lec 13: monocular aberration Flashcards
What kind of optical defects do eyes have?
- Sphere and cylinder refractive error
- Chromatic aberrations (LCA & TCA)
- monochromatic aberration (zernike forms)
- Diffraction
are chromatic and monochromatic aberrations easily correctable?
no
seidel aberration
off axis astigmatism, curvature of field, distortion
zernike describes what?
wavefront aberrations
is focal point a single point in real eye?
no! its aberrated
Impact of higher-order aberrations on light entering the eye
causes multiple focal points
Impact of higher-order aberrations on light exiting the eye is what?
the same as that entering
in real eye, wavefront is spherical but after light leaves it, its not planar anymore
zernike low order aberrations
1 and 2 order
zernike high order aberrations
3,4,5 order
zenike polynomials

set of basic shapes that are used to fit the wavefront
in clinic what are we measuring for zernike polynomials?
second order (sphere and cylinder)
do optometrists correct high order aberrations?
no
first order of zernike polynomials
prism (not important to us)
how do we write zernike polynomials?
Z (m/n)
Each individual aberration will have a unique impact on what?
on retinal image quality
whats the dominant optical aberration?
2nd order
relationship of zernike order to aberration of the optics
inverse relationship
2nd order has more of an effect on eye’s optics compared to 5th order
terms are not similar in any way, so the weighting of one term does
not depend on whether or not other terms are being fit
orthogonal
the RMS (root-mean-square) wave aberration can be simply calculated as the vector of all or a subset of coefficients
normalized
Zernike shapes are very similar to typical aberrations found in the eye (ie principal components)
efficient
properties of zernike polynomials
orthogonal
normalized
efficient
Recent data confirm that correcting HOAs (monochromatic aberrations) can dramatically improve what?
monochromatic VA, but need an AO mirror.
20/10 is not possible with normal levels of monochromatic aberrations (t/f)
true (optics always have aberration)
By correcting aberrations, can clinician see finer detail in the fundus image?
cannot see capillaries or individual cells
principle of adaptive optics
deformable mirror corrects wavefront
does AO improve cone image?
yes
increase defocus, astigmatism, and spherical aberration, will decrease what?
optics ((lower zernikes) defocus, astigmatism, and spherical aberration have the most effect compared to other zernikes)
lower zernikes affect optics more than higher zernikes (t/f)
t
spherical aberration (zernike number)
12, (z4,0)
present in all spherical optical surfaces/lenses) is variation in focus (power) as a function of ray height from pupil center.
spherical aberration
Spherical aberration in diopters, LSA, equals what?
difference in vergence between peripheral rays (y2) and paraxial (central) rays (y1) in image space.
Spherical aberration is considered _______ if peripheral rays focus in front of paraxial rays.
positive
Magnitude of spherical aberration is much larger in classical schematic eyes (with spherical surfaces) than
in what?
real human eyes
peripheral rays focus more (in front) than central rays (t/f)
t
n variation in fish lens _______ its optical power and ________ spherical aberration
increases, reduces
gradient refractive index does what to positive SA?
reduces
n is high in center of lens, what does that do?
central light rays focus more (in front), reduces positive SA
Isolated crystalline lens has more power near the optical axis than near the equator. This cause ______ spherical aberration (SA) of crystalline lens itself.
negative
For crystalline lens, accommodation causes a larger increase in refractive power centrally than at the margins. Therefore, spherical aberration becomes_________ during accommodation.
even more negative
Positive spherical aberration of _______ is balanced partially by negative aberration of lens.
cornea
center of cornea is steep, while periphery is what?
flat
Cornea is ____ curved at its periphery than at its center.
This asphericity reduces the amount of positive spherical aberration.
less
Typical eye has _________ when relaxed, but _________ when accommodating.
positive SA, negative SA
index of refraction of lens is _____ near its power surface than at its center. This gradient of index produces _______.
lower, negative SA
effect of accommodation on spherical aberration
increase accommodation, decrease spherical aberration (becomes negative)
SA is minimized by about ____ accommodation on average.
1.5 D
This change in sign (when accommodating) is due what?
the change in balance between positive corneal aberration and negative lens aberration.
3rd order aberrations can
be used as sensitive
method for diagnosing ___________.
keratoconus
Zernike Spectrum for normal eyes without correction of lower order aberrations (sphere and astigmatism). Notice that 93% of RMS comes from ___________!
sphere and cylinder in uncorrected eyes
How are aberrations measured subjectively?
! Vernier alignment
! Spatially resolved refractometer
! Subjective aberroscope
How are aberrations measured objectively?
! Objective aberroscope
! Laser ray tracing (Aberrometer)
! Shack-Hartmann wavefront sensor
Principles of the Shack-Hartmann Wavefront Sensor
Sub-divide the wavefront with micro-lenslets.
Local slope determines spot position on video sensor.
what happens when wavefront is aberrated using the shack hartmann sensor?
will have new focal point from where it should be
The local slope (or the first derivative) of the wavefront is determined at what?
lenslet location
• The _______________ is determined by a least squares fitting of the slopes to the derivative of a polynomial selected to fit the wavefront
corresponding wavefront
____________ is one of the most commonly used way to describe eye’s wavefront aberrations
zernike polynomial
which instrument is likely to give the most info about the optics of the eye?
aberrometer
in most eyes high order aberrations are large compared to low order aberration. (t/f)
false
high order aberrations decline with radial order and vary significantly from person to person (t/f)
true
secondary astigmatism is one of the 4th order aberrations
true
12th mode in zernike polynomials is what?
spherical aberration
