2 - Optics of CLs Flashcards
Why do we have to convert from spec rx to ocular rx
Bceause the BVD goes from 12mm to 0mm
How to calculate ocular Rx
- if above 4Ds then use table
- if there is a cyl, calculate the difference between the 2 sph values and that is the new cyl power
when convertimg, always prescribe the
least negative
where is refractive astigmatism located?
- cornea
or - crystalline lens
how is corneal astigmatism calculated
from keratometry or typography
(instruments used to determien shape of cornea)
what does keratometry measure
the curvature along the 2 principle meridians of cornea over 3-4mm
what does it mean when the CA is a bigger number?
- circle is bigger
- the curvature is flatter
what does it mean when the number is smaller
- circle is smaller
- curvature is steeper
0.05mm difference between 2 principle meridians means
there is 0.25D of corneal astigmatisn
0.05mm difference between 2 principle meridians means
there is 0.25D of corneal astigmatisn
calculate the CA wtih 7.50mm and 7.30mm
7.50 - 7.30 = 0.20mm
= 1.00 CA
0.20 / 0.05 = 4
4 X 0.25 = 1
CA when its 8.00mm and 7.60mm & what is sign
0.4mm difference
0.4 / 0.05 = 7
8 x 0.25 = 2 CA
negative cyl = flattest k = bigger circle
WTR astigmatism
verically steeper ( so smaller radius)
ATR astigmatism
horizontally steeper
if all astigmatism is located cornea it means…
px is excellent candidate for RGP
