Concepts Flashcards
IMAGE DISPLACEMENT
Image displacement — produced by the total prismatic power acting in the reading position (the total prismatic power of the lens plus the bifocal segment).
Critical angle
Only occurs when light passes from a higher index to a lower index medium.
Light ray passing through mediums (lower/higher) index of refraction to (higher/lower) index of refraction
When a light ray passes from a medium with a lower refractive index (n) to a medium with a higher refractive index (n1), it is bent toward the normal.
When passing from a higher refractive index (n1) to a lower refractive index (n), it is bent away from the normal.
Image jump
Image jump — produced by sudden introduction of prismatic power at the top of the bifocal segment. The object which the eye sees in the inferior field when looking straight ahead suddenly jumps upward when the eye turns down to look at it
If the optical center of the segment is at the top of the segment, there is no image jump:
Diopters
diopters — the reciprocal of the distance, in meters, to the point where the rays would intersect if extended in either direction.
Chromatic aberration
Simple plus lenses likewise bend blue rays more than red rays, leading to the optical aberration known as chromatic aberration. The blue rays come to focus closer to the lens than the red rays.
Light with shorter wavelengths are SLOWED UP more in media (compared to longer wavelengths)
Principle behind duochrome test.
Which color light is bent more due to refraction?
Blue = bent more due to refraction.
Longer wavelength, lower energy, refracted lens.
Duochrome test
RAM-GAP
Red side better, refraction = too hyperopic (ADD MINUS)
Green side better, refraction = too myopic (ADD PLUS)
Chromatic aberration occurs strongly in the human eye, with almost 3.00 D difference in the focus of the far ends of the visible spectrum (1.50 D is usually stated in textbooks).
basis of duochrome test (aka bichrome test).
Sphere is adjusted until the black letters on the red and green halves of the test chart are equally clear, indicating that the red rays are focused as far behind the retina as the green rays are focused in front. Yellow light, midway between the red and green, will then be in perfect focus on the retina, the optimum focus when viewing with white light.
The red and green filters usually used create a chromatic spherical difference of only 0.50 D, requiring visual acuity of 20/30 or better to distinguish a blur difference. Balance with the red-green test should always be approached from the fogged direction (red clearer) to minimize accommodation.
Vergence
vergence = measure of the curvature of the wavefront. (The more curved the wavefront, the greater the vergence.)
Curvature = reciprocal of the radius
Lenses add vergence to light.
power of the lens (diopters) = amount of vergence added to the light
Negative vergence: rays diverging
Positive vergence: rays converging
Which color light is bent more due to diffraction?
Red is DIFFRACTED MORE
Focal point
all parallel rays cross at the focal point (part of definition). Focal point = reciprocal of the lens
Circle of least confusion of a lens
take the spherical equivalent of the prescription. Then take the reciprocal of the diopter = distance of the circle of least confusion.
How should glass prisms be held with the line of sight?
Easy way to think about it: hold it the usual way; plastic prisms (usual ones) are parallel to the FACE
Prentice position: Glass prisms should be held with the back surface PERPENDICULAR to the line of sight (parallel to the eye)
If you don’t hold it this way, then you measure more deviation then what the patient actually has (therefore if you use these incorrect measurements, you will over-correct)
Dk/L
best at describing how much oxygen can pass through a contact lens and reach the cornea. Good way of comparing different brands of contact lenses.
Dk = oxygen transmisibility of a material.
L = central thickness of the lens
How should plastics prisms be held with the line of sight?
Easy way to think about it: hold it the usual way; parallel to the face
plastics prisms should be held with the rear surface in the frontal plane. For near fixation objects, the rear surface should be angled in slightly so as to be perpendicular to the direction of the fixation object.
reduced schematic eye numbers
distance between anterior corneal surface and nodal point of the eye = 5.5 mm
distance between nodal point of eye and retina =17 mm
total axial length of the eye = 22.5 mm
total power of the eye = 60 D
index of refraction of schematic eye = 1.33
what do you use the reduced schematic eye for?
solving problems involving:
1) calculating image size that an object projects on the retina
2) calculating the size of the projected blind spot when given the size of the optic disc.
