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8/14/2019 Clinical optics review
http://slidepdf.com/reader/full/clinical-optics-review 1/69
OpticsBoard Review
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Optics
Light behaves like wave and particle
Physical optics – wave properties of light
Geometrical optics – light as raysQuantum optics – interaction of light and
matter (wave and particle characteristics)
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Physical Optics
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Physical Optics
Wavelength: distance between crests
Amplitude: height of wave crest / maximum value attained
by electric field
Frequency: number of wave crests passing a fixed point
per second
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Photon Energy
Wavelength x Frequency (λ x ν) = c
λ is inversely proportional to v
Energy per photon (E) = h v Wavelength: blue < red
Frequency: blue > red
Energy: blue > red
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Electromagnetic Spectrum
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Interference
Constructive interference: crests of two waves coincide
Destructive interference: crest of one wave coincides with trough of other
wave
Coherence: measure of the ability for two light waves to
interfere
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Interference: Applications
Laser inferometry: Evaluates retinal function in pt w/ cataract
Laser beam split into 2 beams
Beams overlap on retina, producing interference
fringes, thus you know retina is functioning
Antireflective coatings
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Antireflective Coating
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Polarization
Plane-polarized
(linearly polarized) light:
waves that all have the
electric field in thesame plane Polarized sunglasses
Stereopsis testing
Haidinger brushphenomenon
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Diffraction
Bending of light rays when they encounter an
obstruction
Diffraction limits visual acuity when the pupilsize is less than 2.5mm
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Diffraction
What is the optimal pinhole aperture?
1.2 mm
Any smaller would greatly increase diffractionand limit the amount of light into the eye
Because of diffractive effects, pinhole vision
is rarely better than 20/25
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Scattering
Isolated molecules absorb light and re-radiateit at same wavelength but different direction
Causes glare (cataracts, AC flare, corneal
haze)
Rayleigh Scattering
Due to scattering of very small particles Sky appears blue because of greater scattering of
shorter wavelengths
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Lasers
Light Amplification by Stimulated Emission of
Radiation
Which of the following features of laser lightenhance its intensity? Directionality
Coherence
Polarization
Monochromaticity
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Laser-Tissue Interaction
Name 3 ways lasers damage tissue: Photocoagulation (Argon)
Photodisruption (Nd:YAG)
Photoablation (Excimer)
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Geometrical Optics
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Geometrical Optics
Refractive Index:
n = speed of light in vacuum
speed of light in material
n is always > 1
Snell’s law of refraction:
n1 sin θ1 = n2 sin θ2
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Refraction
A fisherman attempts to
spear a fish as shown
at right.
Should he aim directly
at the fish, in front of
the fish, or behind the
fish as he sees it?
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Refraction
He should aim in front of thefish.
When a light ray passesfrom a medium with a
higher refractive index to amedium with a lower refractive index, it is bentaway from the normal.
When passing from a lower refractive index to a higher refractive index, light is benttoward the normal.
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Total Internal Reflection
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Vergence
A measure of the
spreading (or coming
together) of a bundle of
light rays.
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Vergence
The reciprocal of the distance, in meters,
from the object point or to the image point.
Units = m-1= diopters (D)
Lenses add vergence to light
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Vergence
Plus lenses are
biconvex and add
+ vergence
Minus lenses are
biconcave and add- vergence
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Thick Lenses
6 “cardinal points” 2 principal points / planes (H and H’)
2 nodal points (n and n’)
2 focal points (F and F’)
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Focal points
Primary (Anterior) focal point
real object virtual object
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Focal points
Secondary (Posterior) focal point
real image virtual image
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Focal Length Distance from lens to each of its focal points. Focal length in meters:
F = n/D
F = 1/D in air
Primary focal length of eye
F = 1/60 = 0.017 m = 17mm
Secondary focal length of eyeF’ = 1.33/60 = 0.0222m = 22mm
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Vergence Formula
U + D = Vvergence of
light entering
the lens
Amount of vergence
added to the light by
the lens (power of
the lens)
vergence of
light leaving the
lens
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Real or virtual?
light
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Upright or Inverted?
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Vergence
An object is located 20 cm to the left of a
-2.00 D lens. Where is the image located?
A) 20 cm to the right of the lens
B) 50 cm to the right of the lens
C) 33 cm to the left of the lens
D) 14 cm to the left of the lens
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Vergence
D) 14 cm to the left of the lens
100/-7 = -14cm
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The intermediate image formed by the concave lens is
A) Real , inverted
B) Virtual, upright
C) Real, upright
D) Virtual, inverted
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B) virtual, upright
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Schematic Eye
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Reduced Schematic Eye
F F’n
H
5.5 mm
17 mm 22.5 mm
17 mm
power = +60 D
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Mirrors
Angle of incidence
= angle of reflection
Convex mirrors add minus vergenceConcave mirrors add plus vergence
Plane mirrors add zero vergence
Image space is reversed: image rays are on sameside as object rays
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Mirrors
Central ray passes through center of curvature (C)
not through center of mirror.
real, inverted
virtual, upright
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Mirrors
U + D = V
F = r / 2
(r=radius of curvature)
Reflecting power
D = 1 / F = 2 / r
What is the reflecting power of cornea? 2/.008 = 250D (-250D)
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Magnification
Transverse Magnification
= Image height / Object height
= Image distance / Object distance
= Object vergence / Image vergence
Magtrans= U / V
For lens combinations the total magnificationis the product of the individual magnifications.
Wh i th i t di t
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+6 -4
50 cm 12.5 cm
Where is the intermediate
image?
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+6 -4
50 cm 12.5 cm
-2 +4
•Is the object virtual or real? Inverted or erect?
•What is the magnification?
