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87 Cards in this Set

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  • Back
Indirect opthalmoscope characeristics
large depth of field
large depth of focus
low mag
direct ophthalmoscope characteristics
small depth of field
small depth of focus
high mag
what is the image of an indirect ophthalmoscope?
real inverted image
retinoscope how it works
light reflected from retina is refracted and focused at far point
goal of retinoscopy
stop motion by using a lens that is located btw retinoscope and patient
equation for Rx
Rx = F - WD
what does a lensometer measure?
back vertex power
prism
how does lensometer work?
light from std lens must converge at the primary focal point of the lens therefore move target till this happens
lensometer setup
target, std lens, test lens
lensometer formula
x = f^2Fv

+ move toward
- move away
x= distance target moved
f = focal length of std
Fv = back vertex power
what does a radiuscope measure?
radius of curvature of CL
how does a radiuscope work?
two locations where the lens can be at where viewer sees clear image therefore just move lens and measure how far it had to be moved this distance is the radius
what does a keratometer measure?
radius of curvature of cornea and refractive power
how does a K work?
cornea acts as a convex mirror casting image of mires on cornea

mires = radius of cornea
what does a lens clock measure?
directly measures sag

indirectly measures power
how does a lens clock work?
directly measures sag by moving the middle pin
downfall of lens clock
lens clock is calibrated for n=1.523 and asumming that it a center material therefore if the lens measured has a different n the power will not be the true power
what is sag?
distance btw point on a circle and mid point of chord to circle
sag approximate formula
s = h^2/2r
sag in relation to power and diamter
directly related
describe the power for a spherical lens
same in all directions
describe power for a cylindrical lens
power is plano in one principal meridian and not in the other
toric lens
power is different in each prinicipal meridian but one doesnt have to be zero
plus cyl form
front surface is toric
minus cyl form
back surface is toric

we use this
lens form

pl-convex/cave, biconvex/cave; equiconvex/cave, meiscus, pl-cyl, toric
the relationship btw the front and back surface curvature of a lens
pl-convex
one surface is plano the other is positive
pl-concave
one surface is plano the other is negative
biconvex
both are positive
biconcave
both are negative
equiconvex and equiconcave
half of total power is due to front surface and half due to back surface
meniscus
convex on front
concave on back
why are meniscus lens good?
reduce aberrations
pl-cyl
one surface is pl, other surface has cyl
toric
one surface toric the other is sphere
cross cyl lenses (JCC)
plus cyl on front and minus cyl on back of same magnitude located 90 apart

both are toric
what is base curve?
std lens curve set by manufacturer
where is base curve always located?
FRONT SURFACE
where is the base curve of a spherical lens
front
base curve of a plus cyl lens
FRONT surface therefore it is the toric surface, pick the FLATTER one (<power)
base curve of a minus cyl lens
FRONT surface therefore it is the sphere part
Vogel's base curve formula for plus lens
BC = FSE + 6
Vogel's base curve formula for minus lens
BC = FSE/2 + 6
spherical equivalent
FSE = Fc/2 + Fs
describe thickness of minus lens
thicker in the periphery
thinner in center
describe thickness of plus lens
thicker in center
thinner in edge
thickness formula
tc - te = s1 - s2

measure s from arc to cord
flat surfaces have a sag of
zero
when does lens thickness become important?
+/- 4 D
what thickness do we want to control for minus lenses
edge
what thickness do we want to control for plus lenses
center
power and sag equation
F = 2(n-1)s/h^2

h= 1/2 chord length (lens diameter)
thickness, power and sag equation
tc - te = (F1 + F2)h^2/2(n-1)
isothickness curves
show curves where thickness is the same

lines close together = thickness is changing fast
change of thickness is what for low power lens
slower

directly related
Datum system
place lens how would sit in frame and draw horizontal lines tangent to highest and lowest edge
datum line
line halfway btw the 2 horiz lines
datum length
width of lens along datum line
datum depth
length of lens along datum line
Boxing system
adds vertical lines in addition to the tangent horizontal lines of highest and lowest edge
geometric center
point on datum line halfway btw the 2 vertical lines which are tan to the edges
eye size (A)
horizontal length of box
B
vertical length of box
frame PD (GCD)
shortest horizontal distance btw lenses
effective diameter
longest diameter of lens
minimum blank size
smallest size of lens blank needed to make lens
MBS formula
MBS = ED + 2d + 2mm

d = decentration
decentration
the amount and direction of the displacement of the pole of the lens from the geometric center of the lens box
decentration formula
d = FPD = PPD/2
decentration does what
The process of edging a lens so that its optical center does not fall at the geometric center of the lens box
decentration used to
control prism
Eliminate undesired prism
Induce desired prism
major reference point
The location on the lens where the desired prismatic effect, if any, should be verified

If no prism is prescribed, the optical center of the lens should coincide with the prism reference point
where should MRP be located?
directly in front of the center of the patient’s pupils when looking in primary gaze
MRP is specified in relation to what
GC of lens
what passes thru the line of sight
MRP
if there is no prism, where is the MRP located?
optic axis
dispersion and n are
directly related
abbe number and dispersion are
inversely related
is a good abbe number high or low?
high abbe --> low dispersion
high index lens have ___ dispersion and ___ abbe and are thus optically ___
1. high
2. low
3. poor
low index lenses have __ dispersion and ___ abbe and are thus optically __
1. low
2. high
3. good
crown glass
n=
abbe =
comment=
n = 1.523
A = 58.9

used in most SVG
great optics
not impact resistant
flint glass
n=
abbe =
comment=
n = 1.58-1.69
A = 30 -40

used for fused segs
barium crown glass
n=
abbe =
comment=
n = 1.54-1.62
A - 55-59

fused segs
CR39
n=
abbe =
comment=
n = 1.49
A = 55

light, impact resistant, many designs
poly
n
A
comment
n=1.586
a=30

excellent impact resistance, poor optics, lightest
high index
n=1.54-1.66
a - 32-47