optics Flashcards
(262 cards)
1
Q
BIO vs DO
- monocular/binocular
- FOV
- Mag
- image
- WD
- influence of RE
- DOF
- uses of DO
A
2
Q
Bruckner - what do the results mean?
A
Brighter reflex: Strap, amblyopia, higher RX
Darker reflex: opacity, normal
3
Q
how does an ophthalmoscope work?
A
note that the image the retina sees is upside down and the brain flips ie. think VF defects
The image is then focused in front of the examiner and now is the new object (virtual) for the Dr.
lenses then help to focus the image on the Dr’s retina
On BIO, the BIO itself virtual object into a real inverted image. In order to see this image we need another lens ie. the 20 D lens
4
Q
How does a lensometer work?
A
It measures the back vertex power of a lens.
The lenometer is made up of a test lens (what we are looking for) + a known standard lens + TS + a moveable target.
We move the dial in order to move the target so that light from the STD lens converges at the primary focal point of the test lens. CAP FAM. This allows for parallel light rays to leave the test lens and the TS system so that parallel light enters our eye allowing us to see the target.
5
Q
Hand neutralization
A
(-) lens = WM
(+) lens = AM
6
Q
Emmetropic Dr w/ DO to see pt’s retina dials in -4.00 D lens to view the retina clearly. What’s the patient’s RE?
A
6
Q
Lensometer- using a +25.00 D STD lens. To achieve focus, we move the target 2.00 mm forward. What is the back vertex power being tested?
A
7
Q
A prescription calls for 3 PD BO OD. How should the lens be centered in the lensometer (before edging) for verification?
A
8
Q
The readout on a keratometer = 45.25 Dk. What is the radius of curvature of the cornea?
A
9
Q
You use a lens clock calibrated w/ n=1.5. You want to measure the power of a lens made from ,material n =1.7. The power readout of the lens clock is +6.00 D. What is the actual power of the lens?
A
10
Q
The front surface of a lens is measured with a lens clock. The lens clock shows a value of +6.00 D based on an assumed index of 1.53. What is the radius of curvature of the lens surface?
A
11
Q
Suppose you have a CR-39 plastic lens with a radius of curvature of 0.0883m. What is the front surface refractive power of this lens?
A
12
Q
Use the conversion factor for polycarbonate to find the front surface refractive power for the front surface of a polycarbonate lens. The lens clock measures a true power of +8.12 D for the lens.
A
13
Q
A lens clock is used on the front and back surfaces of a lens. The front surface is +2.00 D. The back surface is -6.87 D in the 90 degree meridian and -6.00 in the 180 degree meridian. The lens has an index of refraction of 1.66. What is the power of the lens expressed in minus cylinder form?
A
14
Q
What is the nominal approximate power of a 1.66 index ophthalmic lens with the following clock readings?
front surface meridian 180 = +3.00 D
Back surface meridian 060 = -10.50 D
Back surface meridian 150 = -7.25 D
A
15
Q
What is the lens material of a lens clock calibrated for 1.53 measures the surface of a +5.75 DS refractive surface and produces a clock reading of +6.12 D?
A
16
Q
A -1.00D trial lens was placed in front of the keratometer and used to measure both the principal meridians. The keratometer reads 45.12 D in the 45 degree meridian and 49.87D in the 135 degree meridian. How do you record the amount of corneal astigmatism and the type of astigmatism?
A
17
Q
Calculate the predicted Refractive astigmatism for a 35 year old with a keratometry of 45.00/47.00 @ 090
A
18
Q
Calculate the predicted refractive astigmatism for a 25 year old with a keratomtery of 41.25/41.50 @ 090
A
19
Q
calculate the predicted refractive astigmatism for a 33 year old with a keratometry of 45.37/43.75@ 090
A
20
Q
When you measure the base curve of an RGP lens/ The first clear image you see is at 2mm. The second clear image you see is at 8.3 mm. What is the base curve of your cls?
A
21
Q
calculate the predictive refractive astigmatism of a 42 year old with a keratometry of 44.37/45.75 @ 085.
A
22
Q
Your pt with a stargardt macular dystrophy can see 10/100 and you want them to be able to see 10/20. What telescope do you begin testing with?
A
23
Q
What is the magnification of a TS with an entrance pupil of 24mm and an exit pupil of 6 mm? what does this mag tell use about the image?
A
24
A doctor with emmetopia uses a direct ophthalmoscope to view the patient's retina. The clinician dials in a -4.00 D lens in order to view the retina clearly. What is the patient's RE ( assuming she is uncorrected?)
power of the lens in DO = PT's RX + Doctor's RX.
-4.00 D = pt's rx + 0
Pt's rx = -4.00 D
25
Let's say we are using a +25.00 STD lens. To achieve approximate focus, we must move the target on the lensometer 2.0 mm forward. What is the back vertex power of the lens being tested?
X = f^2(Fv)
+.002m = (1/25D)^2(Fv)
Fv = +1.25 D
26
How does a radioscope work?
This measures the radius of curvature/back vertex power of a rigid gas permeable cls lens.
When looking through the radioscope we see 3 images.
first image when clear is the power at the surface of the lens
filament
the second clear image is the power located at the center of curvature of the cls lens.
In order to find the base curve we take the difference between the 2 images.
27
We are measuring the power of an unknown lens using hand neutralization. With a +5.00 D lens on top of the unknown lens we get no motion. What is the power of the unknown lens?
Lunknown + Lknown = No motion (0)
Lunknown + (+5.00) = 0
unknown lens = -5.00 D
28
How does a keratometer work?
see image
A keratometer measures the radius of curvature/ power of the center of the cornea at different axes.
It converted the radius to curvature to D assuming that the retractive index of the cornea is 1.33.
ie. uses the equation:
F = (n'-n)/r
29
How can you expand the range of the keratometer by 8.00 D?
add a +1.25 D lens over the aperture on the patient's side.
30
The readout on a keratometer is 45.25 Dk. What is the radius of curvature of the cornea?
F = 337.5/r (mm)
45.25 = 337.5/ r
r = 7.46 mm
31
How does a lens clock work?
NOTE: directly measures the SAG.
32
What are the parts of the slit lamp biomicroscope?
eyepiece = keplarian ts
a galilean Ts magnifies the image more.
Know the slit lamps magnify images.
33
Why does a higher powered fundus lens result in less magnification?
The funduss lens creates a keplarian (+ ocular and + objective lens) TS system.
with the lens being the objective lens and the cornea being the ocular lens.
to calculate the magnification it is similar to calculating the mag of a ts
Mag = - Foc/Fobj = - Fcornea/Flens
F cornea = +60 D
know that the higher the power of the lens, the increased FOV and lower mag
34
What is special about the 60 D funds lens.
