AAM refractive error & emmetropism Flashcards
refraction
human eye refracts at 2 ocular structures
- cornea - fixed focusing power
- lens - accommodation changes lens shape
elements that determine eye’s ability to focus light
- shape of cornea
- power of lens
- length of eyeball
- cornea accounts for 2 thirds of total refraction of eye
- lens accounts for 1 third of refraction of eye
total power of eye = +60D
- cornea = 40D
- lens = 20D
far point
the furthest distance that a person can see without the help of spectacles or contact lenses
- can be in one of three places: at infinity, at some distance in front of the eye and at some distance behind the eye
focal point
the point where parallel light rays meet after passing through the lens
- can be in one of three places: macula, in front of the retina and behind the retina
what is refractive error?
mismatch between the power of the eye’s optical system + the length of the eye
- results in blurred vision or eyestrain + headaches
ametropia
- when light rays entering eye don’t focus on the retina
- rx not a disease but rather normal differences in visual ability`
3 types of ametropia
- myopia
- hyperopia
- astigmatism
emmetropia
no rx
emmetropic eye
- focal point on retina
- far point at infinity (6m regarded as infinity)
- light rays from distant object are parallel
- retinal image of object on fovea
myopic eye
- light rays from distant object are parallel
- focal point in front of retina
- far point in front of eye
- image formed in front of retina?
why myopia?
- cornea too curved (REFRACTIVE ametropia)
- lens too powerful (RA)
- eye too long (AXIAL ametropia)
- or combination
hyperopic eye
- light rays from distant object are parallel
- focal point behind retina
- far point behind eye
- image formed behind retina
why hyperopia?
- cornea too flat (REFRACTIVE ametropia)
- lens too weak (RA)
- eyes too short (AXIAL ametropia)
- or combination
astigmatism
- usually occurs w myopia or hyperopia
- irregular curvature of cornea/lens
- light rays focus in different locations
w the rule:
- cornea more curved in the vertical meridian
against the rule:
- cornea more curved in the horizontal meridian
near objects
when looking at near objects, you must accommodate to bring object into focus
refractive error unit of measure
- measured in DIOPTRES (D)
- used to describe the rx of the eye as well as the power of opthalmic lenses
- dioptre = reciprocal of distance between the lens and its focal point
- 1 / focal length (in metres)
- in eye care, we work in quarter dioptre units of power (0.25D)
correction of rx
- amount of power in dioptres, needed to bring rays of light back to focus on retina
- positive, convex lenses used to converge light rays entering eye to move focal point up the plane of retina
- negative, concave lenses used to diverge the light rays to move the focal point back to plane of retina
myopic eye
rays focus in front of retina
- corrected w aid of negative, concave lens to focus rays on retina
hyperopic eye
rays focus behind retina
- corrected w aid of positive, convex lens to focus rays on retina
astigmatism
- correct 1 meridian w sphere
- correct 2nd meridian w cylinder
power in 1 meridian only?
emmetropisation
rapid reduction in refractive error over first few years of life towards emmetropia
- co-ordination of ocular components
- human eye programmed to achieve emmetropia in childhood
- eye maintains emmetropia w advancing years
despite changes in all eye dimensions during period of growth + continuing growth of lens throughout life?
GO THROUGH LECTURE RECORDING
