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87 Cards in this Set
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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
plconvex/cave, biconvex/cave; equiconvex/cave, meiscus, plcyl, toric 
the relationship btw the front and back surface curvature of a lens


plconvex

one surface is plano the other is positive


plconcave

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


plcyl

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(n1)s/h^2
h= 1/2 chord length (lens diameter) 

thickness, power and sag equation

tc  te = (F1 + F2)h^2/2(n1)


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 = 1.581.69
A = 30 40 used for fused segs 

barium crown glass
n= abbe = comment= 
n = 1.541.62
A  5559 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=1.541.66
a  3247 