This gives us 1x magnification ie.
- (+60)/(+60) = -1X
So when we measure the size of say the ONH on the slit lamp using this lens, we are measuring the true size of the ONH.
35
What are the markings on safety eye wear mean?
H + WURL V + Z87.1/2
36
what is the minimum coverage area for safety glasses?
40 X 33 mm
37
lens form
determines the relationship between the front and back surface of a lens.
plano - concave: one is flat/ other is concave
biconvex/biconcave
Concave - wraps around a lower n
convex wraps around a higher n
38
equiconcave/ equiconvex
half the total power is due to the front surface and half is due to the back surface
39
meniscus
convex front surface and concave back surface
40
What is the base curve on glasses?
the curve put on the front of glasses by the manufacturer. this is a spherical curve.
41
(+) cylinder form curves
42
(-) cylinder form curves
43
If a lens has a power of +3.00 -2.00 X 180 and is to be ground in (+) cylinder form, what front and back curves would be used if a base curve of +6.00 D were chosen?
44
If a lens power of -1.50 -1.00 X 090 and is to be ground in minus cylinder form, what front and back curves would be used if a base curve of +5.00 D were chosen?
45
When rotating the lens measure on the front surface of a crown glass lens, it is found that all meridians read +6.50 D. On the back surface, if the three contract points are horizontally aligned exactly along the 180- degree meridian, a maximum value of -7.50 D is found. When the three contact points are oriented in the vertical (090-degree) meridian, a minimum value of -6.00 is found. What is the nominal power of the lens? Is the lens made in (+) cylinder or (-) cylinder form?
46
What is the resulting sum of 2 cylinder lenses, both having a power of pl-2.00 X 180?
47
What is the sum of the following lenses?
pl -2.00 X 180
pl -2.00 X 090
48
A patient with a distance PD of 60 mm is fit with a frame with a PD of 68 mm. The lens effective diameter is 58 mm. What is the minimal blank size?
Decentration(d) = (Frame PD - Pt's PD)/ 2
MBS = ED + 2 (decentration) + 2mm
d = (68mm - 60 mm)/ 2 = 4 mm
MBS = 58 mm + 2(4mm) + 2mm = 68 mm
49
Where is the base curve on the reading segment?
on the front on the lens, why we place the front on the lens on the lensometer when checking segment power.
50
how do you calculate sag and relate it to radius?
sag = h^2/(2r)
h = half the chord length (m)
r in m
51
When should you specify the base curve in a lens?
when replacing the one in a lens pair
when ordering an identically powered second pair
52
add together the contact lenses with the filling RX:
+1.00 -0.50 X 085
+0.50 -0.50 X 100
53
add together the contact lenses with the filling RX:
Pl -2.00 X 180
-2.00 +2.00 X 135
54
add together the contact lenses with the filling RX:
-1.00 -2.00 X 020
-2.50 -3.00 X 080
55
T/F: the sum of the spherical equivalents if the two obliquely crossed spherocylinders will always equal the spherical equivalent of the resultant lens
true
56
T/F: if the axes of either two plus cylinders or two minus cylinders lenses are the same, then the resultant power will be the sum of the two cylinders
- answer: true because
pl -2.00 X 180
pl -2.00 x 180
——————-
pl -4.00 x 180
57
if the axes of two cylinders are very close to one another, what can be said about the power of the new cylinder?
answer: resultant magnitude = sum of the 2 cylinders
the sphere will stay basically the same
example:
pl -2.00 X 002
Pl -2.00 X 178
———————
-0.02 -3.96 X 180
58
if the axes of two equally powered cylinders are 090 degrees away from one another, what will be the resultant?
- answer: the cylinder power will be zero and the sphere power resulting from the two combined cylinder components
_will change by the full power of the cylinder
example:
pl -2.00 X 090
Pl -2.00 X 180
——————
-2.00 DS
59
T/F: if the powers of two obliquely crossed cylinders are equal, the axis of the new cylinder will be halfway In between the two.
- answer: true
example:
pl -2.00 X 030
Pl -2.00 x 070
———————
-0.47 -3.06 X 050
60
if the cylinder powers of two obliquely crossed cylinders are unequal, what happens to the axis of the cylinder?
- answer: the resulting cylinder axis will be pulled in the direction of the axis of the stronger cylinder
example:
pl -2.00 X 030
Pl -1.00 x 070
———————
-0.31 -2.39 x 042
61
if the axes of two equally powered Plano cylinders are very close to being 090 degrees away from one another, what will be the result in terms of sphere and cylinder powers?
the cylinder power will be close to zero, and the sphere power change resulting for, the two combined cylinder components will change by nearly the full power of the cylinder. The cylinder axis of the resultant cylinder will be halfway between the axes of the original cylinders.
example:
pl -2.00 X 088
Pl -2.00 X 002
———————
-1.86 -0.28 X 045
62
if the axes of two unequally powered cylinders are 090 degrees away from one another, what happens to the sphere and cylinder powers?
- answer: the new cylinder power will be the difference between the two cylinder powers, and the sphere power will increase by the amount of the smaller cylinder.
example:
pl -2.00 X 090
Pl -1.00 X 180
————————
-1.00 -1.00 X 090
63
if the axes of two unequally powered Plano cylinders are very close to being 090 degrees away from one another, what happens to the resulting sphere and cylinder powers?
answer: the cylinder power will be close to the difference between the two cylinder powers. The sphere power will increase by close to the amount of the smaller cylinder. (The axes will be close to the axis of the lens with the higher powered cylinder.)
example:
pl -2.00 x 088
Pl - 1.00 x 002
———————
-0.99 -1.02 X 084
64
A certain lens surface has a radius of curvature of 83.7 mm. The lens has a diameter of 50 mm. What is the sag of the front surface of the lens?
s = h^2/ 2r = (.050/2)^2/ 2 (.0837) = .0037m = 3.7 mm
65
Suppose a lens has a true base curve of +7.19 D. If the diameter of the lens is 52 mm, what is the sag of the front surface at the full 52 mm diameter?
F= n'-n/r 7.19 = 1.53-1/r
r = .0737 m
s = h^2/2r = (.052/2)^2/ 2 (.0737) = .00468 m = 4.68mm
66
What is the geometry of a biconvex, biconcave, and meniscus lens?
66
What is the geometry of a biconvex, biconcave, and meniscus lens?
67
How convert between lens power, radius, sad, edge thickness and center thickness?
Lens power -----(F = (n'-n)/r)-------> radius
radius ---------(s = h^2/(2r)--------> sag
sag ------> ET, CT, etc using geometrical relationships
68
A biconvex lens is clocked. The lens has the following properties: diameter = 44mm, sag of the front surface =4.5 mm, the sag of the back surface = 3.2 mm, n = 1.50. What is the power of each lens surface?