silicone oil replacing vitreous in pseudophakic and aphakic eye
Pseudo/phakic eyes: hyperopic shift of 3-5 D (b/c oil presses up and molds to lens as a CONCAVE shape and acts as a negative powered lens) –> hyperopic shift
Aphakic eye (silicone oil has a CONVEX shape –> + lens –> myopic shift)
constructive/destructive interferance
electromagnetic waves combine to produce wave of greater/lesser intensity
temporal coherence
one part of a beam of light is able to interfere with another part of the same beam of light
spatial coherence
when 2 parts of the SAME wave can produce interference. This is how diffraction gradings are produced.
Knapp’s law
Spectacles in anterior focal plane (~15.7mm) induce no aniseikonia
If anisometropia (difference in refraction) is due to an axial length difference, spectacles placed in the anterior focal plane of the eye will NOT induce an anisekonia (magnification or minification of the image).
photochromic lenses (transition lenses) work
work by darkening with the absorption of UV light (300-400 nm). Automobiles have a UV absorbing coating that will prevent transition glasses from working properly.
U+P = V
U = object vergence, which is ALWAYS NEGATIVE P = power of lens/mirror V = vergence of light exiting the lens/mirror to form an image
Object and image vergences = reciprocal of their distances from the lens/mirror respectively
OCT is based on the ____
Michelson interferometer = which has a movable mirror from which light (reference beam) is directed at the retina simultaneously with a light source (objective beam). All of the light from the objective beam is cancelled out through destructive interference except the reference beam.
Because the movable mirror position is known, the layer of the retina can be inferred and the light reflected can be interpreted to give information about that specific layer of the retina. Moving the mirror up and down and tilting the mirror allows a 2D view of the retina.
what wavelength of light is absorbed by fluorescein
B lue B egins the FA (blue is absorbed by fluorescein)
G reen G ets out of the eye
Absolute hyperopia
Minimum (non-cycloplegia) plus correction required for clear VA at distance
Manifest hyperopia
Maximum (non-cycloplegia) plus correction the eye can accept without blurring
Facultative hyperopia
Manifest hyperopia - absolute hyperopia
Person submerged in water - what type of optical error?
A: refractive hyperopia = decreased power of the cornea
Increase in index of refraction of water over air induces a decrease in the power of the cornea.
Latent hyperopia
Cycloplegia hyperopia - manifest hyperopia
Vergence of light of an emmetropic eye
Emmetropic eye emits zero vergence light from the retina.
Keplerian telescopes
form inverted images
lenses separated by sum of their focal lengths.
Makes the lenses longer than Galilean telescopes.
Prince rule
measures accommodative amplitudes
(amplitude/near point/far point of accomodation)
NOT accomodative convergence
Other ways of calculating accomodative amplitudes: method of spheres (minus or plus lenses used), near point of accommodation (near card moving)
Galilean telescopes
positive OBJECTIVE, and negative eyepiece.
Upright images. Lenses separated by DIFFERRENCE in their focal lengths
therefore Galilean telescopes = smaller than Keplerian telescopes
3 major classes of visual field deficits
cloudy media (gives more low vision in conditions of high/low lighting) central VF deficit peripheral VF deficit
Loupes good for…
high-detail near vision
stand magnifiers good for…
reading in patients with a tremor
hand magnifiers good for…
“spot” viewing aka reading labels in a store
telescopes good for…
activities that do not require ambulation while doing them
Diverging mirror
diverging mirror can only form virtual images and the power of a diverging mirror is always negative
juvenile onset myopia vs. adult onset myopia
juvenile onset myopia (7-16 yo) generally 2/2 increased AL. risks: FH, prematurity, near work, esophoria, against-the-rule astigmatism
Adult onset myopia: increased AL, increased K power, increased lenticular power. Highly correlated to intense near work.
Hyperopia far point
located behind the lens
Radiometry
measures light as ENERGY OR POWER. Unit = Watt. Measurement of irradiance
Astigmatic dial refraction STEPS (#1-4)
1) fog the patient to 20/60 with plus sphere
2) ask patient which line of astigmatic dial looks darkest/sharpest
3) add minus cylinder perpendicular to the axis or plus cylinder parallel to the axis until all lines are equally sharp
4) reduce the sphere using snellen chart until vision is clearest.
