prism Flashcards
how much vertical imbalance is induced when the patient looks down 10mm to read?
OD: +3.00 DS
OS: -3.00DS
ADD: +2.50 lined bifocal
6BU OD
OD: 3(1cm)=3PD BU
OS: 3(1cm)=3PD BD
add together=6PD BU OD or 6PD BD OS
if the vertical imbalance is corrected with reverse slab off, which lens will have induced prism, and what is the base direction of the induced prism?
OD: +3.00 DS
OS: -3.00DS
ADD: +2.50 lined bifocal
OD, BD
vertical imbalance
- diff prism induced where looking down between 2 eyes
- looking down
- prentices rule
- need: diet from OC that eyes move, power of lens in vertical meridian (do power cross if cyl involved)
- dont need: add power or type
how much vertical imbalance isa problem
> 1.5PD
things that can correct for vertical imbalance
dissimilar segments
separate single vision reading glasses
slab off
contact lenses
dissimilar segment for vertical imbalance
two different bifocal designs are used in each lens; this method reduces vertical imbalance because each bifocal segment will have a different distance from the top of the segment to the segment optical center
slab off for vertical imbalance
induces BU prism in the most minus lens
BUMMM=BU most minus meridian
reverse slab off
another method to correct vertical imbalance. induces prism on the most plus lens in the 90 meridian in order to counteract the BU prism induced by the plus lens
what are PALs good for fixing
image jump
each diopter of anisometropia produces ___ difference in the size of the retinal image
1%
aniseikonia > ____ prevents the patient from fusing
10%
aniseikonia > ___ is bothersome to the patient
3%
Knapps law
tells us whether glasses or CL are best for axial vs refractive anisometropia
Knapps law and axial anisometropia
correct with spectacles
give them an “axis”
knapps law and refractive anisometrpia
correct with CL
“refractive are active” give them CL
decreases SM in plus lenses
decrease vertex distance
decrease thickness
decreased BC
increase n
decreases SM in minus lenses
increases vertex distance
decreases thickness
increase n
decrease BC
spec mag
Power Factor= (1/(1-(h+3))(P))
shape factor= (1/1-(t/n)(F1))
the patient notes her vision is distorted when she looks through the peripheral portion of the lens. which of the following factors is least likely contributing to he complain?
- CA
- incorrect BC
- SA
- radial astigmatism
- curvature of field
SA
chromatic aberrations
shower wavelengths (blues) are refracted more than longer wavelengths (reds) as they pass through an optical system, causing blue waves to focus closer to the cornea and red waves to focus closer to the retina. this results in color fringes around objects that is more pronounced when viewing through the periphery of the lens. higher powered lenses will also have more chromatic aberrations
monochromatic aberrations
SA coma radial astigmatism curvature of field distortion
what monochromatic aberrations distort image quality
SA
coma
radial astigmatism
what monochromatic aberrations distort image plane
curvature of field
distortion
spherical aberrations
marginal rays bent more
on and off axis points
blur circle
pupil fixes, corneal fixes
night myopia
SA + pupil size
coma
monochromatic abberation. occurs only for off-axis point sources due to a difference in magnification for incident rays of different heights
radial astigmatism
monochromatic Aberration
light rays can strike a lens surface obliquely, resulting in a distorted image due to induced cylinder power. can be reduced by selecting the optical base curve from the Tscherning ellipse (pick BC to decrease marginal astigmatism. always use Oswald curve)
distortion
monochromatic aberration
caused by differences in magnification of a point object based on the distance of the object from the optical axis. it causes a straight line object to form a curved line image
barrel
minus lenses
pincushion
plus lenses
practical application of SA and coma
generally don’t affect vision because the pupil is small enough to block marginal rays that cause these aberrations. high plus lenses (>+7.00D) would be deigned as aspheric lenses in order to minimize these aberrations
practical application of radial astigmatism
minimized by selecting the optional base curve for the specific lens power using the T curve
curvature of field practicality
can be minimized by using the manufacturers recommended BC
CA can be reduced by
using a shorter vertex distance and having sufficient panto tilt
point focal lens corrects for
oblique astigmatism
Percival form lens corrects for
curvature of field
field of view
increased with decreased vertex distance
with plus lenses, the higher power, the smaller FOV
with minus lenses, the higher power, the larger the FOV