Biconvex = (+) lens
d = 44 mm -> h = 22 mm
s1 = +4.5mm
s2 = - 3.2 mm
n= 1.50
F1? F2?
S1 = h^2/2r +0.0045m = .022^2/2r
r = .0537 m
F1= n'-n/r = 1.5 -1/.0537 = 9.3 D
S2 = h^2/2r -0.0032m = .022^2/2r
r = -0.0756
F1 = n'-n/r = 1.0-1.5/-0.0756 = 6.61 D
69
What is GCD?
frame PD = A size + DBL
70
decentration Per lens equation
(Frame PD - Pt's PD)/2 = d
(+) d = decentered nasally
(-) d = decentered temporally
71
minimal blank size equation
MBS = ED + 2d =+ 2mm
d = decentration Per lens
2mm is to account for any chipping of the lens
72
What is MRP?
Where the pupil looks through
73
What is the value of the distance from the top of the segment to the near optical center for a Flat top 28 or less?
5 mm
74
What is the value of the distance from the top of the segment to the near optical center for a Flat top 35
4.5 mm
75
What is the value of the distance from the top of the segment to the near optical center for a Franklin seg
Franklin seg = executive seg -> 0 mmm
ie. near optical center is located at the bifocal line
76
What is the value of the distance from the top of the segment to the near optical center for a round seg?
also called kryptok
radius on the seg
77
What is the power of a trifocal?
1/2 the near add
78
What is seg drop?
the vertical distance between the MRP and the top of the seg
79
What is an inset? How do you calculate for it?
the horizontal distance between the geometrical center of the lens and the major reference point.
Frame PD - pt's PD /2
80
what is the seg inset? How do you calculate for it?
the horizontal distance between the MRP and the OC of the near add
distance PD - near PD/2
81
what is the total inset? how do you calculate for it?
The horizontal distance between the Geometrical center and the near add
frame PD - near PD/ 2
82
A Frame measures 54 X 18. The patient's distance PD is 66 mm and the near PD is 62 mm. What is the seg inset? What is the total seg inset?
A = 54
DBL = 18
PDdistance = 66 mm
PD near = 62 mm
seg inset = distance PD-near PD /2 = 66-62/2 = 2 mm --> positive so closer to the nose.
total seg = Frame PD - near PD/2 = ( (54+18) - 62 )/2 = 5 mm - > positive so closer to the nose.
83
How to fix a seg that seems too high?
What does the patient complain of?
distance vision is blurry
increase panto
decrease vortex distance
84
A certain lens surface has a radius of curvature of 83.7 mm. The lens has a diameter of 50mm. What is the sag if
the front surface of the lens?
r = 83.7 mm
d = 50 mm
s?
S = h^2/2r = (50/2)^2/ 2(83.7) = 3.73 mm
85
Suppose a lens has a true base curve (TBC) of +7.19 D. If the diameter of the lens is 52 mm, what is the sag of the
front surface at the full 52 mm diameter?
F = 7.19 D
d = 52 mm
s?
use n = 1.53
F = n'-n/r 1.53-1/r = 7.19 D
r = .074 m = 74 mm
s = h^2/2r = ((52/2)^2)/(2*74) = 4.57 mm
86
What is the lens thickness for a lens with the following dimensions?
Lens power = +3.00 D
Index of refraction = 1.53
Lens is Plano convex: +3.00 D on the front curve and a Plano back surface.
Lens diameter is 50 mm
Edge thickness is zero. The lens has no decentration.
CT = ET + S1
F = n'-n/r +3.00 = 1.53-1/r
r = .176 m = 176 mm
S = h^2/2r = (50/2)^2/ 2(176) = 1.77 mm
CT = 0 + 1.77 = 1.77mm
87
A -2.00 D lens has the following parameters:
F1 = +6.00 D
F2 = -8.00 D
n = 1.53
CT = 1.5 mm
It is edged to a horizontal oval shape, where
A = 50 mm
B = 30 mm
How thick are the lens edges both in the horizontal and vertical?
ET? vertical
ET = CT - S1 + S2 (get based on drawing)
F = n'-n/r +6.00 = 1.53-1/r
r, = .088m = 88mm
F = n'-n/r -8.00 = 1-1.53/r
r2 = .066m = 66 mm
s1 = h^2/ 2r = (30/2)^2/2(88) = 1.28 mm
s2 = (30/2)^2/ 2(66) = 1.7
ET vertical = 1.5 - 1.28 +1.7 = 1.92 mm
S2 h = h^2/2r = (50/2)^2/2(66) = 4.73 mm
s1 h= (50/2)^2/2(88) = 3.55 mm
ET horizontal = 1.5 -3.55 +4.73 = 2.68 mm
88
What to do with glasses if the seg is too high? What will the patient complain of?
complain of blurry vision at distance
increase pantoscoptic tilt
decrease the vertex distance
spread the nose pads
more the nosepads up by adjusting the pad arms
stretch the bridge
89
panto and seg height
for every 2 degrees of panto that you add, must raise seg height by 1 mm
90
What to do with glasses if the seg is too low? What will the patient complain of?
complain of near blur
narrow the pads
move the pads down by adjusting the pad arms
increase the vertex distance
reduce panto
shrink the bridge
91
glasses fall down nose
pull the temples in
bend down temple tips
pull in nose pads to tighten the fit
92
one lens feels closer to the face than the other
straighten the temples
93
right lens is higher than my left
raise the right temple
94
What does abbe value tell us
chromatic aberrations a patient may experience
the higher the abbe value the less the chromatic aberrations
95
crown glass n and abbe
n = 1.523
abbe = 58.9
96
CR-39 n and abbe
n = 1.5
abbe = 58
97
polycarbonate n and abbe
n = 1.586
n = 30
98
trivex n and abbe
n = 1.53
abbe = 43-45
99
Why do chromatic aberrations occur and that are the different types?
Caused by marginal rays (ie. rays at the edge of a lens) bending more. The lens in the periphery acts as a prism and splits the light into different wavelengths. Smaller wavelengths bend more.
Longitudinal/axial abberations: the idea that white light is spread and so the image will spread (occurs because different wavelengths are refracted differently)
lateral chromatic aberration: idea that as the image travels, the bigger it gets.
100
How do you prevent chromatic aberrations from occurring?
make sure the patient is looking through the optical center
101
Why do spherical aberrations occur?
same idea. as chromatic aberrations. that marginal rays focus closer to the lens vs paraxial rays.
So a point object is no longer forming a point image.
typically don't have to worry about this due to pupil size
102
Why does a coma aberration occur?
This occurs when the object you are looking at is located of axis (ie. causing you not to view it using the optical center).
