Clinical Optics Flashcards
Given lens power in diopters, object distance from lens and location of image. How would you calculate the distance from the image to lens?
U + P= V
U = object vergence (always negative)
P = power of lens/mirror
V = vergence of light exiting the lens/mirror to form image
How do you find the object and image vergences?
Reciprocal of distances from the lens/mirror respectively
Spherical equivalent given principle meridians
Average cylindrical powers of a lens.
How do spherical lenses focus light rays?
Focus light rays around the same point producing a BLUR CIRCLE that can be mathematically described by a point spread function.
Image height divided by object height
Transverse magnification
Simple magnifiers
Angular magnification
How do you calculate the magnification produced by simple magnifiers?
Magnifier is held 25 cm from objects and magnification produced is the dioptric power of magnifier divided by 4
Axial magnification
AKA longitudinal magnification
Calculated by square of transverse magnification
How do you calculate transverse magnification?
Transverse magnification (“M”) is given by M = U/V
U is incoming vergence (often from object)
V is output vergence of light leaving lens
“MUV’in out” – M = U/V = i/o “i” is image distance and “o” is object distance from lens of interest.
What is the vergence of light rays emanating from an object left of the lens?
Negative, and reciprocal of its distance from the lens
Kestenbaum’s rule
initial add will be the inverse of the Snellen acuity for the BETTER SEEING EYE (if Snellen acuity were 20/40 OD and 20/100 OS, then add would be +2.00)
Refractive change when oil is placed in eye
Phakic or pseudophakic - hyperopic shift of 3-5 diopters because molds to back of lens producing concave shape (negative powered lens)
Aphakic - variable but convex shape acting as positively powered lens and produces a myopic shift.
Refractive index of crown glass
1.523
Refractive index of aqueous
1.336
Refractive index of the cornea
1.376
Refractive index of acrylic
1.46
Refractive index of silicone oil
1.438
tendency for light to turn or bend when passing by an edge
Diffraction
How does a pinhole work?
limiting light rays entering the eye to those which are mostly perpendicular to the lens and cornea and therefore not refracted before projecting on to the retina.
Uses chromatic aberration to help refine the spherical component of a person’s refractive error
Duochrome test
How do you interpret results of a duochrome test?
if red side is better, current refraction is too hyperopic and more minus power must be added. If Green better, too myopic and more plus power added.
“RAM-GAP”
Shorter wavelengths (green) focused more anteriorly. Longer wavelengths (red) focused more posteriorly.
Duochrome can’t be used on this patient
VA worse than 20/30, since small distance (0.50 diopters) between red and green sides is too small to distinguish.
Where does a positive cylinder lens extert focusing power?
exerts all focusing power 90 degrees away from axis of placement and no focusing power along its axis of placement.
Principle of AcuFocus corneal inlay
Pinholes allow increased depth of field. Ring inserted under LASIK flap in nondominant eye after LASIK. Opaque and matches pupil size. At near, pupil constricts only light rays inside inlay brought into focus, increasing depth of field.
Myopic glasses help
convergence insufficiency - when glasses wearing myopic patient is accomodating, the lenses act as base in prism
Hyperopic glasses help…
Accomodative esotropia - lens act as base out prism
normal AC/A ratio
3:1 to 5:1
How would you place a prism to correct a left hypotropia?
Base-UP prism in front of eye. Apex of prism is pointed toward the eye deviation. Apex down, base up.
Formula needed for multiple lens ray-tracing problems. Given power of lens and mirror and distance of first object, calculate distance of new object..
U + P = V
U - vergence of object, always negative (reciprocal of distance from lens)
P = power of lens
V = vergence of light exiting lens to form an image (reciprocal of distance from lens)
Calculate magnification given lens power, distance of object from lens with virtual image.
M = i/o where i is image distance from lens and o is object distance; distances are negative left of lens
or
M = U/V where V = U + P (vergence is reciprocal of distance from lens) U is vergence of object (always negative) and V is vergence of image.
Positive magnification means upright image
How do you make a galilean telescope?
Positive (low power) objective and negative (high power) eyepiece. Lenses separated by differences in their focal lengths. Form upright images.
How do you build a Keplerian (astronomical) telescope?
