Optics Flashcards
Photons of like behave as both ______ and ______.
waves and particles
What is the speed of light proportional to?
speed = velocity (v) which is directly proportional to wavelength and frequency.
What it the speed of light?
Given a constant speed of light in a given medium, what is the relationship of wavelength and frequency?
Which are the only two substances that light does not slow down in?
When light slows down in a medium, what happens to the frequency and the wavelength?
Frequency remains the same
Wavelength shortens
Name the 6 categories of light in the elctromagnetic spectrum from longest wavelength to shortest
- Radio (AM/FM)
- Infrared
- Visible Light (350-750)
- UV light
- X-ray
- Gamma rays
What is the relationship of Energy to frequency and wavelength?
Directly proportional to Frequency
Inversely proportional to wavelength
Define the index of refraction
ratio of the speed of light in a vacuum to the speed of light in a specific material
Give the index of refraction of the following materials
- Air
- Water
- Aqueous and vitreous
- Cornea
- Crystalline Lens
- IOL (silicone, acrylic, PMMA)
- Glass
- High index lenses
- Air 1.00
- Water 1.33
- Aqueous and vitreous 1.34
- Cornea 1.37
- Crystalline Lens 1.42
- IOL (silicone 1.41, acrylic 1.55, PMMA 1.49)
- Glass 1.52
- High index lenses
Define Optical interference
Overlapping of light waves
What is Constructive Interference?
when the peaks of 2 waves overlap, resulting in maximum intensity
What is Destructive Interference?
when the peak of one wave overlaps with the tough of another,
Explain how anti-reflective coatings wor?
Coating uses destructive interference 1/4 wavelength apart
How do interference filters work (i.e. FA filters)
allow only green light out of the eye by using destructive interference to block all other colors
What is Coherence?
the ability of two light beams to cause interference (a large white source has a coherence close to zero)
What is the best clinical example of coherence?
OCT
Explain the polarization of light
Each light wave has an electrical field with a particular orientation
What is non-polarized light?
electrical field of each wave has a random orientation
What is polarized light?
All electrical fields have the same orientation
What are Haidinger brushes, what are they used for and how do they work?
polarizing filter rotating in front of a blue background produces a rotating image like a double ended brush or propeller
This is a type of endopic phenomenon which tests macular function
Name 3 clinical examples of polarized light
- Titmus stereo testing
- polarized microscopy
3.
Define Defraction
Bending light waves around edges; change in direction of light waves is related to wavelength (the shorter the wavelength the less change in direction)
What determines the amount of diffraction?
related to the size of the aperature (the smaller the aperature the greater the diffraction)
Interference of new waves ith original rays forms a diffraction pattern
What is an airy disc?
Diffraction pattern produced by a small, circular aperature.
Occurs when the pupil size is <2.5mm. Diameter of the central disc increases as pupil size
How does a pinhole occluder work?
Reduces refractive error and improves vision by increasing depth of focus, but limited by diffraction
What is th eoptimal size of a pinhole?
1.2mm
How many diopters of refractive error can be corrected with a pinhole occluder?
What happens when the pinhole is smaller than 1.2mm?
smaller aperature limits visual acuity
How does squinting improve vision?
For which disorders can a pinhole improve vision? Decrease vision?
Improve vision in eyes with corneal or lenticular irregularities
Decreases vision in
What is the scattering of light?
Which wavelengths scatter more?
Disruptions of light by irregularities in light path
By what mechanism does corneal scar or cataract decrease vision?
They scatter light causing glare and image degredation
What is Rayleigh scattering?
scattering of light in the atmosphere involves blue particles and blue light
(blue light scatters the most
What is Optical Reflection?
The bouncing of light off of optical interfaces
What determines the amount of reflection?
- The greater the refractive index difference between the two media, the greater the reflection
- Also varies with the angle of incidence
What is a clinical example of reflection?
Asteroid hyalosis
(asteroids reflect light back into the examiners eye creating glare, but the patient is asymptomatic)
What is the transmission of light?
the percentage of light penetrating a substance
What can cause transmission of light to to vary?
wavelength
How is the absorption of light expressed?
expressed as Optical Density
OD= log
What is Illumination?
What is luminance?
Te measure of reflected or emitted light(lumen/
What is an apostolib?
diffusing surface with luminance of 1 lumen/m2
What is an apostolib used in?