As a result the rays that pass through the off axis location will cause a magnification difference and produce a blurred circle of the image. The more zones the light passes through the larger the blur.
The clearest image will be the one that is passing though the optical center/
103
Why does an oblique astigmatism aberration occur? What are other names for this?
marginal/radial astigmatism
Light is passing though as off axis point (ie. an oblique point) so it is no longer focusing on a single point.
This creates a symmetrically warped image with 3 different curves:
Teacup - tangential
Saucer - saggital
Plate - petzal
104
How do you avoid oblique astigmatism from occurring?
1. choose the right base curve
- on Tschering's elipise we want to pic the flatter base curves ie. the ozwalt curve.
2. tilt the lens so that light passes through it perpendicularly
105
what happens when you tilt a lens?
1. sphere increases
2. cylinder is induced
- (+) cylinder - induces (+) cyl
- (-) cylinder induces (-) cyl
3. axis = the meridian you are rotating around
106
Why does an curvature of field aberration occur? What are other names for this?
= power error
if you have a flat object projected onto a screen, you will get a curved image (ie. image with the center clear, but edges curved and blurry). The farther that you move the projector away from the screen the more out of focus the edges will be.
Our retina is also curved so it can help match things up.
107
How to avoid spherical aberration and coma in high powered lenses
use aspheric lenses
108
When is the off- axis light really a problem causing coma, spherical, and chromatic aberrations?
only in high powered lenses Ie. +10 or greater
109
How do you avoid curvature of field aberration from occurring?
choose the correct base curve
110
tscherning Ellipse
X- axis = back vertex power
y - axis = base curve
flatter curve = Oswalt
steeper cuve = Wollaston
111
What is distortion aberration? Types? when do we not get these?
the idea that magnification is unequal. around the lens.
(+) lenses - magnify things
- points that are father away from the optical center will be bigger
e. get a pincushion distortion
(-) lenses minify- points farther from the optical center will be more minified.
ie. get a barrel distortion
straight line objects form straight line images only if they are in the center of the lens
112
ROYGBIV
Longer wavelength to shorter wavelength
113
why can we use red/green balance?
due to chromatic aberration
114
what is the chromatic aberration equation?
CR = F/abbe
CR = chromatic aberration
F = power of the lens
115
What is achromatic doublet?
it is a way to minimize or eliminate chromatic aberrations
we take a (+) lens with a certain abbe number and combine it with a (-) lens of a certain abbe number
116
Equation for prism power based on how much a light bean is deviated
Y /X = Prism
Y = amount light is deviated from starting point in cm
X = how far away wall is in meters
117
Definition of what a prism does
bends light to the base and image at apex.
118
deviation angle equation relating to the apex angle
DAN-1 equation
ie. d = A(n-1)
d = deviation angle
A = apex angle
119
How many prism diopters is 1 degree of deviation?
1.75 PD
120
A patient views a letter on a screen 6 m away. A 4 PD BO prism is placed in front of the patient's left eye. How far does the prism laterally displace the image of the letter on the screen?
x = 6 m
prism = 4 PD
y= ?
prism = Y/X
4 = y/6
y = 24 cm to the right
121
equation relating power and prism thickness
prism = (100 g(n-1))/ l
g = thickness difference between the apex and the base
l = length of the prism
can use any units, just make sure they are the same for both
122
4. equations that do not use meters
1. MBS equation - uses mm
2. y/x = prism ; y-> cm, x -> m
3. 337.5/r = D ; r = mm
4. prism = dF, d -> cm
123
Find the prism at the center of the following lens: -6.00 D with n =1.523. Assume the lens has A = 50 mm, B = 30 mm, a nasal thickness of 4.0 mm and a temporal thickness of 6.0 mm .
n = 1.523
g = 6-4 = 2mm
l = 50 mm (using this because talking about nasal and temporal thickness, use the B size if referring to superior and inferior thickness)
Prism = 100g(n-1) / l
= 100(2)(1.523-1) / 50
= 2 PD BO
124
What is the effective power equation?
FB = FA / 1- d FA
125
What is the prentice rule equation?
prism = dF
d = decentration in cm
126
A prescription calls for a +4.00 D lens for the right eye, and it also calls for 3 BO. By how much should the lens be decentered?
F = +4.00
prism = 3 BO
prism = dF
3 = d (4)
d = 0.75 cm
(+) lens -> so move to the left/ nasal
127
Find the vertical prism induced in the right eye when a patient looks 10 mm below the optical center of the following lens:
OD: +2.00 -1.50 X 135
d = 10 mm = 1 cm
F - draw a cross and because they are exactly 090 degrees away the F value is the middle of the values of the cross ie. middle of +2.00 and +0.50. so +1.25
prism = dF =(1cm)(1.25) = 1.25 PD base up
128
What is the total prism that results from combining 2 PD BU and 1 PD BO? What about 2 BU and 1 BU?
129
What is vertical imbalance
the idea that each eye has a difference prism amount when looking away from the optical center.
130
how do we add prism between the 2 eyes?
Horizontal:
- both in same direction -> add
- opposite directions -> subtract
vertical:
- both in same directions -> subtract
- opposite directions -> add
131
what is yolked prism ? What are these used for?
both prisms are in the same direction
used to help with field enhancement ie. for patients with a stroke. Example bilateral left VF defect -> want base toward the defect so use BO OU
help with midline shifts
also help with nystagmus -> ie. so eyes stay in null point. If null point is to the right use BO OS
132
A patient has a prescription calling for OD: +5.00 D and OS: +3.00 D spherical lenses. How much vertical imbalance is induced where the patient looks down 10 mm to read?
OD:
d = 10 mm = 1 cm
F= +5.00
prism = dF = (1)(5) = 5 PD BU (because + Lens)
OS:
d = 1 cm
F = +3.00
prism = dF = (1)(3) = 3 PD BU
total prism -> since vertical and same subtract 5-3 = 2 PD BU OD or 2 PD BD OS
133
if a (+) lens is decentered down, the patient is looking through what direction of prism.
BD
134
What are the 6 ways to correct vertical imbalance and how do they work?
1. slab off : BUMM - BU more (-) in the 090 axis
idea is that the lens shape is changed so that the lower hand of the lens has more base BU.
2. dissimilar segs - using different seg heights/ shapes to correct the imbalance
3. compensating R seg - more cosmetically appealing dissimilar sed
4. multiple glasses - having a DVO and NVO glasses to remove the problem
5. cls - the optical center moves with the eyes
6. fresnel prism
135
what is reverse slab off?
BDMP
BD more (+) in 090 axis
136
when does a patient usually experience symptoms from vertical imbalance ?
if the imbalance is 3 PD or more
137
What is image jump? What do you need to know for this?
how does the image jump or more as you go right across the bifocal line?