If using plus cylinder phoropter, for every 0.50 diopter of cylinder you add, you must subtract 0.25 diopters of sphere.
Brewster’s angle
= polarization angle. All light polarized in parallel with the incident plane is transmitted at the Brewster angle. Polarized sunglasses use the principle of Brewster’s angle to reduce glare from the sun reflecting off horizontal surfaces such as water.
Photometry
measures light in terms of RESPONSE OF THE EYE. Units = LUMENS. Ex: eye is more sensitive to yellow light, so yellow light has HIGHER lumens if power is kept constant
Other units = lux, apostlibs, candelas.
Lens tilt with glasses in 2 varieties:
1) tilt along horizontal axis (180 axis) for pantoscopic tilt. Induces cylinder change along 180 axis.
2) tilt along the vertical axis (90 axis) for face form. Induces cylinder change along the 90 axis.
If lens is POSITIVE, the cylinder change along the axis will be POSITIVE
if lens is NEGATIVE, the cylinder change along this axis will be NEGATIVE
Sphere changes in direction of the sphere power
Positive lenses will ADD sphere power
Negative lenses will add NEGATIVE sphere power.
Tilting a lens vs. rotating a lens about its optical axis
Rotating a lens about its optical axis makes no difference to its spherical equivalent power
Tilting a lens will affect its power
Amount of hyperopia in newborns vs infants
Newborns have 3 diopters of hyperopia.
At 1 yo: 1 diopter of hyperopia
Laser interferometry
Laser interferometry projects two separate laser beams onto the retina. These two light beams interfere with each other forming “interference fringes” on the retina. The patient sees wavy light and dark lines. Therefore gets around media opacities such as cataract.
Potential acuity meter (PAM)
PAM projects Snellen chart through a tiny aperture. This is then moved around a patient’s pupil until a clear path is obtained and the patient can see the project Snellen chart
Does changing frequency or wavelength affect the lens’ refractive index or power?
The refractive index of a lens material is higher for higher frequency (shorter wavelength) light. Therefore, if you increase the frequency of light from the source, you will increase the lens’ refractive index or power.
Geometric optics
Geometric optics deals with light as a ray (mathematical construct)
Physical optics and Quantum optics
Physical optics deals with light as a wave per ophtho?s (and photon per LMO)
per ophtho?s: Quantum optics deals with light as BOTH a wave and particle.
index of refraction affects what property of light?
When light passes through any transparent medium besides a vacuum, its speed or velocity is reduced by the index of refraction of that medium
Frequency of light is constant in a refractive medium. Therefore wavelength changes.
How does AcuFocus work?
Pinholes allow an increase in the depth of field
AcuFocus - when patient is focusing at distance, pupil dilates PAST the ring, peripheral light rays brought into focus for distance vision. When the patient is looking at a near object, pupil constricts and only light rays inside the inlay are brought into focus.
optical center of bifocal and near vision
2 mm below the center of the distance optical axis is the top of the bifocal add generally placed
ppl on average look 8 mm down and 2 mm in from the optical centers of their lenses for near vision.
Types of contact lenses (hybrid and hard)
Hybrid CL = SynergEyes-KC
Hard CL for keratoconus = Rose-K lens, McGuire lens, Soper cone lens
Orthokeratology fluorescein pattern
Orthokeratology lenses = used to reduce myopia
Bull’s eye pattern: dark-green-dark (center is flatter and therefore pushes fluorescein out of the way)
RGP: apical bearing
apical bearing: CL rests on the CENTRAL cornea. 2ndary to the lens being fit too flat
Perceptual completion
Perceptual completion - brain fills in central scotoma with information from the edges.
Slit lamp: 5 components
Slit lamp: 5 components (AGIOB)
“Astronomical Galileo I/O binocular”
1) astronomical telescope = produces an inverted image AND MOST OF THE MAGNIFICATION
2) Galilean telescope = concave and convex lens. ALLOWS ADJUSTMENT OF THE MAGNIFICATION. Magnification levers control whether light goes through the convex or concave lens first. If light goes through CONVEX lens first, the image is magnified.
