Vision
Refractive Error
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Refractive Error
, Refraction, Emmetropia, Hyperopia, Farsighted, Astigmatism
Definitions
Refraction
Measured in Diopters
Bending of light rays
Junction of two transparent media
Media must have different densities to bend light
Human eye refracts light at two locations
Cornea
(66% of eye's focusing power)
Fixed focusing power
Crystalline lens (33% of eye's focusing power)
Accommodation changes lens shape to focus objects
Refractive power of lens
Reciprocal of focal length measured in meters
Examples
One diopter lens has focal point of 1 meter
Two diopter lens has focal point of 0.5 meters
Emmetropia (normal vision)
Eye focusing power perfectly matched to globe length
Image focused precisely on
Retina
Normal vision confers focal length of infinity
Myopia
(
Nearsighted
ness)
See
Myopia
Hyperopia (Farsightedness)
Normal in infants (+0.50 to +2.50 Diopters)
Vision normalizes by age 5 to 8 years old
Light rays focus behind
Cornea
Cornea
too flat or lens too weak for globe
Near objects not seen clearly
Correction: Convex lenses (convergent, plus power)
Astigmatism
Non-spherical
Cornea
l surface
Parts of surface (
Meridia
ns) are steeper than others
Objects blurry at any distance
Assessment
See
Visual Acuity
Management
Refractive Error Correction
Non-Surgical Options
Eye Glasses
Contact Lens
es
Refractive Surgery
Laser In-Situ Keratomileusis
(
LASIK
)
Intrastromal corneal ring
(ICR or
Intacs
)
Phakic Intraocular Lens
es
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