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