Humphrey and Goldman visual field testing
What is constrast sensitivity?
the ability to detect
What does laser stand for?
- Light
- Amplification
- by
- Stimulated
How does a laser work?
- excited material releases photons of the same wavelength and frequency
- released photons are in phase (constructive interference)
- Produces monochromatic, coherent, high intensity polarized light
How can you increase the power of a laser?
Power can be increased by increasing the energy or decreasing the time (P=E/t)
Name two methods of increasing laser power
- Q- switching
- mode locking
(types of shutters that synchronize light phase and compress output in time)
What happens to light when it travels from one material to another with a change in refractive index?
- Explain the direction that light bends when it passes from a medium of lower refractive index to a higher index medium?
- How about
- Bends toward the normal
- Bends away from the normal
Why does light refract when traveling through different mediums?
Higher refractive index materials are more difficult for light to travel through, so it takes a shorter path
(closer to the normal)
Describe Snell’s Law
n sin(i) = n’ sin (r)
n= refractive index of material
i = angle of indence (from the normal)
What is the critical angle?
the angle at which incident light is bent exactly 90 degrees away from the normal (medium of higher to lower n)
What is the critical angle of glass/air interface?
What is the critical angle of the cornea?
What is the law of total internal reflection?
angle of incidence exceeds the critical angle, so light is reflected back into material with higher index of refraction
How do prisms deviate light?
Prisms have non parallel surfaces
- bends light toward the base
*
How is a Prism diopter measured?
The distance (in cm) that a light ray is displaced when passing through a prism. Measured 100cm (1m)
- How many centimeters is a 15 PD prism displaced at a distance of 1 cm?
- How about a 1PD prism?
How many prism diopters are in 1 degree?
2 PD per degree
What is the angle of minimum deviation?
The total angle of deviation is least when there is equal bending at both surfaces of the prism
How are plastic prisms calibrated?
calibrated by the angle of minimum deviation
How are glass prisms calibrated?
Calibrated in Prentice position:
back surface perpendicular to visual axis
When a prism is placed in fornt of the eye in which direction is the phoria created?
What type of phoria does a base out prism induce?
How do you correct it?
Exophoria
to correct use prism with apex in opposite direction
When holding a prism, which direction is the apex of the prism pointed to neutralize the deviation?
Apex is pointed in direction of deviation
Base in or base out for esotropia?
Base in or base out for exotropia?
Base out
Base up or base down for hypertopia?
When using prisms, how do you add prism power together?
Stacking prism is not additive
1 prism over each eye is
What is a Risley prism?
Two right angle prisms positioned back to back
0-30PD
What is a Risley prism used for?
used to measure prismatic correction for tropias
What is a fresnel prism?
Thin, stick-on prism composed of side-by side strips of small prisms
What determines the prism power of a fresnel prism?
prism power is related to the apex angle, not the prism
What is the limitation and down side of fresnel prisms?
disadvantage is reflection and scatter at prism interface
How does the lens vergence of glasses impact light rays passing through them?
all glasses induce prism
all off axis rays are bent toward or away from axis depending on lens vergence
What kind of prism is made by plus lenses?
Plus lenses act like two prisms base to base
What kind of prism is made by a minus lens?
Minus lenses act like two prisms apex to apex
What kind of retinoscopic motion is produced by a plus lens?
Plus lenses induce “against” motion (the target moves in opposite direction as the lens)
What kind of retinoscopic motion is produced by minus lenses?
Minus lenses produce “with motion” (target moves in same direction)
What determines the amount of motion on retinoscopy?
The amout of motion is proportional to the power of the lens
How do you calculate the effect of glasses on strabismic deviations?
percent difference = 2.5 xD
Do minus lenses make a strabismic deviation appear larger or smaller?
Deviation appears larger with minus lenses
“Minus measures more”
Do plus lenses make a strabismic deviation appear larger or smaller?
smaller
What is the Prentice’s rule equation for the prismatic power of a lens?
Á = hD
h = distance from the optical axis of lens (cm)
Does the prismatic power of a lens increase or decrease as one moves farther away from the optical center
Increases
Does the power of a lens increase or decrease as one moves farther away from the optical center?
