Ophthalmic Equations Flashcards
Newton’s formula
x=f^2 *Fv
x- distance target moved
f= focal length standard lens
Fv=BVP
Magnification
m= I/O
m=o/i
Converting K to power
F=(1.3375-1)/r=0.3375/r
F=337.5/r (where r in mm)
Lens clock
F lens= (n lens-1)/(n lensclock-1)*F lens clock
Fresnel law
R= |(n2-n1)/(n2+n1)|^2
Reflection of light boundary of two indices
Transmittance
Ts=1-R
CL BVP
Fv=F2+ F1/{1-(t/n)*F1}
CL power from spec RX
Fc=Fs/(1-dFs)
Fs=refraction at vertex distance
d=vertex distance
Fc=refraction measured at the cornea
d=0 corresponds to K
Total power req at K
Rc= Fcl+Ftl
Rc=power req at K
Fcl= power at K
Ftl= power tear lake
_____ is the distance btwn the peripheral edge of the GP CL and the K. Adjusted by changing the peripheral radii in ___mm steps.
Edge lift; 1.0mm
Excessive edge lift is characterized by ___ peripheral pooling of fluorescein. desiccation, increased awareness, decreased CL centrationand 3&9 o’clock pooling
excessive
In adequate edge lift characterized by minimal pooling of fluorescein in the ____ results in debris trapped underneath CL
periphery
The more ______ center of gravity the better contraption of CL
posterior
_____ fit- negative TL; center of the CL will have K bearing with broad ring of mid-periphery and periphery of CL
Flatter than K
_______ fit fluorescein with pool in the center of the CL due to due to apical clearance with K bearing (minimal fluorescein) in mid periphery and periphery of CL
Steeper than K fit
Spherical fit over ____ astigmatism- fluorescein will pool across vertical (steepest) meridian with a horizontal dumbbell shaped pattern (dumbbell where touching)
WTR
Spherical fit over ____ astigmatism- fluorescein will pool across horizontal (steepest) meridian with a vertical dumbbell shaped pattern (dumbbell where touching)
ATR
Bitoric GP lens - ______ K astig and ______ Residual astig. Back surface is ___ astig and remaining astig is on the front. (>/= 2.50D K astig)
increase; increase; K
Bitoric GP-
_____ fit is equal alignment in both principal meridians btwn CL and K.
_______unequal alignment btwn principal meridians- fit on K -0.25 flatter than K for flat K meridian
Saddle fit; low topic simulation
_______ toric lens- indicted in >/= 2.50 D K astigmatism AND the magnitude of spec astig is = 1.5xK astig; (need more ref astig than K bc Back surface toric d
Back surface Toric GP
_______ toric lens- spherical back surface and topic front surface- K astig <2.50D unacceptable mag of residual astg
Front surface Toric GP
______ incorporates BD prism in order to minimize CL rotation during blink
Prism ballasting
_______ CL progressively flatten toward periphery of CL similar to peripheral flattening- improved alignment, decreased magnitude
Aspheric GP CL
Back surface aspheric CL indicated for ppl _____ astig, irregular K astig
ATR
Front surface aspheric CL indicated for ppl with excessive ____ astig for ppl hypercritical
excessive
Primary disadvantage of fitting a aspheric GP is _____ tear exchange due to decrease in CL movement
decreased
Multifocal RGP
______ design- D&N located pupil at same time (centration critical)
______design-CL segmented different focal zones for different viewing distances;
simultaneous; translating
Center thickness ____ lenses have more flexure
thinner
Magnitude of K astig K astig cause for increase flexure with >/= ____D K astig
1.50D
Flexure only measures on ____; measure w keratometer
the eye
OZD increases the sag depth of the CL which ____ flexure
increases
CL material ___Dk materials tend to have more flexure
higher
Base curve ____ BC often results in more flexure
steeper
Plus GP K lenses tend to maintain their shape as do minus- with center thickness greater than ____mm
0.13mm
Flexure ____ the amt of residual astig bc LL provides less astig correction compared to magnitude of the K astig
increases
Soft CL have ____ flexure bc it completely conforms to the shape of the K resulting in a plano lacrimal lens power
total
Warped GP lens has a ____ BC
toric
GP CL flexing will have ____ BC
spherical
If K astig is = ____D; fit 0.50 D flatter than K
0.75D
For every ____D increase in K astig, steepen the BC by ___D until bitoric GP lens is necessary
0.50D; 0.25D
Every 0.4 mm increase (decrease) in OZD/OAD flattens(steepens) the BC by ____D in order to achieve same fit
0.25D
Increase thickness, _____ oxygen
decreases
Gas permeability
P = Dk
Myopes ____ Dk (25-50) for daily wear; ____ Dk (51-99) for extended wear
low; high
Hyperopes and aphakes _____Dk for daily wear and _____Dk (>100) for extended wear
high; hyper
Toric lens- every clock hour movement ____ degrees axis change; ____
30 degrees; LARS
Silicone hydrogel decrease in water content, ______ oxygen
decrease
BC selection-
BC that is ___D flatter than K
4.00D
SCL- Average K value- fit median BC or the ___ BC
flatter
SCL- For steeper K value (>45) , fit ____ BC
steeper
SCL-For flatter K value (<45), fit _____ BC
flatter
FDA soft CL Group 1- \_\_\_ water content; \_\_?\_\_\_ ionic Group 2-\_\_\_\_water content;\_\_?\_\_\_ ionic Group 3-\_\_\_\_\_water content; \_\_?\_\_ionic Group 4-\_\_\_\_\_\_water content; \_\_\_?\_\_ionic Group 5-
Grp 1- low; non-ionic
Grp 2-high; ionic
Grp 3- low; non-ionic
Grp 4-high; ionic
High water content, ____ silicone content;_____ permeability
high; low; low
Deposits are more likely in ___lenses with a ____ water content
ionic; high
Hyperopes accom ____ with CLs compared to specs
less
Myopes accom ___ with CLs compared to specs
more
Hyperopes have ___ retinal image size with CL compared to specs due to ___ spec mag
smaller; less
Myopes have____ retinal image size with Cl compared to specs due to ___spec mag
larger; less
RGP- apical touch ____ BC
steepen