1. add type (ex. FT -28 -> 5mm)
2. add power
all image jumps will be with BD PRISM with a (+) add power
138
Find the image jump in bifocals, given a prescription +2.00 -1.25 X 180, Add = 2.00 D. In this case the seg is a FT 28 seg. The seg drop is 4 mm
all we are doing is prentice's rule with image jump.
d for a FT 28 = 5mm = 0.5 cm
F -> add power -> 2.00 D
prism = dF = (.5)(2.00) = 1.00 BD
ALL IMAGE JUMPS WILL HAVE BD PRISM!!!
139
What is total prismatic effect?
the sum of the prism induced looking away from the seg OC (ie. the prism induced when looking away from the near OC) and the prism induced from looking away from the distance OC.
in 1 eye
140
What is the total vertical prismatic effect in the right eye when a patient looks through a bifocal at a reading level of 15 mm below the MRP?
OD: +1.50 -1.00 X 090
OS: +0.50 -0.75 X 180
add: +2.00
seg drop: 4 mm
seg type: FT 28
PD: 64/59 (distance/near)
Distance prentice rule:
d = 15 mm = 1.5 cm
F = +1.50
prism = dF = (1.5)(1.50) = 2.25 PD BU
near prentice rule:
d = what is the distance from the near OC to the reading level? know seg drop = 4 mm and with FT 28 know that 5mm from top to the OC
15 - 4-5 = 6 mm = 0.6 cm
F = 2.00
prism = dF = (0.6)(2) = 1.2 PD BD
total:
2.25 BU + 1.2 BD = 3.45 BU OD
141
A patient presents with a round 22 mm bifocal with a power of +2.00 D. What is the image jump?
d = radius of a round -> 11 mm = 1.1 cm
F = +2.00
prism = dF = (1.1)(+2.00) = 2.2 PD BD
142
what is spectacle magnification?
corrected image size/ uncorrected image size
143
what are the spectacle magnification equations we need?
1. SM = (shape factor) X (power factor)
2. shape factor = Ms = 1/ (1-(t/n)F)
3. power factor = Mf = 1/ (1-(hFv'))
t= thickness
F1 = the front surface power
Fv' = back vertex power
h = vertex distance + 3mm
everything in m
144
(+) lenses What increases spectacle magnification?
increasing t, increasing BC/F1, increasing h
145
(-) lenses What increases spectacle magnification?
increasing t
increasing BC
146
(+) lenses What decreases spectacle magnification?
increasing n
147
(-) lenses What decreases spectacle magnification?
increasing h
increasing n
148
Find the spectacle magnification for a patient who wears a +10.00 D lens (n = 1.5, 11mm thick) at 11 mm vertex distance. The front surface of the lens is +16.00 D
Shape factor = 1/1-(t/n)F1 =
1/ 1- (.011m/1.5)(+16.00D) = +1.13x
h = vertex distance + 3 mm = 11 +3 = 14mm
power factor = 1/1-hFv' = 1/1-(.014)(+10.00) = 1.16x
spectacle magnification = shape factor X power factor
SM = 1.13X1.16 = 1.31x
149
What is relative spectacle magnification?
RSM = corrected eye retinal image/ standard eye retinal image
150
knapps law
ametropia = different eye rx
if axial amertropia - too long or too short , correct with glasses (because glasses alter image size)
- axial myope - long eye - large image
- axial hyperope - short eye - small image
if refractive ametropes - best corrected in cls
- image size will be all the same
151
you have a 4 diopter uncorrected axial myope , will they have a larger or smaller retinal image size than an emmetrope?
larger
152
If you are a refractive hyperope/myope how will glasses affect the image size?
hyperope - (+) make image larger
myope - (-) make the image smaller.
153
which has a larger retinal image size? a spectacle corrected refractive hyperope or a spectacle corrected refractive myope?
refractive hyperope -> will make image larger with glasses
154
what is anisekonia?
the difference between the image sizes between the right and left eyes
155
what are the 3 types of anisekonia?
1. anatomical = due to a discrepancy in the density of the photoreceptors
2. induced - optics of the corrected eye
- ie. induced by wearing glasses
3. meridional- difference in the amount of astigmatism between the 2 eyes
156
how much anisekonia is there for every 1 D of power difference between the eyes?
1 percent
157
at what percentage will anisekonia become a problem?
3%
158
what are we worried about with anisometropia?
anisekonia - difference between image sizes between the right and left eyes
amblyopia
159
who is more likely to develop amblyopia? anisometropic hyperope or an aniometropic myope?
anisometropic hyperope
160
what do you want to prescribe for a patient with small differences in RSM between the eyes?
equal base curve and equal thickness lenses
161
what do you want to prescribe for a patient with large differences in RSM between the 2 eyes?
lens with high RSM - more magnified eye
- thin flat lens ie. low t and low F1
eye with lower RSM
- steeper and thicker lens
162
if a patient has a small amount of ametropia in one eye while the other eye has more than 4 D of ametropia. what do you rx?
bad eye is likely dye to axial
Rx glasses
163
if a patient has a small amount of ametropia in one eye while the other has less than 4.00 D of ametropia. what do you rx?
bad eye likely due to refractive
rx cls
164
what do you rx for large anisekonia due to astigmatism ?
usually anisekonia from astigmatism is due to cornea - rx cls
165
accommodation and anisometropia. Does hyperopia or myopia anisometropia cause amblyopia more and why?
accommodation occurs equally between both eyes.
- in hyperopia - the less hyperopic eye will always see more clearly at all distances,(because the eye will accommodate equally by the least hyperopic eye) the more hyperopic eye will be blurry at all distances.
- why more likely to get amblyopia early on.
- for myopia, a person will typically learn to use the less myopic eye for distance and the more myopic eye for near vision.
166
risk factors for aniometropic amblyopia
and isometropic amblyopia
anisometropic amblyopia
myope: -3 D
hyperope: +1.00 D
astigmatism: -1.50 D
isometropic amblyopia
myope: -8
hyperope: +5
astigmatism: -2.50
167
What normally happens as light passes through the center of a lens?
some percent will be reflected by the front of the lens
some percent of light will be absorbed by the middle of the lens
some percent of light will be reflected by the back of the lens
168
how do you calculate how much light ends up passing all the way through the lens?
a. Reflectance at the front surface of the lens = (n2-n1/n2+n1)^2
NOTE: if surrounded by the same media - reflectance and front and back surface = same
b. transmittance at each surface= 1- reflectance
c. light absorbed by the medium =
1- amount of light absorbed by the lens
d. total transmittance = T = (Ts1)(Ts2)(Tm)
Ts1 = transmittance at the front surface
Ts2 = transmittance at the back surface
Tm = light absorbed by the medium
169
A lens (n=1.5) 2.00 mm thick absorbs 30% of incident light, ignoring reflection. Find the Total amount of light transmitted through the lens?