Niether, power of the lens is constant
Induced prism in anisemetropia
Calulate the induced prism power if the patient looks 1cm below the optical center of the following refraction?
OD: -3.00
OS: +1.00 +3.00 at 90
OD Prism power = 1cm x 3D =3PD BD
OS Prism power vertical meridian = 1cm + 1D =1PD BU
Net prismatic effect is 4PD (BD over OD or BU of OS
How do you treat the prismatic effect of anisemtropia?
- CLs (optical center moves with the eyes)
- Lower optical centers (reduce induced prism)
- Prescribe slab off prism
- Single vision reading glasses
What is image jump associated with the prismatic effect of bifocals?
sudden prismatic power at top of bifocal segment. Image osition suddenly shifts up because of BD prismatic effect
What is image displacement of bifocals?
displacement of the image by total prismatic effect of lens and bifocal segment
How do you minimize image displacement?
minimized when the prismatic effect of the bifocal segment and the distance lens are in opposite directions
What type of prismatic effect do hyperopic lenses induce?
Base up prism- image moves progressively downward in downgaze
What type of prismatic effect do myopic lenses induce?
Base down prism, causing image to progressively upward in downgaze
What is the prismatic effect of a ROUND TOP bifocal segment?
acts like a BD prism
Do round top bifocals have much image jump?
yes. maximum image jump
When using a ROUND TOP bifocal, is the image displacement worse for a myope or hyperope?
Displacement is worse for a myope than hyperope
What is the prismatic effect of a FLAT TOP bifocal?
Acts like a BU prism
Do FLAT TOP bifocals have much image jump?
No. Minimal image jump
When using a FLAT TOP bifocal, is the image displacement worse for a myope or hyperope?
Displacement for for a hyperope than myope
Do Executive type bifocals have much image jump?
No image jump
Do progressive lenses have any image jump?
No image jump
Where is the optical center of executive and progressive bifocals?
At the top of the segment
If you have a plus lens, what is the best bifocal to use to minimize image jump?
Choose a round top
If you have a minus lens, what is the best bifocal type to minimize image jump?
Choose flat top or executive type
What bifocal type should you avoid in myopes?
Avoid round tops in myopes.
image jump is very difficult to ignore because it is in the same direction as image displacement
What variable causes prismatic effect to vary?
Wavelength
Which wavelengths bend further through a prism, long or short?
What does this cause?
Short wavelengths bend further
Causes chromatic aberration
When white light shines though a prism, which color is closest to the base? the apex?
- Blue rays closest to the base (bend farthest)
- red rays are closest to the apex
In the eye, which color of rays come to focus closest to the lens?
Blue rays come to focus closer than red rays
What is the doptric power difference between red and blue rays in the eye?
- Blue 1.5D
- Red 3D
What is the Duochrome test?
Red/green light filters are used to create a 0.5D difference
What is the duochroem test used for?
Used to check accuracy of refraction
What does it mean when the red letters are clearer when doing the Duochrome test?
The focal point is in front of the retina
(eye is “fogged” or myopic)
What does it mean when the green letters are clearer when doing the Duochrome test?
Focal point is behind the retina
(eye is overminused, or hyperopic)
How do you perform the Duochrome test?
Start with the red side clearer and add minus sphere in 0.25D steps until red and green are equal (focal point on the retina)
What is the mneumonic to remember with the Duochrome test?
RAM-GAP
red add minus
green add plus
Does the Duochrome test work on color blind patients?
Yes! because it works by chromatic abberation rather than color discrimination
How would you add a 3BU prism with a 4BO prism?
prismatic deviations in different directions are addative based on pythagorean theorem
(a² +b² = c²)
What is vergence?
The amount of spreading of a bundle of light rays (wavefront) emerging from a point source
What is the conventional specified direction of light when considering vergence?
the direction of light must be specified (by convention left to right )
What is convergence?
plus vergence, very rare in nature, must be produced by an optical system
What is divergence?
Minus vergence
How much vergence do parallel rays have?
Zero vergence
Define a diopter
A unit of vergence that is the reciprocal of the distance (m) to point at which the rays intersect
- reciprocal of the foacl length of the lens
What is the effect of a lens as light passes through it
adds vergence to light
amount of vergence = power of the lens (D)
How does a plus lens effect the vergence of light?
