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