A. light reflected at the front surface of the lens:
(n2-n1/n2+n)^2 = (1.5-1/1.5+1)^2 =0.04
AA. how much is transmitted by the front surface ?
1- R1 = 1-0.04 = 0.96 = Ts1
since both media same = Ts1 = Ts2
if 30% absorbed -> 70 percent transmitted
T= (Ts1)(Ts2)(Tm) = (.96)(.96)(.7) = 0.65 = 65%
170
What is the back vertex power of a cls equation?
When do you use this equation ?
only use if the thickness of a lens is given
t is in m
171
A cls has a back surface radius of 7.4 mm, a front surface radius of 7.8 mm, a center thickness of 0.16 mm, and an index of refraction of 1.45. What is the back vertex power of the cls?
F2?
r = 7.4 mm =.0074
n'-n/r =F = 1-1.45/.0074 = -60.81 D
F1
r = 7.8 mm = .0078m
n'-n/r = 1.45-1/.0078 = 57.69 D
t = 0.16 mm = .00016 m
n = 1.45
Fv1 = F2 + F1/1-(t/n)F1
Fv1 = -60.81 +
59.67/ 1- (.00016/1.45)59.67= -2.75 D
172
What happens to the effective power when we move from glasses to cls? How do we account for this in our cls rx?
becomes more minus the closer the lens is
CAPFAM
- add more (+) to cls RX
Note: only need to make a difference if > 4.00 D
173
What is the effective power equation?
Feff = F/ 1- dF
d = (+) when move toward the eye
d = (-) when move away from the eye
174
A patient has a spectacle prescription of a -7.00 DS OD at a vertex of 12 mm . What contact lens power is required?
Rule of thumb
4.00-> 0.25 D difference
6.00 -> 0.50 D difference
8.00 -> 0.75 D
10.00 -> 1.00
CAPFAM
-7.00 + 0.50 = -6.50 ish or you can calculate with the equation
175
What are the 3 components to the GP cls model?
1. Gas permeable CL
2. tear film = lacrimal lake = tear lens
3. cornea
176
What are the GP lens equations?
Tear layer power = BCR - K(diopters)
-> BCR = flattest K in both meridians
cls power = vertex (spherical) refraction - tear lens power
177
What type of tear lens power will a steep GP cls cause and what about a flat GP cls cause? What if the cls fit perfectly on the cornea, what is the tear lens power?
steep cls -> NaFl thicker in the middle -> induces (+) power
flat cls -> No NaFl in the middle -> induces (-) power
if perfect fit = 0
178
The following information is obtained for a patient:
corneal curvature (K readings): 7.5 mm or +45.00 DK
subjective refraction (at the cornea) = +1.00 DS OU
What power of GP cls should be chosen for an on- K fit?
179
Given:
corneal curvature = 7.5 mm (spherical)
subjective refraction = +1.00 DS
what power of GP cls is required if you chose a 8.0mm as the new cls base curve?
180
How do change the power of the cls when the base curve of the GP lens is changed.
Whenever the GP lens base curve is changed, the power of the lacrimal lake will change. So the GP lens must be modified to compensate for the change in the lacrimal lake power.
SAM FAP
- if you steepen the BC -> the lacrimal lake is more (+) -> add minus power to the GP Cl power
- if you flatten the BC -> the lacrimal lake is more (-) -> must add plus power to compensate for the GP CL power
181
How much will the GP cls power change for every 0.1 mm change in the base curve?
0.1mm change in the base curve will cause a 0.50 D change in the GP cls power and the Lacrimal lake power
182
A patient is fit with a GP contact lens with a base curve of 8.00 mm and Fv = -3.00 D. If the base curve is steepened by 0.1mm to improve the fit, what is the new power of the cls?
For every 1 mm of steepening, the power changes by 0.50 D
SAMFAP
-> add minus because steeper
-3.00+-0.50 = -3.50 D
183
What is residual astigmatism and how do you calculate it? Why is this important?
residual astigmatism = lenticular astigmatism
RA = RXcyl - Kcyl
Amount of RA that is tolerated = 0.75D
- so a patient can be put in a sphere lens if less than or equal to 0.75
- if > 0.75 -> then need to put in a front surface topic lens
184
A patient presents with the following information:
K readings: 43:00 D @ 180/45.00 D @ 090
subjective refraction = pl -2.00 X 180
the patient is fit on K with a GP cls. What is the amount of the residual astigmatism?
185
Javal's rule
refractive astigmatism = 1.25(measured corneal astigmatism + (-0.50 X 090)
186
Consider a patient with the following keratometry findings:
K readings: 43.00 @ 180/ 46.00 @ 090
How much total refractive astigmatism would you predict for this patient?
43-46 = -3.00 X 180
1.25(-3.00 X 180) + -0.50 X 090
1.25(-3.00 X 180) + (+ 0.50 X 180)
-3.75 X 180 + 0.50 X 180
-3.25 X 180
187
Given the information below, what is the anticipated over-refraction for this patient?
K readings: 43.00 DK
trial GP cls BC = 7.94 mm
Trial GP cls power = -3.00 D
corneal refraction = -5.00 DS
188
GP cls parameters
OZD = area of usable optics
peripheral curve - prevents the edge of the cls from bearing, promotes tear exchange, and cls contraption
- edge lift:
too much decreases cls centration, increased awareness of cls -> can get 3 and 9 staining (cls moves too much)
too little edge lift - > not get enough pooling of NaFL, greater change of debris stuck underneath cls, poor movement, poor tear exchange
- center thickness:
thinner lens -> better centration, better oxygen to the eye
center of gravity:
for a cls to have better centration, we want the center of gravity to be more posterior.
189
How is Sag, OZD related and BC related ?
If we increase the OZD, we increase the SAG -> ie. lens is steeper
when this happens we need to flatten the BC to compensate
for every 0.4 mm change in the OZD, the BC should be adjusted by 0.25 D in order to maintain the same fitting relationship
" think 4 quarters per dollar"
190
What kind of change would be needed to be made in center thickness to make a significant amount of change in the fitting relationship of the cls?
0.03mm steps
191
A patient presents with K readings of 42.00 D @ 180/44.00 D @ 090. What is the total refractive astigmatism predicted for this patient?
192
steep lens fit
pooling = clearance
- can lead to SEAL OFF = lack of tear exchange
193
flat lens fit
194
spherical fit over WTF astigmatism
WTR astigmatism: horizontal = flatter so bearing
vertical meridian - pooling
195
spherical fit over ATR astigmatism
note dark areas = area of touch
ATR astigmatism: vertical is flatter - so bearing here
pooling horizontally
196
How do you decide what GP lens to fit your patient into?
