Lecture 5 - optics of a CL Flashcards
why is a CL considered a thick lens not a thin lens?
CL has a small thickness and small radii (thin lens = radii are large compared to thickness)
what 3 things do you need to consider for the power calculation of a CL?
front and back surface radii, center thickness and refractive index
what can you do to ovoid measurement errors when measuring the back vertex power of CL?
use a small lens stop
how does the front vertex power compare to the back vertex power?
the front vertex power is always a little less than back vertex power (*except for high plus lenses)
how do you use the effective power equation in sphero-cylinder lenses?
if one meridian is more than 4D = break equation into optical cross - apply equation to each meridian and then put back into Rx
when can you use the square rule approximation for effective power calculations?
if the vertex distance given is 12mm
how are diopters and mm related?
inversely related = when mm increase, diopters decrease
how many diopters is 7.50mm?
45.00D
if you change by 0.1mm - how much does it change in diopters?
0.1mm = 0.5D (1mm = 5D)
what happens to the BCR of a soft lens when placed on the eye?
the back surface of the lens takes on the K-reading of the cornea (= no optically meaningful LL)
when is a LL optically meaningful in GP lenses?
when the BCR is selected flatter or steeper than the K-readings
what is the power of the LL if the BCR is flatter than K-reading?
minus power LL
what is the power of the LL if the BCR is steeper than K-reading?
plus power LL
if the patients Rx = -3.00D and GP fits 0.50D flatter than the Flat-K, what is the necessary GP lens power?
-3.00D - (-0.5D LL) = -2.50D GP lens
if your patients Rx = +3.00 -2.00 x 180 and GP fits 0.25D steeper than Flat-K, what is the necessary power of the GP lens?
*use spherical component
+3.00 - (+0.25D LL) = +2.75D GP lens
what is the SAM-FAP rule?
if GP fits steeper = add minus
if GP fits flatter = add plus
what astigmatism does a spherical soft CL correct for?
none (after 0.50D use toric soft lens)
what astigmatism does a spherical GP correct?
almost all of the corneal astigmatism (about 10.6% remains - corrects about 90%)
when should you use a toric GP lens?
for more than 2.50D of corneal astigmatism
what is internal astigmatism?
the refractive astigmatism - corneal astigmatism (K’s)
do spherical GP CL correct internal astigmatism?
no - only corneal
if a patient has 1.50D x 180 corneal astigmatism and 2.50D x 180 refractive astigmatism, should they use a spherical GP lens?
2.50 - 1.50 = 1.00D x 180 internal astigmatism (no more than 0.75D residual)
what type of lens can be used to compensate for residual astigmatism?
front toric GP lenses or bitoric lenses
what is the reference location for checking for unwanted rotation of a toric lens?
the 6 o’clock position
if a lens rotates to the left - how do you compensate to avoid poor vision?
left rotation = deviation is added to original axis
(right rotation = deviation is subtracted to original axis)
LARS rule
if a toric lens Rx = -2.00 -0.75 x 180 and rotated 15 degrees to the left, what should the new Rx be?
-2.00 -0.75 x 015 (subtract 15 from 180)
is the accommodative demand greater for a myope wearing glasses or CL?
accommodative demand is greater with CL
is the accommodative demand greater for a hyperope wearing glasses or CL?
the accommodative demand is greater with glasses
do CL induce prismatic effects when converging?
no - CL should always be centered on the pupil
what are prism-ballasted CL?
created by varying thickness from superior to inferior - so lenses are stabilized against rotation (1.5-3.0 BD)
how do you calculate prism-ballasted CL?
use P = 100 g(n-1)/l
when does unintentionally induced prism occur?
when a lens de-centers on the eye (use prentice rule)
what is the spectacle magnification in CL correction?
close to 1
which type of ametropia do CL help reduce magnification?
refractive (axial = specs)
