)Weak FixedAdjustable Problems: MyopiaPresbyopia HyperopiaYellowing AstigmatismCataracts

Lens A positive lens collects light over a large area Cones of light. Point of cone: focused Cone diameter varies from point

Light Refraction in the Eye Cornea -chief refractive surface of the eye Lens -shape is changed by accommodation to focus images on the retina.

Emmetropia (Normal) When the eye is emmetropic, the eye is exactly the right size so that the image formed by the optics in the front falls on the retina.

Myopia (Nearsighted) When the eye is myopic, the eye is too long and the point of focus is in front of the retina. A blurry image falls on the retina.

Myopia When the eye is myopic, objects close to the eye will be in focus.

With any lens, objects at different distances are imaged at different distances behind the lens. In myopia, near objects may be in focus. Myopia = near-sighted

Negative Lens: Spreads Light Out.

Myopia corrected Myopia is corrected with a negative lens that compensates for the excess power of the cornea and lens. The focus point is moved backwards.

Surgical Corrections RK: small cuts around edge of cornea PRK: laser reshapes surface of cornea LASIK: laser reshapes internal layers of cornea Implantable lens and rings

Hyperopia (Farsighted) When the eye is hyperopic, the eye is too short and the point of focus is in behind of the retina. A blurry image falls on the retina.

Positive Lens: Focuses Light; Magnifier

Hyperopia corrected Hyperopia is corrected by placing an additional, positive lens in front of the eye. The stronger the lens, the more the focus point is moved forward.

Spherical Aberration Human cornea is “aspherical” Human lens has progressive index of refraction

Current Research Question: Why are some eyes myopic or hyperopic? Why are so many eyes emmetropic? Not random as after birth % emmetropic increases. Most cases of myopia start later, in teens. Associate with “close-work”. Two theories: –Extra muscular forces for accommodation etc. –Misguided growth

Astigmatism Christman, 1971, p.150.

Zakia, 1997, p.220. Radial

Accommodation

Marmor & Ravin, 1997, p.16 The Lens & Accommodation The lens changes shape to focus objects at different distances. Can also compensate for some myopia, hyperopia

Marmor & Ravin, 1997, p.17 Presbyopia

Gregory, 1997, p.39. Accommodation vs Age Fixed Focus by 50 “Presbyopia”

Hyperopia and Accommodation When young, hyperopia may compensated by making the lens more round.

Myopia and Accommodation When young, myopia may not be obvious as it can be compensated by flattening the lens.

Presbyopia Yellowing Opaque (Cataracts) The Lens and Aging

. Cataract: young