197
What is an alignment fit?
even distribution of NAFL
198
when do we fit a biotic lens?
increased corneal astigmatism
- back surface of the topic lens will match the corneal astigmatism and correct it
increased residual astigmatism
- front surface of the cls will match the internal/residual astigmatism
199
when do you fit a back surface toric? why are these hard to fit?
increased corneal astigmatism
- toric back surface
Rxcyl = 1.5(Kcyl)
- because index of gp cls (n=1.5) and index of tears (1.33) = diff, so the black surface of a back toric lens will overcorrect the corneal astigmatism due to index diff.
if you fit someone with back toric lens that doesn't follow the second rule -> will lead them to overcorrect the astigmatism leading to induce astigmatism and as a result will not be able to see through the lens clearly.
hard to fit because hard to find patient with these parameters
200
when do you rx a front surface toric? why are these hard to fit?
decreased corneal astigmatism
- because using a spherical back surface on the cls and use the tear lens correct the astigmatism
- residual/ lenticular astigmatism - placed on the front surface of the cls
why hard to fit? - rotation- > rotating will make vision worse.
201
what are aspheric cls? pros? cons?
these flatten at the periphery of the lens mimicking flattening of the cornea, but does not affect the overall prescription.
benefits - increased alignment, increased comfort, better centration and less aberrations
cons - lower risk of tear exchange and potential of seal off
202
types of multifocal gp cls
simultaneous design
translating design
203
what is a simultaneous multifocal gp cls design?
both the distance and near vision simultaneously go through the pupil
need perfect centration for this
204
what is a translating design?
think like FT 28 bifocal
will see a segmented line on the cls. when patient looks down to read, lens will move up and patient will look through the bifocal part
205
flexure vs warpage
flexure - occurs only one the eye.idea that GP cls alters the shape of the cornea when on the eye. take cls off the eye and the cls goes back to its original shape
warpage - occurs on and off the eye.
- main cause due to excessive digital cleaning
206
what causes flexure to increase?
ie. there the lens is higher on the eye
- thinner CT
- increased OZD -> increased sag -> lens steeper_> sits higher
- steeper BC
- higher K astigmatism - ie. greater than equal to 1.50 D
- higher DK
207
What is the gas permeability of GP cls equation?
P = Dk (permeability - how much o2 gets through)
T = P/t = Dk/t
P = permeability
t = thickness of the cls material in cm
208
What are gas permeability trends?
as thickness increases oxygen to the eye decreases and vice versa
209
soft cls rules?
no lacrimal lens - so no SAM FAP
LARS
210
when can you fit a patient with a spherical cls?
<1.00 WTR astig
<0.75 ATR astig / oblique astig
211
what is prism ballasting?
base down prism incorporated into the cls
this weights the cls down and keeps it from rotating
212
what is periballasting?
base down incorporated into the cls outside the optical zone
213
what is dynamic stabilization?
thinning the superior and inferior portions of the cls - helps center the cls lens due to the eyelid pressure and keeps it from rotating
214
what is eccentric lenticulation?
combination of dynamic stabilization and periballasting
215
what is truncation?
cut off the lower half of the cls so its sits on the lower lid
NOTE: mostly seen in GP lenses
216
LARS
LARS has to do with toric soft lens rotation from 6'clock position of the lens
Left add, right subtract
(always from the Dr's perspective)
1. CW = Left rotation , modify based on pt's axis NOT trial lens
1. CCW = right rotation
217
you fit a soft toric cls OD with the Rx +1.00 DS -1.00 DC X 150. On the slit lamp examination you note that cls rotates 10 degrees ccw with the blink, What new axis should be ordered to compensate for this rotation?
LARS
CCW = rt -> subtract
150-10 = 140
218
hydrogel vs sihy cls
hydrogel:
- more water and more oxygen to the eye
- less water and less oxygen to the eye
sihy:
more water, less oxygen to the eye
vice versa
219
classification of soft cls
group number:
1 - non-ionic, low water content
2. non-ionic and high water contact
3. Ionic and low water content
4. Ionic and high water content (more likely go get deposits on lenses)
5. sihy
220
fitting guide for soft cls
diameter: HVID + 2 mm
221
rigidity of a soft cls
higher modulus - > increase stiffness
higher thickness -> increase stiffness
higher h20 -> decrease stiffness
222
accommodation through cls for hyperopes and myopes
hyperope: accommodate less with cls vs glasses
myope: accommodate more with cls vs glasses
- for patients who experience symptoms with cls vs glasses, use a MF cls or use low powered reading glasses over cls
223
magnification though cls and for hyperopes vs myopes
hyperope: they have a smaller retinal Image size with cls due to less SM
myope: have a larger retinal image see with cls compared to glasses
224
vergence though cls and for hyperopes vs myopes
hyperope: converge less with cls compared to spectacle lenses due to less induced BO prism
myope: converge more with cls compared to spectacles fur to less induced BI vergence.
225
How should the power of a soft cls be modified when the Base curve is steepened?
No change in power is needed
226
A patient with a spectacle prescription of +1.25-1.25x180 is fit with a soft contact lens prescription of +1.25-1.00x180. The contact lens rotates 5 degrees clockwise. What new axis should be ordered to compensate for this rotation?
5
227
The incorporation of base down prism at the bottom of a soft contact lens and outside the optic zone is termed:
periballasting
228
4 types of low vision magnification
-lateral magnification (L/L')
- relative distance mag (original object to eye distance)/ new)
closer you bring something , mag increases to your eye
think DON
- relative size mag (new/original)
material at same distance and changing letter size ie. making it larger
think SNO
- angular mag (think lens (optical system) being used to create magnification ie. hand held, stand magnifier)
229
hand held magnifier - what does it do?
how do you look though it?/ use it?
how do you calculate total mag?
how do you calculate the HHM field of view
collimating magnifier. ie. parallel light leaves the magnifier.
think primary focal point - object at the primary focal point with diverge light hit the lens and leave the lens as parallel light
hold object at focal point of (+) lens
no accommodation or add is needed since parallel light is leaving, doesn't matter how far you are from the magnifier either (but FOV increases the closer it is to the eye)
total magnification of a HHM = |d|F
d = distance of object to the eye in m before you started using the magnifier
F = power of the lens
if d not given - usually 0.25 cm -> this is how get store labeled magnification = effective magnification
w=d/(Fel)
w = field of view
d = diameter of lens in m
Fe = equivalent power of the lens
I = distance between magnifier to eye
230
stand magnifiers
what is it?
what equations do we use?
what is the max mag you can get with a stand magnifier?
What is the normal mag of a stand magnifier?
