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Optical Science 6 > lec 13: monocular aberration > Flashcards

lec 13: monocular aberration Flashcards

1
Q

What kind of optical defects do eyes have?

A
  1. Sphere and cylinder refractive error
  2. Chromatic aberrations (LCA & TCA)
  3. monochromatic aberration (zernike forms)
  4. Diffraction
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2
Q

are chromatic and monochromatic aberrations easily correctable?

A

no

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3
Q

seidel aberration

A

off axis astigmatism, curvature of field, distortion

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4
Q

zernike describes what?

A

wavefront aberrations

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5
Q

is focal point a single point in real eye?

A

no! its aberrated

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6
Q

Impact of higher-order aberrations on light entering the eye

A

causes multiple focal points

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7
Q

Impact of higher-order aberrations on light exiting the eye is what?

A

the same as that entering

in real eye, wavefront is spherical but after light leaves it, its not planar anymore

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8
Q

zernike low order aberrations

A

1 and 2 order

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9
Q

zernike high order aberrations

A

3,4,5 order

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10
Q

zenike polynomials

A



set of basic shapes that are used to fit the wavefront

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11
Q

in clinic what are we measuring for zernike polynomials?

A

second order (sphere and cylinder)

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12
Q

do optometrists correct high order aberrations?

A

no

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13
Q

first order of zernike polynomials

A

prism (not important to us)

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14
Q

how do we write zernike polynomials?

A

Z (m/n)

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15
Q

Each individual aberration will have a unique impact on what?

A

on retinal image quality

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16
Q

whats the dominant optical aberration?

A

2nd order

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17
Q

relationship of zernike order to aberration of the optics

A

inverse relationship

2nd order has more of an effect on eye’s optics compared to 5th order

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18
Q

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

A

orthogonal

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19
Q

the RMS (root-mean-square) wave aberration can be simply calculated as the vector of all or a subset of coefficients

A

normalized

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20
Q

Zernike shapes are very similar to typical aberrations found in the eye (ie principal components)

A

efficient

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21
Q

properties of zernike polynomials

A

orthogonal
normalized
efficient

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22
Q

Recent data confirm that correcting HOAs (monochromatic aberrations) can dramatically improve what?

A

monochromatic VA, but need an AO mirror.

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23
Q

20/10 is not possible with normal levels of monochromatic aberrations (t/f)

A

true (optics always have aberration)

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24
Q

By correcting aberrations, can clinician see finer detail in the fundus image?

A

cannot see capillaries or individual cells

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25
Q

principle of adaptive optics

A

deformable mirror corrects wavefront

26
Q

does AO improve cone image?

A

yes

27
Q

increase defocus, astigmatism, and spherical aberration, will decrease what?

A

optics ((lower zernikes) defocus, astigmatism, and spherical aberration have the most effect compared to other zernikes)

28
Q

lower zernikes affect optics more than higher zernikes (t/f)

A

t

29
Q

spherical aberration (zernike number)

A

12, (z4,0)

30
Q

present in all spherical optical surfaces/lenses) is variation in focus (power) as a function of ray height from pupil center.

A

spherical aberration

31
Q

Spherical aberration in diopters, LSA, equals what?

A

difference in vergence between peripheral rays (y2) and paraxial (central) rays (y1) in image space.

32
Q

Spherical aberration is considered _______ if peripheral rays focus in front of paraxial rays.

A

positive

33
Q

Magnitude of spherical aberration is much larger in classical schematic eyes (with spherical surfaces) than
in what?

A

real human eyes

34
Q

peripheral rays focus more (in front) than central rays (t/f)

A

t

35
Q

n variation in fish lens _______ its optical power and ________ spherical aberration

A

increases, reduces

36
Q

gradient refractive index does what to positive SA?

A

reduces

37
Q

n is high in center of lens, what does that do?

A

central light rays focus more (in front), reduces positive SA

38
Q

Isolated crystalline lens has more power near the optical axis than near the equator. This cause ______ spherical aberration (SA) of crystalline lens itself.

A

negative

39
Q

For crystalline lens, accommodation causes a larger increase in refractive power centrally than at the margins. Therefore, spherical aberration becomes_________ during accommodation.

A

even more negative

40
Q

Positive spherical aberration of _______ is balanced partially by negative aberration of lens.

A

cornea

41
Q

center of cornea is steep, while periphery is what?

A

flat

42
Q

Cornea is ____ curved at its periphery than at its center.

This asphericity reduces the amount of positive spherical aberration.

A

less

43
Q

Typical eye has _________ when relaxed, but _________ when accommodating.

A

positive SA, negative SA

44
Q

index of refraction of lens is _____ near its power surface than at its center. This gradient of index produces _______.

A

lower, negative SA

45
Q

effect of accommodation on spherical aberration

A

increase accommodation, decrease spherical aberration (becomes negative)

46
Q

SA is minimized by about ____ accommodation on average.

A

1.5 D

47
Q

This change in sign (when accommodating) is due what?

A

the change in balance between positive corneal aberration and negative lens aberration.

48
Q

3rd order aberrations can
be used as sensitive
method for diagnosing ___________.

A

keratoconus

49
Q

Zernike Spectrum for normal eyes without correction of lower order aberrations (sphere and astigmatism). Notice that 93% of RMS comes from ___________!

A

sphere and cylinder in uncorrected eyes

50
Q

How are aberrations measured subjectively?

A

! Vernier alignment
! Spatially resolved refractometer
! Subjective aberroscope

51
Q

How are aberrations measured objectively?

A

! Objective aberroscope
! Laser ray tracing (Aberrometer)
! Shack-Hartmann wavefront sensor

52
Q

Principles of the Shack-Hartmann Wavefront Sensor

A

Sub-divide the wavefront with micro-lenslets.

Local slope determines spot position on video sensor.

53
Q

what happens when wavefront is aberrated using the shack hartmann sensor?

A

will have new focal point from where it should be

54
Q

The local slope (or the first derivative) of the wavefront is determined at what?

A

lenslet location

55
Q

• The _______________ is determined by a least squares fitting of the slopes to the derivative of a polynomial selected to fit the wavefront

A

corresponding wavefront

56
Q

____________ is one of the most commonly used way to describe eye’s wavefront aberrations

A

zernike polynomial

57
Q

which instrument is likely to give the most info about the optics of the eye?

A

aberrometer

58
Q

in most eyes high order aberrations are large compared to low order aberration. (t/f)

A

false

59
Q

high order aberrations decline with radial order and vary significantly from person to person (t/f)

A

true

60
Q

secondary astigmatism is one of the 4th order aberrations

A

true

61
Q

12th mode in zernike polynomials is what?

A

spherical aberration

Optical Science 6 (13 decks)

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