Plus lens (convex) ADDS vergence
How does a minus lens effect vergence?
minus (concave) lens SUBTRACTS vergence
Describe the basic lens formula and all of its components
U = vergence of light entering the lens
D = power of the lens
V = vergence of light leaving the lens
How do you calculate the power of a spherical surface in a fluid?
(n’ - n) = difference in refractive indices
r = radius of curvature (in meters)
What is the power of the cornea (front and back)
front = -5.7D
back = =52.9D
Total 47.2D
What is the equation for the power of a thin lens immersed in fluid?
refracting power of a thin lens is proportional to the difference in refractive indices bewtween lens and medium
What are object rays?
Which side of lens?
Rays that define an object, always on the incoming (left) side of the lens
What are image rays?
Which side of the lens?
Rays that define and image
Always on the outgoing (right) side of the lens
Which side of the lens are objects and images located?
Either side of the lens:
real if on same side as respective rays
vitrual if on opposite side from rays
How does adding power to an lens system move an image?
- Add plus power = pulls image against light
- Add minus power = pushes image with might
What is the focal length of a lens?
distance between the lens and focal points
(reciprocal of lens power)
f = 1/D
What is the focal length of a 20D lens
1/20 or
What is the spherical equivalent?
average spherical power of a spherocylindrical lens
SE = sphere + 1/2cyl
(places circle of least confusion on the retina)
What is the circle of least confusion?
circular cross section of the conoid of strum which ies halfway between the two focal lines at which image is least blurred
What do you convert cylinder notation from plus to minus?
- New sphere = old sphere + old cyl
- New Cyl = magnitude of old cyl but with opposite power
- New axis = change old axis by 90·
What is a power cross diagram?
Depicts two principle meridians of lens with power acting in each meridian (90º from axis), rather than according to axis
Put the following cross synder notation onto a power cross
Put the following power meridian notation onto a power cross:
How do you convert cross cylinder notation to spherocylindrical notation?
ADD or SUBTRACT the crossed cyl
Where is lenticular abberations occur?
Lenses only behave ideally near the optical axis
(perheral to this, paraxial region, abberations occur)
What is spherical aberration?
- shape dependent aberration
- periphery of lens has increasing prismatic effect
- peripheral rays refracted farther than paraxial ones
- blur is produced along the optical axis
what kind of retinoscopic reflex is produced by spherical aberation?
bulls-eye reflex
How do you reduce spherical aberration?
- Avoid a biconvex lens shape
- Use plano convex, meniscus, or aspheric lens surface
What are 3 ways that the eye handles spherical aberration?
- smaller pupil size eliminates greater number of peripheral rays
- cornea progressively flattens in periphery
- nucleaus of the crystalline lens has higher index of refraction
What is Coma aberration?
comet shaped image deformity from off axis peripheral rays
What is curvature of field in optics?
WHen a spherical lens produces a curved image of a flat object
What is astigmatism of oblique incidence?
When you tilt a spheical lens it induces astigmatism
(oblique rays encounter differnt curvatures at front and back of lens surfaces)
What is an example of astigmatism of oblique incidence?
pantoscopic tilt (the amount of induced sphere and cylinder depends on power of lens and amount of tilt)
What is optical distortion?
Differntial magnificantion from optical axis to lens periphery alters straight edges
How does the shape of disortion relate to the shape of the lens?
Shape of distortion is opposite the shape of the lens
Plus lens produces pincushion
Minus lens produces barrel
How does distortion change with increased power of the lens
What is chromatic aberration?
light of different wavelengths is refracted by different amounts
(shorter wavelengths are bent farther; chromatic interval between blue and red is 1.5 to 3.5)
What is the chromatic interval between blue and red?
Name one example of chromatic aberration?
At night Purkinje shift occurs- chromatic aberration moves the focal point of the eye anteriorly
What is transerve magnification of an image?
magnification of image size away from the optical axis
- ratio of image height to object height (or image distance to object distance)
- if image is inverted magnification is negative
What is axial magnification?
magnification of depth (along the optical axis)
equal to the square of transverse magnification
What is angular magnification?
the magnification of angle suspended by an image with respect to an object
- useful when object or image size cannot be measured
Name two examples of magnification in daily life?
telescope to moongaze
direct ophthalmoscope - the eye acts as a simple magnifier MA =60/4= 15x
What effects the size of an image seen though glasses?
