3.4- Ametropia and its correction Flashcards
What is the causes of spherical ametropia?
- axial length of eye incorrect
- refractive power of eye incorrect (due to surface curvature or refractive index)
What causes ametropia?
- failure of co-ordination of eye during growth of the optical components of the eye e.g axial length may get too long for power of eye or the power of eye too much for the axial length during the growth phase.
- Sometimes pathology e.g nuclear cataract
What are the causes for ametropia larger than 4D?
eye is usually too long or too short
What does a nuclear cataract cause?
a myopic shift
What are the changes in ametropia with k’ (axial length)?
Equation showing change in ocular refraction
AK= (-K’2/n’) x AK
A= Delta K
What is the equation for a standard emmetropic eye K’= +60D and k’ is in mm?
AK= -2.7 Ak’
change in ocular refraction is -2.7 times the change in axial length in mm
-this means axial length increase by 1mm, eye becomes -2.7D myopic- sensitive to small changes
What is the length of a cone photoreceptor?
0.07mm
AK=2.7 X .07= -0.189D
How can we correct myopia via spectacle correction?
- need a negative spectacle lens to diverge the infinitely distant incoming light.
- Light from an object at far point will strike the eye at the correct vergence (K) and form a sharp image on the retina
- Light diverge at spectacle lens and forms a sharp image on retina
- light travelling parallel is diverged by spectacle lens and strikes the eye with divergence value K , the value after refraction, leaving eye K’ and sharp image formed on retina.
What is the correlation with the Fsp’ and the MR?
- So as light comes through infinitely through the negative lens it is parallel to the optical axis
- However it diverges light leaving it.
- You extrapolate the diverging ray backwards and that is the second focal point of spectacle lens
- (If eye is fully corrected it must coincide with the far point) F sp’ coincides with Mr- a negative lens required for a myope
- Rays incident on the eye appear to come from MR and thus form a sharp image onto the retina.
What is the spectacle correction of hypermetropia?
-Second focal point of spectacle lens Fsp’ coincides with MR- Positive lens required
-Rays incident on the eye appear to come from MR and thus forms a sharp image on the retina.
On the diagram- the positive lens converges the light coming from an infite object. IT then extrapolated forwards , in front of the retina forming the Fsp’ which coincides with the MR. Hence forms a sharp I’m,age on the retina.
What does the vergence of eye have to be?
equal to the ocular refraction (K) to be able to form a sharp image on to the retina.
When does vergence change?
as light travels through space.
Where are the spectacles placed?
placed at the vertex distance , d, from the eye
What will the spectacle plane refraction be?
will not be the same as ocular refraction
What is ocular refraction (K)?
thin lens power of a contact lens needed to correct ametropia
What is spectacle refraction?
thin lens power of a spectacle lens needed to correct
what is the effectivity?
i.e. the vergence at the eye is different to when it is leaving the spectacle lens
A spectacle lens produces an effective power that varies at different distances from the eye
What is the equation for effectivity?
L2= Fsp/ 1-dFsp
What happens if only the eye is corrected fully?
L2 (vergence of the eye) becomes the ocular refraction K- now have a relationship of the power of the eye Fsp and K.
K=. Fsp/1-dFsp
What is the magnitude of the spectacle lens power for myope and hypermetrope?
is higher in a myope than it is for ocular refraction(K)
and in hypermetrope is opposite (lower)
How is the magnitude of the spectacle lens power is higher in a myope than the ocular refraction?
- Converges slightly more by the time it reaches the eye
- e.g of spectacle lens effectivity- the effect of spectacle lens at the eye is different to when it is leaving the eye
What does spectacle correction require?
the second focal point of the spectacle lens to be coincident with the far point.
What is Fsp?
spectacle refraction and it is different from ocular refraction
What is effectivity?
- the vergence of light leaving a spectacle lens is different from that striking the eye
- power changes when leaving the lens when before.