3) inverting prism = Porro-Abbe prism = series of two triangular prisms that reflect light several times and TURN THE IMAGE UPRIGHT without losing clarity, with minimal light loss, and no magnification.
4) objective lens
5) binocular eyepieces
Disadvantage: some light lost between the objective and the eyepiece
Preferred retinal locus (PRL)
Preferred retinal locus (PRL) = preferred alternative point of extrafoveal fixation in pts with foveal damage
Constructive interference
Constructive interference: electromagnetic waves add to produce a more intense wave
Destructive interference
Destructive interference: electromagnetic waves combine to produce a wave of less intensity
Spatial coherence
Spatial coherence: 2 parts of the same WAVE can produce INTERFERENCE. This is how diffraction gradings are produced
Temporal coherence
Temporal coherence occurs when ONE part of a beam of light is able to interfere with another part of the same beam of light.
Spasm of near reflex
Spasm of near reflex: combination of 3 things = miosis, convergence, accommodation.
Also have: myopic refraction, esotropia from convergence, miosis
3 basic elements of a laser
3 basic elements of a laser:
1) medium
2) energy pump to excite the medium = causes the majority of molecules in the medium to attain an activated state (aka POPULATION INVERSION)
a photo of the proper frequency can then cause the excited molecule to emit an identical photo (stimulated emission)
3) mirrors to amplify the desired wavelength (selectively amplify only the photos released by the stimulated emission = OPTICAL FEEDBACK)
CL toric mark has shifted
LARS:
if mark has shifted LEFT, then you ADD the corresponding amount of cylinder axis
If mark has shifted RIGHT, then you SUBTRACT the corresponding amount of cylinder axis
1 clock hour = 30 degrees
What are most glasses made of (material)?
Most glasses made of CR-39.
ways to minimize meridional magnification
All of the following MINIMIZE meridional magnification
- grind the cylinder onto the back of the spectacles
- minimize the vertex distance
- change the axis so that it is more horizontal or vertical (towards 90 or 180 degrees)
- decrease the power of the cylinder
Flat top - image jump and displacement
Flat top = minimizes image jump
Majority of people are myopic, and therefore flat top minimizes image displacement for these patients.
Steps used in the cross-cylinder technique
Steps used in the cross-cylinder technique (think going anterior –> posterior (axis then power))
1) adjust sphere so best VA obtained
2) adjust the cylinder axis
3) adjust the cylinder power
4) refine the sphere, cylinder axis, cylinder
How does a keratometer work?
keratometer works by
1) directly measuring the REFLECTING power of the cornea to
2) CALCULATE the radius of curvature and assumes an index of refraction to
3) CALCULATE the refracting power of the cornea
What type of mirror is the cornea?
cornea = CONVEX mirror
Fusional amplitudes
we have very small VERTICAL fusional amplitudes (< 2 prism diopters) convergence amplitudes (~14 diopters) divergence amplitudes (~6 diopters)
Diverging mirror power and image
Diverging mirror = automatically means it has NEGATIVE power and that it forms a VIRTUAL image (to the right of the mirror)
Light passing through a window pane…
Light that passes through a window pane undergoes lateral displacement ONLY
- there is no overall refraction
- light that exits the piece of glass is PARALLEL to the incoming light ray but laterally displaced.
Fermat principle
Fermat principle: light will take the shortest effective distance between any two points.
Pupil size/Airy disc
Airy disc: Diffraction pattern produced by pinhole; as when pupil < 2.5 limits a person’s VA. Reason is that < 2.5 mm aperature begins to cause enough spreading to have effect (diffraction) on VA
Principle of Airy disc. Airy disc = image formed on the retina from a distant small source that takes the form of concentric light and dark rings surrounding a bright central disc.
d = 2.44fA/a
d= central disc
f = focal length of the eye
A = wavelength
a = iris diameter
If you are to the left of the lens, are you in front or behind the lens
Left of lens = IN FRONT OF THE LENS
Pincushion and barrel shapes
Pincushion and barrel shapes are distortions from plus and minus lenses.
