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Page 3.34. Visual Optics. Chapter 3 Retinal Image Quality. The Monochromatic Wavefront Aberration: Key Points so Far. Corneal refractive surgery can increase (conventional) or decrease (wavefront-guided) ocular aberrations Aberrometers measure the eye’s wavefront aberration

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visual optics

Page 3.34

Visual Optics

Chapter 3

Retinal Image Quality

the monochromatic wavefront aberration key points so far
The Monochromatic Wavefront Aberration:Key Points so Far
  • Corneal refractive surgery can increase (conventional) or decrease (wavefront-guided) ocular aberrations
  • Aberrometers measure the eye’s wavefront aberration
  • Adaptive Optics systems compensate for the eye’s wavefront aberration:
    • feedback about aberrated wavefront drives deformable mirror shape change to compensate
    • Used in ophthalmic imaging systems (e.g. SLO)
    • Used to demonstrate potential acuity by smoothing the eye’s aberrated image (e.g. keratoconus)
  • Zernike function (polynomial expansion) breaks net wavefront aberration into a series of components
    • Each component describes a feature of the overall wavefront
slide3

This

becomes

This

q1 the purpose of the deformable mirror in an ocular ao adaptive optics system is to
Q1. The purpose of the deformable mirror in an ocular AO (adaptive optics) system is to:
  • Compensate for the eye’s wavefront aberration
  • Remove diffraction from the eye’s PSF
  • Sculpt the patient’s cornea using high energy excimer photons
  • Measure the eye’s wavefront aberration
555 nm mono light 6 mm pupil
555 nm Mono- Light; 6 mm Pupil

Outer Functions produce LESS acuity loss

seidel third order monochromatic aberrations
Seidel (Third Order, Monochromatic) Aberrations

Page 3.40

  • Seidel approach more manageable
  • Produces less terms (5 only)
  • Covers central Zernike terms (SA, coma, secondary astigmatism); the ones producing greatest image degradation
slide7

Seidel Approach: Wavefront Shape in Exit Pupil & Image Plane

  • Paraxial Optics predicts that an axial point object produces an axial point image

r

Page 3.40

Figure 31 – Relationship between wavefront coordinates in the (exit) pupil plane (x, y, z) and image plane (x0, y0, z0). r = wavefront radius of curvature.

slide8

Seidel Approach: Wavefront Shape in Exit Pupil & Image Plane

For the ideal wavefront, all locations in the exit pupil would converge to (x0 y0 z0 ) at the paraxial image point

r

Page 3.40

Figure 31 – Relationship between wavefront coordinates in the (exit) pupil plane (x, y, z) and image plane (x0, y0, z0). r = wavefront radius of curvature.

slide9

An aberrated wavefront does not converge to x0 y0 z0 (paraxial image point).

Different parts of the wavefront converge to different locations in image space

slide10

x

x0

Defining Wavefront Shape in Exit Pupil Plane

Based on page 3.40

Exit Pupil

Paraxial image plane

Object plane

Most important wavefront attributes to quantify mono-chromaticaberrations:

1. Aperture (): distance from center of ExP

2. Meridian (): in exit pupil (measured CC-wise from horizontal)

3. Off-axis position (): must cover both on- and off-axis object points

slide11

y

x

Coordinates in Exit Pupil: Wave at Oblique Angle

Page 3.40

Paraxial image plane

W

z

Object plane

Exit Pupil

Defining wavefront position as a longitudinal distance (W) from the exit pupil plane at aperture height () and meridian ()

slide12

y

x

x0

x

Coordinates in Exit Pupil (and displacement in image plane): Off-axis Object Point

Page 3.40

Paraxial image plane

Object plane

Exit Pupil

For an off-axis object point, how does the image point vary from the paraxial prediction, x0 ?

slide13

y

x

Ideal Wavefront Shape in Exit Pupil

Page 3.40

W

Paraxial image plane

W

z

Object plane

Exit Pupil

The ideal longitudinal distance (W) from the exit pupil plane for all apertures () and meridians () would match that of a spherical wavefront centered on the corresponding paraxial image point

slide14

Ideal Wavefront (Spherical)

Actual Wavefront – Seidel Aberrations (third order)

Ideal vs Aberrated Wavefront

Page 3.41

Generate monochromatic aberration by replacing paraxial simplification of Snell’s Law: ni = n i with true form: n sin i = n sin i

slide15

Seidel Aberrations: Aperture (), Angular () and Object Height (0) dependence

Page 3.41

Which aberrations are aperture-dependent?

Spherical aberration and Coma (aperture dependence > 2)

slide16

Seidel Aberrations: Aperture (), Angular () and Object Height (0) dependence

Page 3.41

Define the off-axis aberrations:

q2 identify the off axis aberrations most complete correct answer
Q2. Identify the “off-axis” aberrations (most complete, correct answer)
  • SA, coma & distortion
  • Coma, OA astigmatism & distortion
  • Coma, OA astigmatism, field curvature & distortion
  • SA, coma, OA astigmatism, field curvature & distortion
slide18

Seidel Aberrations: Aperture (), Angular () and Object Height (0) dependence

Page 3.41

Define the off-axis aberrations:

Coma, off-axis astigmatism, field curvature and distortion (all have an 0 term).

slide19

Seidel Aberrations: Aperture (), Angular () and Object Height (0) dependence

Page 3.41

Which are the meridionally-dependent aberrations?

Coma, OAA ( cos2; greatest meridional variation) and distortion

Significance of no meridional dependence of SA and field curvature?

Symmetrical image

slide20

Off-axis Astigmatism

Coma

Point-spread functions: Which are the meridionally-dependent aberrations?

Spherical aberration

Ideal wavefront

Airy disc pattern

slide22

Spherical Aberration: Ray Diagram

Page 3.43

Figure 3.36 – Spherical aberration

quantifying spherical aberration
Quantifying Spherical Aberration

Page 3.44

  • Longitudinal Spherical Aberration (LSA)
  • Transverse Spherical Aberration (TSA)
slide24

Longitudinal Spherical Aberration (LSA)

Page 3.44

Ideal spherical wavefront

Note: in Geometrical Optics, the symbol “y” is often used for aperture diameter instead of 

slide25

Marginal Focus

Marginal Focus

Marginal Focus

LSA

Figure 3.37 – LSA for (a) small, (b) medium, and (c) large pupil

Page 3.45

NOTE: figures assume a spherical reduced surface

slide26

Q3. How does the assumption of spherical reduced surface curvature affect the estimate of longitudinal spherical aberration (compared to a typical real eye)?

  • Underestimated
  • Accurately estimated
  • Unrelated
  • Overestimated
slide29

Calibration Sphere:“Power” vs. Incident Height

Myopic Real Cornea: “Power” vs. Incident Height

54.00 D  46.91 D = 7.09 D

50.75 D  44.71 D = 5.04 D