- Front and back surface curvature
- Center thickness
What increases the image size for any corrective lens?
Increase either the front surface curvature or the lens thickness
How much will changes in front surface curvature change the image size in a correct lens?
every D of change will change image size by 0.5%
How much will changes in the central lens thickness of a corrective lens change the image size?
Every millimeter of change in thickness will change image size by 0.5%
(magnification decreases while lens
Which produces smaller images, minus or plus lenses?
Vertex power (refractive power):
Minus lenses produce smaller lenses than do plus lenses
How does vertex distance effect the magnification of lenses?
an increase in vertex distance will increase the magnification of a plus lens and decrease the magnification (increase the minification) of minus lenses?
How much does an increase in vertex distance effect magnification of lens power?
- PLUS LENS: every millimeter increase in vertex distance will increase magnification by 0.1% per diopter of lens power.
- MINUS LENS: every millimeter increase in vertex distance will decrease magnification by 0.1% per diopter of lens power.
How much does a spectacle lens change the retinal image size?
spectacle lens changes the retinal image size by 2% per diopter of power
What is anisemetropia? what does it produces as it increases?
Anisemetropia: difference in power between two eyes, every 1 D produces approx 2% aniseikonia
What is aniseikonia?
Difference in image size between eyes from unequal magnification of correcting lenses
How much aniseikonia is usually tolerated in percentages? in diopters?
Who tolerates most aniseikonia?
Up to 6% to 7% is usally well tolerated; corresponds to approximately 3D of spectacle anisemetropia.
What is an example of anisekonia?
unilateral aphakia: 25% enlargement with spectacle lens; 7% with CL;
What is Knapp’s rule?
when the proper corrective lens is positioned at the anterior point of the eye, retinal size will be equal in both eyes, no matter what the degree of anisemetropia
How do telescopes work?
magnify objects by increasing angle that objects subtends on retina
How does a Keplerian telescope (Astronomical) work?
Which lens is the eyepiece?
Image inverted or upright?
It is the combination of 2 plus lenses where the focal points coincide in an image plane. The distance between the lenses is the sum of focal lengths. the higher powered lens is the eyepeice. Creastes an inverted image.
How does a Galilean telescope work?
It is a combination of a weak plus lens (objective) and a strong minus lens (eyepiece)
The distance between lenses with the differnce of the focal lengths.
Upright image
Surgical loupes
What do Keplerian and galilean telescopes have in common?
angular magnification is the same for both
How would use us a galilean telescope to help treat a monocular aphakic eye?
for uniateral aphakia, overcorrect aphakic, CL by +3D, then correct induced myopic error with spectacle of -3D. THis produces an inverse Galilean telescope system that results in significantly smaller magnification difference between the two eyes than occurs with CL alone.
What is the angle of incidence?
angle of reflection (measured from the normal)
How do you know if an object is real or virutal in a mirror system?
real if locate on the left
How do you caluclate the focal length of a mirror?
One quarter the radius of curvature (f =r/2)
How do you caluclate reflecting power?
reciprocal of focal length (Dr = -1/f = -2/r)
Does a convex mirror have positive or negative radius of curvature?
pos or neg vergence?
what kind of image is produced?
positive radius of curvature (R)
adds minus vergence
produces virtual erect minified image (“VErMIn”)
Name two examples of a convex mirror?
- rear view mirror
- cornea- reflecting power MUCH stronger than refracting power)
Features of concave mirrors:
positive or negative radius of curvature?
plus or minus vergence?
real or virtual image?
upright or inverted?
magnified or minified?
What determines this?
negative r
adds plus vergence
can be either real or virtual
either upright or inverted
either
object location with respect to
An object is twice the focal length from the center of a concave mirror— describe the image
real, inverted, same size
An object is between the focal length and the center of a concave mirror— describe the image
real, inverted, magnified
An object is at the focal length of a concave mirror— describe the image
at infinity
An object is inside the focal length of a concave mirror— describe the image
virtual, upright, magnified
What are the features of an image produced by a plano mirror?
- vergence?
- real or virtual?
- upright or inverted?
- size?
- field of view?
- no change in vergence
- virtual
- upright
- same size
- field of view is double the size of the mirror
Name an example of a plano mirror
a dressing mirror needs to be only half the body length to view entire self
describe the central ray
passes through center of curvature of mirror, not center of mirror.