Pincushion: when central light ray is LESS magnified than off-center light rays in plus lenses
Barrel shapes: when central light rays are MORE magnified centrally than off-center rays in minus lenses
Anti-reflection coating on glasses reduce glare through what time of interference?
Destructive interference.
Anti-reflecting coating = 1/4 of the wavelength of incoming light.
Different coatings needed for different wavelengths.
After light ray is reflected from the lens surface itself, it is slowed by another 1/4 of a wavelength on its way out of the eye.
Total = 1/2 of wavelength out of phase. Perfectly destructively interferes with light rays that are reflected from the surface of the antireflective coating.
Lighthouse Distance VA test
modified ETDRS chart has 5 letters for each line MC used chart tests at 10 ft equal line difficulty equal spacing between letters and lines more lines of letter in low vision (20/125 and 20/160)
Retina lenses: contact or non-contact
Contact lenses: neutralize K refractive power (high power - or + lens)
Non-contact lenses create an astronomical telescope (90D, 60D lenses, etc)
Non-contact lenses
Non-contact lenses create an astronomical telescope (90D, 60D lenses, etc)
Real, magnified, inverted
Contact lenses with SLE
Contact lenses: neutralize K refractive power (high power - or + lens)
DO NOT magnify image.
Ex: Hruby, Goldman 3- mirror, Zeiss 4- mirror, Goldman fundus contact lens
The high + contact lenses nullify K power but also add a plus lens so that an astronomical telescope is formed –> magnifying image
Ex: Rodenstock CL, panfundoscope CL
=Real, magnified, inverted image.
error in Axial length has a greater effect in shorter or longer eyes?
Answer = shorter eyes.
spherical lenses
focus light rays to around the same point producing a BLUR CIRCLE (mathematically described by a point spread fxn)
AC/A ratio
gradient method: patient fixates on Snellen and you measure the patient’s deviation.
Place minus diopter sphere in front of the patient and re-measure the deviation.
Difference in deviation = AC/A ratio
Or have patient fixate on 33 cm target and measure deviation before and after a +3.00 D lens placed before the eye
Length of mirror to see yourself
Mirror just needs to be half the size of the person for them to see themselves head to toe
Normal AC/A ratio in prism diopters of convergence to diopters of accomodation
3:1 to 5:1
Luminous intensity
Brightness
measured in candela
Luminous flux
Light flow
measured in lumens
Illuminance
Light on surface
measured in lux
Luminance
Light emitted or reflected
measured in apostlibs (1 lumen / m2)
Purkinje shift
At night, decreased wavelength leads to myopic shift
Haidinger’s brushes
Humans only able to see polarized light by this phenomenon.
When polarizing filter rotated in front of a blue background.
Bruch looks like a propeller.
Entoptic* phenomenon that demonstrates light polarization.
Ex: Can be seen if one wears polarized sunglasses and stares at a solid blue background (rotate head back and forth). See faint yellow lines perpenticular to the axis of polarization. OR look @ white area on LCD flat panel screen (polarization effect of the display)
*Entopic images = physical basis in the image cast upon the retina (unlike optical illusions = perceptual effects that arise from interpretations of the image by the brain)
Lens effectivity formula
Da = Db (nlens – na / nlens - nb)
D, power in diopters; n, refractive indices
Focal points
Primary: object location for image at infinity
Secondary: image location for object at infinity
Thick lens formula
D = (Db – Df )(t / n)
Df, power of front surface; Db, power of back surface; t, thickness in meters; n, refractive index
Diameter of field for indirect lens?
Volk 20D: 12mm
Volk 28D: 14mm
What kind of telescope is a slit lamp vs lensmeter?
Slit lamp: Astronomical + Galilean
Lensmeter: Astronomical
Which are additive vs. muliplicative? Lens or Prisms?
