Clinical optics review

8/14/2019 Clinical optics review

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OpticsBoard Review



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)



Physical Optics



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



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



Electromagnetic Spectrum



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



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



Antireflective Coating



Polarization

Plane-polarized

(linearly polarized) light:

waves that all have the

electric field in thesame plane Polarized sunglasses

Stereopsis testing

Haidinger brushphenomenon



Diffraction

Bending of light rays when they encounter an

obstruction

Diffraction limits visual acuity when the pupilsize is less than 2.5mm



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



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



Lasers

Light Amplification by Stimulated Emission of

Radiation

Which of the following features of laser lightenhance its intensity? Directionality

Coherence

Polarization

Monochromaticity



Laser-Tissue Interaction

Name 3 ways lasers damage tissue: Photocoagulation (Argon)

Photodisruption (Nd:YAG)

Photoablation (Excimer)



Geometrical Optics



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



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?



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.



Total Internal Reflection



Vergence

A measure of the

spreading (or coming

together) of a bundle of

light rays.



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



Vergence

Plus lenses are

biconvex and add

+ vergence

Minus lenses are

biconcave and add- vergence



Thick Lenses

6 “cardinal points” 2 principal points / planes (H and H’)

2 nodal points (n and n’)

2 focal points (F and F’)



Focal points

Primary (Anterior) focal point

real object virtual object



Focal points

Secondary (Posterior) focal point

real image virtual image



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



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



Real or virtual?

light



Upright or Inverted?



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



Vergence

D) 14 cm to the left of the lens

100/-7 = -14cm



The intermediate image formed by the concave lens is

A) Real , inverted

B) Virtual, upright

C) Real, upright

D) Virtual, inverted



B) virtual, upright



Schematic Eye



Reduced Schematic Eye

F F’n

H

5.5 mm

17 mm 22.5 mm

17 mm

power = +60 D



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



Mirrors

Central ray passes through center of curvature (C)

not through center of mirror.

real, inverted

virtual, upright



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)



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



+6 -4

50 cm 12.5 cm

Where is the intermediate

image?



+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



+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



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



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)



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



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



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



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



Prisms: True or False?

Glass prisms are

calibrated while held in

the angle of minimumdeviation.

False, it’s Prentice position



Prisms

Real images created by prisms are deviated toward

the prism base.



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



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?



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



Fresnel Prisms

Fresnel prisms are equivalent to side-by-side

strips of long, narrow, thin prisms.

fresnel prism



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.



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



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



(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.



Astigmatism

Each focal line is formed by the power of the lens

acting 90° away from the focal line.



Conoid of Sturm

Spherical equivalent = sphere + ½ cylinder



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 + +



Maddox Rod



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



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



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



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



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)

Clinical optics review

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