The location where primary and secondary focal points collide.
What are the 4 Purkinje sanson images?
Name the location of all 4 and describe the image
- front surface of cornea (image of object at infinity is located at the focal point. virtual erect minified)
- back surface of cornea (virtual erect minified)
- front surface of lens (virtual erect, minified)
- back surface of lens (real inverted minified)
How do Purkinje images work in an IOL?
images 3 and 4 are taken from the front and back surfaces of the IOL, respectively.
Useful in assessment of mild phacodonesis
What are the critical measurements of Gullstrand’s simplified schematic eye?
power: +60D
F =17mm
F’=22.6mm
What is minimum visible vision?
presence or absence of a stimulus
-Depends on light striking the photroeceptors
What is minimum discriminable vision?
resolving power of the eye- depends on ability to detect differences in light intensity
What is minimum seperable vision?
smallest angle at whcih two seperate objects can be discriminated -detection of a break in a line
What is vernier acuity?
spatial discrimination; ability to detect misalignment of two lines (8 seconds of arc; smaller than diameter of photoreceptor)
Describe Snellen acuity
Describe the ETDRS chart
5 letters per line
space between letters is equal to the size of letter on that line
geometric proportion of optotype height
what must be recorded with near acuity?
testing distance
Name 5-6 ways to measure visual acuity in children
Optokinetic nystagmus (OKN drum)
CSM (central steady maintained)
preferrential looking
allen pictures
HOTV
visual evoked potential (VEP)
what factors other than disease can reduce visual acuity?
uncorrected refractive error
eccentric viewing
decreased contrast
smaller pupil size
older age
Define legal blindness
VA = 20/200 or worse or visual field <20’ in better seeing eye
How does pupil size effect visual acuity?
- larger pupil size limits vision owing to spherical and chromatic aberration
- smaller pupil limits vision owing to diffraction
What is the optimal pupil size?
3mm
What is a laser inferometer?
- A helium laser beam is split and projected onto the retina
- produces interference fringes (varied spacing)
- retinal function is estimated by narrowest fringes discernable
What is contrast sensitivity?
ability to detect changes in luminance
How does refractive error occur?
The secondary focal point (F’) of the eye is not located on the retina (accomodation must be completely relaxed)
What is the far point?
describe the focal lines produced by astigmatism
produces two focal lines rather than a single focal point
Describe “with the rule astigmatism”
- cornea is steepest in vertical meridian
- axis of plus cyl is 90
- usually in young patients (elastic lids press top and bottom of cornea)
Describe “against the rule” astigmatism
- cornea is steepest in the horizontal meridian
- axis of plus cyl is 180
- older patients
What are 3 causes of a hyperopic shift due to decreased effective axial length?
- retrobulbar tumor
- choriodal tumor
- CSCR
What are 5 causes of a hyperopic shift due to decreased refractive power?
- lens changes (IOL dislocation, aphakia, DM)
- drugs (chloroquine, phenothiazines, antihistamines, benzos)
- poor accomodation (tonic pupil, drugs, trauma)
- flat cornea (CL)
- intraocular silicone oil
What conditions or substances can increase lens power thus inducing aquired myopia?
- osmotic agents (DM, galactomsemia, uremia, sulfa drugs
- NS cataracts
- anterior lenticonus
- change lens position or shape (miotics)
- anterior lens dislocation
- over accomodation
Which conditions can induces myopia by increasing corneal power?
- keratoconus
- congenital glaucoma
- CL induced corneal warpage
Which conidtions can induce myopia by increasing axial length?
- congenital glaucoma
- posterior staphyloma
- scleral buckle
- ROP
What is night myopia?
increased myopia in the dark
How does pupil dilation contribute to night myopia?
larger pupil incresaes spherical aberration, uncovers irregular astigmatism
What is a Purkinje shift?
spectral sensitivity shifts toward shorter wavelengths at lower light levels, and chromatic aberration moves the focal point anteriorly
Describe dark focus
no accomodative targert in dark, therfore tend to overaccomodate for distance and underaccomodate for near
How does the length of the refraction lane effect your refraction?
a lane shorter than 20 ft produces 1/6 D undercorrection (add minus 0.25D to final refaction)
Name 7 causes of acquired astigmatism
- lid lesion (tumor, chalazion, ptosis)
- pterygium
- limbal dermoid
- corneal degeneration/ectasia
- surgery (corneal or cataract)
- lenticular astigmatism
- Ciliary body tumor
How does the eye accomodate?
eye gains plus power when crystalline lens becomes more convex
What is the amplitude of accomodation?