Prisms are additive
Lenses are multiplicative
Magnification/minification from spectacle lens (per diopter)
2% per diopter (@ 12mm vertex)
Most can tolerate to 8% difference
Fusional amplitudes
Vertical 3 PD
Divergence 6 PD
Convergence 18 PD
Prism diopter
∆ = displacement measured in cm at a distance of 1m
1° is about ~2∆ if
Prismatic effect of lenses (effect on strabismus)
& Error = 2.5 x D
D, power of lens in diopters
Minus measures More
E.g. 40∆ XT wearing -10.00D SPH glasses will measure 2.5 (10) = 25%or 50 ∆ XT
Drugs causing Myopic shift:
Topamax
Sulfa
Tetracycline
Drugs causing Hyperopic shift:
Chloroquine
Phenothiazine
Anti-histamines
Marijuana
Kestenbaum’s rule
Estimate of reading add for low-vision patients
Reciprocal of Snellen acuity
i.e. 20/200 → 200/20 = +10
Astigmatism Adjustments (suture/pterygium)
Suture steepens axis
Pterygium flattens axis
Retinoscopy
Against motion: needs minus or head forward
With motion: needs plus or head backwards
Minimum visible and minible resolvable VA
Minimum visible 10 arc sec (spot on wall)
Minimum resolvable: 1 arc min (Snellen)
Snellen 20/20 = 5 arc min @ 20ft; 8.73mm high
20/20 “E” = 5 arc min; 1 arc min per leg of letter
tan 1 arc min = 0.0003
ETDRS Chart .. 3 lines…
3-lines = doubling or half of visual angle
Amsler
Tests central 10 degrees of vision
Vernier acuity:
2-3 arc sec (break in line)
pt to discern the minimum distance that keeps 2 line segments visually separable
Loss of Accomodation
Age 40: 6D
Age 44: 4D
Age 60: 1D
Pinhole size
= 1.25 mm (optimum size)
limited by diffraction if
Moving lens (glasses)
Forward (towards nose) : more (+) sph
Backwards (towards eyes): more (-) sph
Bifocals:
Myopics: flat top, executive
Hyperopics: round top
Move optical center of add close to top of segment
Loupes
Power calculation for RGP?
Prefer trial lens + over-refraction If not available: Measure MRx + K Convert MRx to (-) cyl & zero vertex Disregard cyl – in tear lens No need to correct for vertex if <4.00 Choose base curve for flatter K Usu. +0.5D steeper to form tear lens to prevent apical touch or tight lens Power = MRx (sph) + tear lens
“SAM-FAP” steeper add (-); flatter add (+)
If +0.5D steeper than K, then add -0.5D
Tilting lens (glasses)
Plus lens: more (+) sph, more (+) cyl
Minus lens: more (-) sph, more (-) cyl
Axis in axis of tilt
CL K change = how many diopters?
K change 0.05mm = 0.25D
Rotated CL (toric) on exam?
LARS: Left Add; Right Subtract
1 clock hour = 30 degrees
Too tight CL?
Too tight?
reduce diameter; then reduce curvature
Central K edema from RGP use?
Sattler’s veil
CL effect on accomodation or convergence demands?
Increases demands on myopes
Decreases demands on hyperopes
Normal AC/A?
3:1 – 5:1
Heterophoria method (moving the fixation target)
AC/A = PD(cm) + [near Δ – distance Δ] divided by
[diopters of accomodation @ near]
Gradient method (with + without lens)
AC/A = [Δ(with lens) – Δ(without lens)]/ [lens diopters]
Distance & near method (not actual AC/A)
near Δ – distance Δ
>10 Δ = high AC/A ratio
About 5 Δ is normal
Indices of refraction:
Air? 1.00
Water? 1.33
PMMA? 1.47
Acrylic? 1.55
Multifocal Lenses?
Alcon Restor (diffractive rings)
B&L Crystalens (position shift)
ReZoom (refractive zones)
Keratometer measures?
directly: reflecting power
Indirectly: radius of curvature
How to calculate power of piggy-back IOL?
Hyperopia: 1.50D per 1.00D hyperopia
Myopia: 1.00D per 1.00D myopia
Convert glasses to CL for presbyopic pt
Myopes accommodate MORE for a given object when wearing CL than when wearing spectacles (reverse for hyperopes)
For myopes: spectacles induce a BASE-IN prism (aids pt in converging therefore less convergence requirement). So it is easier for myope to read in spectacles.