Positive objective and positive eyepiece. Form inverted images. Lenses separated by SUM of their focal lengths (longer)
Prism diopter bends light
From optical axis at 1 meter in centimeters
Rule for fitting rigid contact lens
‘SAM FAP’
Steep add minus
Flat add plus
For every 0.05 mm of radius, the sphere is adjusted +0.25 diopter
(If you fit to the steep or flat axis)
Q value of cornea
Asphericity of cornea. Q = 0 is spherical. Q < 0 prolate (flatter periphery); Q > 0 oblate (steeper periphery)
Ideal Q value to produce stigmatic image
-0.50, average cornea is -0.26. All light rays come together at central point on retina
Wavelength of light inside a coating?
wavelength = wavelength in air / n where n is index of refraction in medium
Ani-reflective coatings require
one quarter wavelength for destructive interference.
Wavelength of excimer laser in refractive surgery
193 nm which is (UV) C range.
Argon-fluoride laser
6.4 eV per photon, low tissue penetrance
Photoablation
Energy required to break carbon-carbon bond
4 eV per photon or greater
wavelength of femtosecond laser
1053 nm (infrared)
Wavelength of Nd:YAG laser
1064 nm
Femtosecond and YAG lasers utilize
photodisruption (tissue is transformed into plasma which causes tissue to rapidly expand and form microscopic cavities)
Convergence amplitudes
~14 prism diopters
Divergence amplitudes
~6 prism diopters
Prisms pointing in same direction
subtract for overall effect
Prisms pointing in opposite direction
ADD for overall effect
Convex mirror can only form
virtual images on the opposite side from the object. Power is always negative.
Galilean telescopes always consist of
lower plus power objective lens and high power negative eyepiece lens
Distance between lenses (Galilean telescope)
1/P of objective - 1/P of ocular (ignore positive or negative signs for power of lenses)
Describes the farthest point an eye can view clearly
far point
Diffraction limits the VA in a person’s eye when the pupil size is below what threshold diameter?
2.5 mm
Image formed on retina from a distant small source that takes the form of concentric light and dark rings surrounding a bright central disk.
Airy disc
d=2.44fA/a (where d=central disc, f = focal length of eye, A=wavelength, a=iris diameter) Resolution is determined by inverse of d
Transverse magnification of any telescope
= power of eyepiece/power of objective
Distance between lenses of an astronomical telescope
two plus lenses are always separated by the sum of their focal lengths
Excimer laser is in this light spectrum
Ultraviolet (UV) 193 nm
Axial magnification (longitudinal) is calculated by
square of transverse magnification
To create a tighter contact lens fit
1) Decrease the base curve (steeper)
2) Increase the diameter
(power change has no effect)
Instrument used to measure accommodative amplitudes
Prince rule
Direction light is predominately polarized after reflecting off a road surface
Horizontally. Sunglasses are vertically polarized.
Lens only used while patient is supine (in OR)
Koeppe lens
Lens that uses 4 mirrors to reflect image without coupling agent
Zeiss gonio lens
Indirect gonio lens that uses a coupling agent making dynamic gonioscopy difficult
Goldmann lens
Non-contact, high powered plano concave minus 55 diopter lens used to vew the fundus, attachment to slit lamp
Hruby lens, creates erect image of retina with high magnification (FOV is one DD)
Degree at which the power acts AT
Axis
Degree in which the power is placed in
Meridian
Lenses are separated in a Galilean telescope by
1/P of objective - 1/P of ocular (ignore positive or negative signs). (Objective is the positive, ocular is the negative)
Lenses are separated by what distance in an astronomical (Keplerian) telescope
1/P of objective + 1/P of ocular
Type of lenses that make up a Galilean telescope
Low power plus objective and high power negative power ocular
Type of lenses that make up a Keplerian (Astronomical) telescope
Low plus power objective and high plus power ocular
Problem with silicone IOLs
If risk for RD and silicone oil in eye, form condensation on posterior surface of lens making visualization of posterior segment difficult.
Power of a mirror
Dmirror = 2/r or 1/f (r is radius of curvature and f is focal length, D is power of mirror)
Image jump
occurs when eyes look from distance correction to the bifocal segment, since eyes encounter new plus lens with different optical center
How do you minimize image jump
flat-top segments, since optical center of bifocal segment is closer to optical center of lens
How do you eliminate image jump
Executive-tpe bifocal segments where optical center is located at top of bifocal segment
Image displacement
prismatic effect induced by combination of bifocal type and power of distance lens prescription in reading position. F
How do you minimize image displacement in a plus-powered lens and minus power lens?
Plus -> round-top bifocal segments
Minus -> flat-top bifocal segment