The total dioptric power that an eye can accomodate
What is Prince’s rule?
combies reading card with a ruler calibrated in cm and diopters to measure amplitude of accomodation?
Describe the technique for Prince’s rule measurement of amplitude of accomodation?
place =3d lens in front of distance correction to bring far point to 1/3mm (33cm) then measure how near the patient can read and convert to diopters. subtract far point from near point to determine amplitude
Describe the method of spheres for testing accomodation?
fixate on a reading target (40cm), successively increase minus sphere until print blurs, then increase plus sphere until blurring occurs again. absolute difference between spheres is the amplitude of accomodation
What is the range of accomodation?
distance between the far point and near point; measured with tape measure or accomodative rule
What is the far point of accomodation?
point on visual axis conjugate to the retina when accomodation is completely relaxed
What is the near point of accomodation?
the point on the visual axis conjugate to the retina when accomodation is fully active
How do you measure the near point for myopia?
near point = amount of myopia + amplitude of accomodation
How do you measure the near point of accomodation for hyperopia?
near point = difference between amplitude of accomodation and amount of hyperopia
What is presbyopia?
loss of accomodation with age, whcih becomes symptomatic in 40s with asthenopic symptoms and need for reading glasses
What are the to different theories of accomodation?
- Helmoholtz
- Tscherning- Schacher
Describe Helmholtz theory of accomodation
- zonular tension decreases
- lens becomes more spherical
- focusing power increases
- presbyopia due to loss of lens elasticity
Describe the Tscherning- Schacher theory of accomodation?
- equatorial zonular tension increases
- lens diameter increases
- central lens steepens
- focusing power increases
- lens growns thoughout life
- decreased distance between lens and ciliary body
- presbyopia is the result of decreased CB effectivity
Donders table shows what?
the average amplitude of accomodation for different ages
How much does accomodation decrease every 4 years before age 40?
accomodation decreases by 1D every 4 year (starting at 14D at age 8)
What is the amplitude of accomodation at age 40?
6D (+/- 2)
How much does accomodation decrease between ages 40 to 48?
by 1.5D every 4 years
How much does accomodation decrease over the age of 40?
By 0.5D every 4 years?
What is asthenopia?
eye fatigue with sustained near effort
What conditions cause asthnopia?
- hypothyoidism
- anemia
- pregnancy
- nutitional deficiencies
- chronic illness
What can cause premature presbyopia?
debilitating illness, diptheria, botulism, mercury toxicity, head injuries, CN3 palsy, Adie’s pupil, tranquilizers,
How do you treat premature presbyopia?
- reading add
- base in prism to help convergence
What are the 3 components of color?
- Hue
- Saturation
- Luminosity (lightness, brightness)
What is hue?
hue is the main component of color perception, hue depends on which wavelength was perceived as dominant.
What is hue descrimination?
the ability to distinguish between adjacent wavelengths
What is saturation?
saturation is the vidiness or richness of colors
adding white desaturates color but the hue stays the same.
What is saturation disctimination?
saturation discrimination is measured by how much of a specific wavelength must be added to white before the mixture can be distinguished from white
What is luminosity?
the sensation produced by retinal illumination
Depends on the realtive luminous efficiency of wavelengths
Filters decrease brigtness
What is a luminosity curve?
a graph that illustrates the ensitivity to different wavelengths
How is a luminosity curve constructed?
Made by asking an observer to increase luminance of lights of various wavelengths until they appear equal in birghtness to a yellow light of fixed luminance.
What is the peak sensitivity of a light adapted eye?
555nm
What is the peak sensitivity of a dark adapted eye?
505 nm (blue)
What is the Bezold-Brucke phenomenon?
As brightness increases, most hues appear to change.
Low intensities: blues greens yellows are greener, reds ad oranges are redder
High intensities: blues, yellows greens are bluer, reds oranges are yellower
What are the 3 exceptions to the Bezold-Brucke Phenomenon?