M = U/V = -2/+4 = -0.5
12.5 cm
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+6 -4
50 cm 12.5 cm 12.5 cm
-2 +4 +8 +4
Mag = U/V = -2/+4 * +8/+4 = -1
Where is the final image?
12.5 cm
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Simple Magnifiers
The (angular) magnification of a simple plus lens is
defined as the ratio of the size of the image produced by
the lens to the size of the object viewed at 25 cm
Magsimplemagnifier = D / 4
Examples:
+ 8D lens is called a 2x magnifier
+20D lens is a 5x magnifier
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Direct ophthalmoscope
What is the angular magnification of a retinal imageusing direct ophthalmoscopy in an emmetrope?
Mag = 60D / 4 = 15x
(the pts retina appears 15x larger than if it were cutout of the eye and held at 25 cm)
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Telescopes
Receives parallel rays from a distant object
and projects parallel rays out.
(i.e. an afocal system)
2 lenses : objective + eyepiece
Transverse magnification is same for every
object regardless of location.
Magtelescope = Deyepiece / Dobjective
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Keplerian Telescope
Objective: low-power plus lens
Eyepiece: high-power plus lens
Separation: sum of focal lengths
Image: inverted, all light from objective is collected
Astronomical telescope
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Gallilean Telescope
Objective: low-power plus lens
Eyepiece: high-power minus lens
Separation: difference between focal lengths
Image: upright, some light collected from objective is lost
Surgical loupes
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Prisms: True or False?
The power in prism diopters is the number of
centimeters that light is displaced perpendicularly for
every centimeter that the light travels.
False, it’s for every 100 cm
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Prisms: True or False?
Glass prisms are
calibrated while held in
the angle of minimumdeviation.
False, it’s Prentice position
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Prisms
Real images created by prisms are deviated toward
the prism base.
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Prentice’s Rule
Except at its optical center, a spherical lens
has prism at every point on it’s surface.
Δ = h x D
Δ = prism diopters
h = distance from optical center in cm
D = diopter power of the lens
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Prentice’s Rule
If a patient with no
ocular misalignment
reads 1 cm below the
optical centers of hissingle vision glasses,
with the different lens
powers as shown, what
prismatic effect isinduced?
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Prentice’s Rule
The powers of the
lenses acting in the
vertical meridians are
used
Total prismatic effect in
the reading position =
4Δ of vertical prism Will induce a left
hypertropia
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Fresnel Prisms
Fresnel prisms are equivalent to side-by-side
strips of long, narrow, thin prisms.
fresnel prism
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Fresnel Prisms
Used to avoid the weight of conventional
prisms.
Plastic Fresnel prisms are available as Press-
On prisms from 0.5Δ to 40Δ.
Visual acuity suffers by one or two lines with
higher power prisms because of glare and
chromatic aberration.
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Bifocal Segments
Image Jump – prismatic power at top of bifocalsegment Executive has no image jump
Image Displacement – total prism in readingposition
What type of add minimizes imagedisplacement with: Plus lenses? Round top
Minus lenses? Flat top
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Question
A patient with congenital nystagmus has a
null point measured to be 10° to the left of
fixation. The appropriate prism prescription
to rectify the induced head turn isa. 10∆ BI OS, 10∆ BO OD
b. 10∆ BI OD, 10∆ BO OS
c. 20∆ BI OS, 20∆ BI ODd. 20∆ BI OD, 20∆ BO OS
e. 20∆ BI OS, 20∆ BO OD
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(Regular) Astigmatism
Curvature of an astigmatic lens has minimum
and maximum values, located in meridians
90° apart.
An astigmatic surface cannot bring light rays
to a point (stigma) of focus.
Instead two focal lines are formed.
Geometric figure is formed called Conoid of Sturm.
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Astigmatism
Each focal line is formed by the power of the lens
acting 90° away from the focal line.
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Conoid of Sturm
Spherical equivalent = sphere + ½ cylinder
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Type of Astigmatism
Location Sphere Sphere + Cyl
CompoundMyope
Vitreous - -
Simple
Myope
One vitreous
One retina
0/- -/0
Mixed Straddle Retina +/- -/+
Simple Hyperope One retinaBehind retina
+/0 0/+
CompoundHyperope
Behind retina + +
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Maddox Rod
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Accomodation
The accomodative amplitude of a 60 yr. old
healthy person is approximately:
1.50 D
Accomodative amplitude:
age 40 = 6.0 44 = 4.5 48 = 3.0
>age 48 decreases by 0.50 every 4 yrs
<age 40 increases by 1.0 every 4 yrs
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Kestenbaum’s Rule
A 72 yr. old patient with bilateral macular
degeneration has a distance acuity of 20/100. The
add required for this patient to read newspaper print
is:
A) +1.00
B) +3.00
C) +4.00
D) +5.00E) +10.00
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Contact Lenses
Obtain Refraction & K'sChoose base curve steeper than low K Usu +0.50 D steeper to form tear lens
Prevents apical touchConvert refraction to Minus cylinder formDisregard the cylinder
Convert to zero vertex distanceSubtract +0.50 spherical tear lens from the
sphere value to obtain the final RGP sphere
Accounting for the tear lens in
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Steeper add minus SAM
Flatter add plus FAP
Power of the “tear lens” is 0.25 D for every 0.05 mm
radius of curvature difference between contact lens
and cornea
Accounting for the tear lens in
RGPs
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The refractive error of an eye is -3.00 D, the
K measurement is 7.80 mm and the base
curve chosen for the rigid contact lens is 7.95
mm. What is the anticipated power of thecontact lens?
Power of tear lens: 7.95-7.80 = 0.15 mm = 0.75 D CL power: -3.00 D + 0.75 D = -2.25 D (FAP)