The total magnification provided by a stand magnifier is due to which of the following 2 types of magnification?
non-collimating magnifiers = non-parallel light leaves magnifiers
- light diverges as it leaves the lens because holding object inside the primary focal point of the lens
- creates an upright, magnified, virtual image
for use to see the image clearly need to accommodate or use add power
Feq = ER * F2
F2 = add, accommodation or uncorrected RE
ER = L'-F1/F2
F1 = power of the stand magnifier
l' = 1/L' = virtual image location
c = f2 - l' = distance between the eye and the stand magnifier.
What is the max mag you can get with a stand magnifier?
Max = Feq/4 +1
What is the normal mag of a stand magnifier? Feq/4
The total magnification provided by a stand magnifier is due to which of the following 2 types of magnification?
- relative distance mag and lateral mag
231
A patient uses a hand held magnifier with a +15.00 D lens to view an object 50 cm away. What is the total magnification? How would this magnifier be labeled in the store?
total mag for HHM = |d|F = (0.5m)(+15) = 7.5x
store mag = (0.25)F = 0.25*10 = 2.5 x
232
A 70 year old patient uses a 40 mm HHM with a power of +10.00 D to view an object 40 cm from the eye. The hand held magnifier is held 30 cm from the eye so that the object is located at the primary focal point of the lens. What is the field of view of the magnifier?
diameter of lens = d = 40mm
Fe = +10.00 D
distance object to eye w/o magnifier = 40 cm
l = distance of magnifier to eye = 30 cm
FOV = w = d/Fel = .040m/ (10*.30m) =13 mm
233
You calculated that you need a power of +10 D and have a stand magnifier with a magnification of 5x. What is the add power you need?
Feq = +10
ER = 5x
F2?
Feq = ER* F2
10 = 5*x
F2 = +2 - ie. need an add, accommodation, or uncorrected myopia of +2 D
234
How much magnification is provided by a +6.00 D hand held magnifier held in front of an object located 40 cm from the eye?
|d|F = mag HHM = .40m* 6 = 2.4x
235
What is the big picture of telescopes?
we want parallel light entering and leaving a TS
236
What are the Telescope equations?
tube length of a TS = d = fobj+foc
- focal length of the objective lens + focal length of the ocular lens
Magnification
- Foc/Fob = M
M = entrance pupil diameter/ exit pupil diameter
" think dent over dext"
237
What does A X B on a ts stand for?
A = magnification
B = diameter of the entrance pupil in mm/ objective lens
238
What is the size of the exit pupil for a 5 x 45 Ts?
5 = Mag
45 = entrance pupil diameter
M = entrance/exit
5 = 45/exit diameter
exit diameter = 9 mm
239
what is the entrance pupil?
image of the aperture stop formed by all the lenses in front of it.
NOTE no lenses in front of the objective lens. So entrance pupil is the
objective lens
240
what is the exit lens?
image of the aperture stop formed by the lenses behind it. The lenses behind the objective lens = ocular lens.
image of the objective lens as seen through the ocular lens
241
What is the aperature stop in a TS?
objective lens = limits the amount of light entering the system
242
Keplarian Ts
(+) objective and (+) ocular lens
- light crosses inside the TS
- exit pupil is behind the ocular lens in free space outside the TS
- final image = inverted and magnified image - because light crosses inside the TS
- longer tube length
- greater weight
- larger FOV because the exit pupil is outside the TS
- images = dim - because small exit pupil
243
Galilean Ts
(+) objective lens (-) ocular
- upright magnified image
- light weight TS
- smaller tube length
exit pupil = inside the TS
- small FOV because exit pupil inside TS
- exit pupil = larger so brighter image
* larger and brighter image
244
when do you get the biggest FOV in a telescope?
when you place your eye at the exit pupil.
- in keplarian - move eye away from the ocular lens
245
A telescope has a +20.00 D objective lens and a -50.00 D ocular lens. What its he magnification and type of TS? What is the tube length of the TS?
Galilean TS
M = -Foc/Fob = -(-50)/20 = 2.5 x
d= fob + foc = (1/20)+(1/-50) = 0.03 m = 3 cm
246
A patient is using a 2.5x Galilean TS to view an object 25 cm away. How far apart should the lenses be set in order for the patient to view the object clearly though the TS?
Fob = +20 Foc = -50
L = n/l=1/-0.25 = -4 D
L + F = L' = -4+ 20 = +16
n'/L' = l' = 1/15 = 0.0625 m
L+F = L' -> 0 = -50 + L
L = -50
n/L = l = 1/-50 = -0.02
0.0625m + -0.02m = 0.0425m = 4.25cm
247
What is a tele-microscope?
Magnification for near tasks like cooking etc with a comfortable working distance
248
to look at an object closer than infinity, How do we adjust a telescope?
we adjust the tube length
reading cap in front = (+) lens that is like a HHM
HHM = parallel light leaving magnifier
- so don't need to accommodate
wear an add power
249
how do you calculate the magnification of a TS?
mag of reading cap?
magnification of reading cap * mag of the telescope
HHM equation - > |d|F
- if d not given, but given standard distance = 0.25 m
250
A patient is using a 4x telescope with a +3.00 D reading cap. What is the total magnification of this telemicroscope? What reading distance will allow the patient to view the object clearly though the telemicroscope?
Mag TS = 4x
reading cap +3.00
d?
reading cap mag = |d|F
d not given use 0.25 m
0.25 m * 3 = 0.75x
0.75x*4x = total mag of the telemicroscope
need reading distance to be 1/3 = .33 cm
251
types of spectacle mounted TS?
center fit - right in the center of their vision - ie. right in sight of light - this is if patient is stationary
bioptic fit - better for mobile - use it. for spotting. ie. driving
252
reverse telescope
- what does it do?
- who uses it?
minify image
expanding patients VF
- use RP, advanced glaucoma
253
patient with some type of VF defect
mirrors - located in nasal portion to view things temporally
prism - shift vision away from the defect to area where have better vision ( base toward the defect)
254
how much PD shifts the visual field
1.75/2 D shifts the VF by 1 degree
255
central vision loss
will not have trouble with mobility
256
peripheral vision loss
mobility issues
257
moderate low vision
- what to do to improve vision?
- how to improve reading vision, how much do you give?
20/60-20/70
- bring this closer - to increase RDM
give prism to this patients to help with convergence (do don't have to converge as well)- ie. give BI prism
how much prism of we give per eye?
2+D = 2 + spherical power of the lens
258
20/200-20/400
severe va loss
monocular viewing reading material
259
legal blindness
can't read any letter on 20/100
vf 20 degrees of less
260
M notation
-what is it
-conversion from mm, and smelling
1 M = size at 1 m of 1 arc min of detail, or 5 arc min of size
1 M = 20/20
1M = 1.45 mm in height
conversion
test distance in meters /M = 20 ft/ snellen