- blue or 478nm
- green of 583nm
- yellow of 578nm
do not change with changes in intensity
What is the Abney effect?
as white is added to any hue, desaturating it, the hue appears to change slightly in color, all colors except yellow appear yellower
What is an afterimage?
after a color is stared at for 20 sec, it begins to fade/desaturate
Then, when gazing at a white background, the complement of the color appears (this is an afterimage)
Why is white perceived as white?
Becuase white paint reflects all photons equally well
Why does charcoal appear black?
it absorbs most of the light that strikes it
Why is a blue flower blue?
blue flowers appear blue because it best absorbs yellow, red and green. Blue is absorbed least, so a greater number of blue photons are reflected
What substance makes a green leaf appear green?
chlorophyll which absorbs red and blue and reflects green.
How could a dress appear purple in incndescent light while looking blue in fluorescent light?
Incandescent/tungsten light emits a relatively greater number of photons of longer (red) wavelength than shorter (blue) wavelength.
Fluorescent light emits a greater number of blue and green wavelengths
What can be best used to uncover full refractive error in children and hyperopes?
cycloplegia
What is the average refraction of infants?
2D hyperopia
Between whuch ages is a myopic shift common in children?
ages 8-13
When should you prescribe glasses for a child with hyperopia?
When should you prescribe glasses for a child with myopia?
When should you prescribe glasses for a child with astigmatism?
What is the optically preferable location for placing the astigmatism correction on a lens?
On the rear surface (closer to the eye)
What is an astigmatic dial?
12 spokes corresponding to clock hours are projected on a screen.
The spokes parallel to principle meridians of eye’s astigmatism are sharp
What should you do with the base curve of new glasses compared with old?
Kepp the base curve the same as the old pair of glasses.
What tool is used to measure the base curve of a lens?
The Geneva lens clock
How does a Geneva Lens clock work?
the direct dioptric power of convex, concave, or aspheric lens surface is read on the dial of the clock
What is the calibration of a Geneva lens clock based on?
Refractive index of crown glass (1.52)
Name 3 methods to try to acheive binocular balance?
- Prism dissociation
- Balanced fogging
- Duochrome testing
What is prism dissociation?
3 base up prism over 1 eye and 3 base down prism over the other eye
(use Risley prism in phoropter)
What is balanced fogging?
fog both eyes and alternate cover until equally fogged.
What is the duochrome test based on?
what VA is required)
Red-green balance of both eyes (VA must be 20/30 )or better
Where should bifocal segments be placed in glasses?
place segments as high as practical in relatiioon to optical centers of distance lenses
What is Kestenbaum’s rule?
used to estimate the strength of a plus lens required to read newspaper print without accomodation
How do you calculate Kestenbaum’s rule?
Add power = reciprocal of best distance acuity
Reciprocal of add power= working distance (in meters)
What are the disadvantages of aphakic spectacles?
- 25% magnification
- altered depth perception
- pincushion distortion
- ring scotoma (prismatic effect at edge of lens causes visual field loss of 20%)
- jack in the box phenomenon (peripherally invisible bjects suddenly appear when gaze is shifted)
What is the SRK formula?
Name the values in the SRK formula
A = A constsant (related to lens type)
L = axial length in mm
K = average K value in D
How much error in lens power will occur if you are off by 1mm on the axial length?
2.5D error in IOL power
How much error in lens power will occur if you K measurements are off by 1D?
1.25D error in lens power
How much error in lens power will occur if you are off in lens power by 1mm?
1.0 D change in power
Vergence equation
Lens power equations (in diopters)
Snell’s law equation
Prismatic power equation
Prentice’s rule equation
Reduce schematic eye equation
(calculations of retinal image size)
Spherical equivalent equation
Refracting power of a spehrical equivalent
Reflecting power of a spherical mirror
Power of a thin lens immersed in fluid
Power of a lens at a new vertex distance:
Linear magnificantion equation:
Axial magnification equation
Angular magnification
Angular magnification equation
Simple magnifier equation
Telescope (Galilean and astronomical)
Total accomidation through a telescope equation
SRK IOL power equation:
AC/A ratio:
(accomodative convergence/ccomodation)
AC/A ratio Heterophoria method:
AC/A ratio
Lens gradient method