For low vision patient with large add, how to help with convergence demand
Correct by DECENTERING the optical centers NASALLY to induce base-IN prism
For low vision patient with magnification - what is problem and hwo to fix?
Reduced VF = consequence of magnification
overcome by telescopes (but expensive)
angle alpha vs. kappa
Alpha: visual axis in relation to optical axis
Kappa: visual axis in relation to pupillary axis
Non-mydriatic camera uses what wavelength of light?
Infra-red light
Pinhole removes refractive error up to…
+/- 5 diopters
For myopic refractive treatment, how much are Ks reduced for every diopter
Ks are reduced 0.8 D for every 1 D of ablation
Central Cornea is weakened, allowing the POSTERIOR cornea to bow forward and add negative power to the cornea
For hyperopic refractive treatment, how much are Ks reduced for every diopter
For hyperopic treatment, Ks are INCREASED 1D for every 1 D of ablation
Minimum legible threshold acuity
Divide denomination by 20 and then use that number to multiply by 5.
For example. Patient can only read 20/60.
60/2 = 3
3*5 = 15
Answer is 15 arcmin
pupillary axis
line going through pupil center
(normal fovea = temporal to pupillary axis)
If you make iris more center (more nasal), then the light reflex will appear to be more temporal –> gives patient appearance of ET
visual axis
fixation point to the fovea
optical axis
goes through the approximate center of all the optical elements of the eye
cornea refractive index
1.376
aqueous refractive index
1.336
air refractive index
1
acrylic refractive index
1.46
silicone refractive index
1.438
cold mirror
allows the infrared portion of light to pass while reflecting visible light
Why are sunglasses vertically polarized
Block out the HORIZONTALLY reflected light from the road surfaces
Minimum legible threshold
pt to discern progressively smaller letters or forms
Minimum visible threshold
pt to discern minimum brightness of a target from its background
Minimum separable threshold
pt to discern minimal angle between two objects that makes them visually separable
Reasons standard methods of IOL power calculation do not work in post-refractive surgery eyes
1) radius error: true central K is flatter than the K readings obtained from today’s instruments (myopic pt’s post-LASIK cornea = flatter)
2) keratometer index error = index of refraction assumes a certain ratio between the radius of curvature of the anterior and posterior cornea surfaces (“gullstrand ratio”) which is no longer true after refractive surgery
3) formula error: IOL power formulas use the measured axial length and K power to predict effective lens position posterotively (these formulas assume the flatter K = shallower AC; but not true in post-refractive patient)
Dz in which patient has surprisingly good contrast sensitivity compared to their Snellen acuity
Macular degeneration
calculating induced prism in glasses
Horizontal prisms (looking across): assume .8 cm x diopters If both prisms are hyperopic (or both myopic) then ADD the two. If 1 hyperopic & 1 myopic, then SUBTRACT the two.
Vertical prisms (looking down): 1 hyperopic & 1 myopic, then ADD the two. If BOTH hyeropic (or myopic), then SUBTRACT the two.
Range of distance that an image is focused to an observer?
Depth of focus
Range of distance that an object is focused to an observer?
depth of field
Reduce anisometropia in glasses
1) reduce the front surface power (decreasing magnification)
2) reduce the vertex distance (distance from the glasses to the eyeball)
3) decreasing the central thickness of glasses (??)
Greatest amount of positive dysphotopic complaints in what type of lens.
High index square-edged acrylic lens with a less steeply curved anterior surface
block the top of the lens
if you block part of the lens, the rays of light do not change direction. Only the amount of light (brightness) will change.
Prism bends light in which direction? In which direction is the image?
Prisms bend light towards the base
Images move towards the apex.
Ideal cornea Q value
Q = -0.50
Looking nasal or temporal to optical center acts like what type of lens?
acts like minus lens
Looking superior or inferior to optical center acts like what type of lens?
acts like plus lens
wetting angle of CL describes?
how water beds up on the surface of the CL
High wetting angle indicates that water readily beads up.
scanning laser ophthalmoscope (SLO)
able to simultaneously plot a VF and correlate it with where the retina was stimulated
preferred retinal locus (PRL)
preferred alternate point of extrafoveal fixation in pts w/ foveal damage
To create a tighter fit for CL
increases sagittal depth
1) decrease the base curve (make it steeper) = first number
OR
2) Increase the diameter (second number)
easing adult transition to astigmatic lens
1) use minus cylinder in the glasses (ground on back of lens)
2) reduce power of clylinder
3) move axis closer to 90 or 180 degrees
4) reduce vertex distance
Fresnel
side by side prisms = add together to have same effect as a large prism
place on the BACK of glasses
Decrease VA 2/2 CHROMATIC aberration
long term UV radiation MC assoc/w/which type of cataract?
cortical cataract
Slab off vs reverse slab off
slab off (bicentric grinding) =grinding the MORE minus (or least plus lens) and creates a base UP prism effect over the reading area of the lens
reverse slab off - adding base down prism to the more PLUS lens (MC used 2/2 manufacturing methods)
Radiuscope
measures base curve of CL
keratoscope
measures K curvature
Q switching a laser
decreases energy, increases power
Patient who is pseudophakic in one eye and phakic in the other eye will have what amount of anisekonia?
2.5%
Which glasses help with convergence insufficiency and accommodative esotropia?
Myopic glasses help convergence insufficiency
-when myopic pt accomodates wearing MRx, the lenses act as a base in prism. Helps pt with convergence insufficiency.
Hyperopic glasses help accomodative esotropia
Hyperopic MRx = base out prism.
Which wavelengths diffracted most?
Red (longer) wavelengths diffracted more than shorter wavelengths.
Which glasses help with convergence insufficiency and accommodative esotropia?
Myopic glasses help convergence insufficiency
-when myopic pt accomodates wearing MRx, the lenses act as a base in prism. Helps pt with convergence insufficiency.
Hyperopic glasses help accomodative esotropia
Hyperopic MRx = base out prism.
Standard way that index of refraction is measured?
using 589 nm yellow light
Which situations would require that the near segment in bifocals be decentered the LEAST amount nasally
A: short IPD (decentered nasally is not helpful)
Helpful to have decentration nasally in:
1) increased convergence demand situations
- shorter working distance
- wider IPD
2) want to induce more base-in prism
- correct exophoria
- more plus power present in distance segment (aphakic spectacles) create greater base out prism effect
Lensmeter
places a standard lens behind the spectacle so that the focal point of the lens overlies the posterior vertex of the spectacle. Makes reading the power of the spectacle linear. Badal principle
Titus stereo testing
stereo testing and binocular testing:
observer wears glasses with lenses polarized at 90 degrees to each other. Each lens only allows the polarized rays from 1 image to pass, therefore, a different image passes through to each eye, and binocular VA is possible.
RGP - tight fitting RGP
Tight hard CL = decentered inferiorly, don’t move when pt blinks. Can also see: central area of fluorescein and absence of peripheral fluorescein*
Problem: Lens has base curve TOO STEEP for the cornea.
need more FLAT base curve so you INCREASE the base curve –> makes lens more flat. lens is vaulted HIGHER over the cornea and fluorescein pools in the CENTER
or, to decrease tightness, you can DECREASE the diameter
- b/c in tight fitting RGP, tends to fit about 2 mm smaller than the corneal diameter. Cornea 2/2 unprotected by the lens, dries out (at the 3:00 and 9:00 position). Steep lenses dig into peripheral cornea and = absence of green.
RGP: apical alignment
apical alignment: hard CL has the same base curve as the cornea. In ideal fit, contact lens has an apical alignment and the upper portion of the CL is under the upper eyelid. With blinking, the upper eyelid moves the CL so there is good tear exchange
RGP: apical clearance
apical clearance: CL is steeper than the central cornea curvature –> therefore the lens is tighter than an apical alignment.
If it is too tight, it will center within the interpalpebral fissure and have minimal movement with blinking. (can be advantage if patient has very large interpalpebral fissure where the lens would be extremely large if it were to rest partially under the upper eyelid
