Basics Of Retinal Image Quality - University of...

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Basics Of Basics Of Retinal Image Retinal Image

QualityQuality

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The optics of the eye are The optics of the eye are the first stage of vision. the first stage of vision.

It is an extremely It is an extremely important stage but not important stage but not

the only stage.the only stage.

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The optical quality of the The optical quality of the retinal image is defined by retinal image is defined by

pupil size and:pupil size and:

DiffractionDiffraction

Wave AberrationWave Aberration

ScatterScatter

Chromatic AberrationChromatic Aberration© RAA

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DiffractionDiffractionWave AberrationWave AberrationScatterScatterChromatic AberrationChromatic Aberration

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DiffractionDiffraction

“Any deviation of light rays from a rectilinear path which cannot be interpreted as reflection or refraction.”

Sommerfeld, ~ 1894

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Wavefronts connect points having the same phase.

Figure 5-1 from MacRae, Krueger and Applegate, Customized Corneal Ablation: The Quest for Super Vision, Slack, Inc. 2001.

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To understand diffraction, To understand diffraction, it is necessary to understand it is necessary to understand the behavior of a wavefront the behavior of a wavefront

as it passes through an as it passes through an aperture or by edge.aperture or by edge.

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This eye can see the light.

aper

ture

aper

ture

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But not seen if viewed from here.

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aper

ture

aper

ture

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But, the light can be dimly But, the light can be dimly seen. seen.

Light is apparently bent by Light is apparently bent by the aperture.the aperture.

How can this be explained?How can this be explained?

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Huygens postulated that every point on a wavefront was the

source of a secondarywavefront.

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For an unbounded system (no pupil), interference effects cause the light to

propagate only in the original direction.

However, for a bounded wavefront (with pupil), the effects do not cancel.

bounded

Pupil

unbounded

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Light from the wavelets can reach the eye even though a straight line from the eye to the point source does not pass through the aperture.

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A special and particularly interesting A special and particularly interesting case of Fresnel diffraction, called case of Fresnel diffraction, called

Fraunhofer diffraction, occurs in the Fraunhofer diffraction, occurs in the focal plane of an aberrationfocal plane of an aberration--free or free or

nearly aberrationnearly aberration--free imaging free imaging system.system.

The Fraunhofer diffraction pattern of The Fraunhofer diffraction pattern of an axial point source defines the an axial point source defines the

appearance of the point source in the appearance of the point source in the image plane.image plane.

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Airy disc

Fraunhofer diffraction defines

the diffraction limited point spread

function (PSF).

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The diameter of the Airy disc varies The diameter of the Airy disc varies with pupil diameter.with pupil diameter.

The radius of the Airy disc The radius of the Airy disc increases as pupil size decreases increases as pupil size decreases

by the following formula.by the following formula.

anfr

''22.1 λ

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As the radius of the Airy disk As the radius of the Airy disk decreases the higher the decreases the higher the

fidelity of the retinal image. fidelity of the retinal image. Said differently, in a perfect Said differently, in a perfect optical system the larger the optical system the larger the pupil, the better the image.pupil, the better the image.

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Diffraction PSFs for pupil diameters 1 - 8

1 mm 2 mm 3mm 4 mm

5 arc min.

5 mm 6 mm 7 mm 8 mm© RAA

Resolved

Rayleigh resolution

limit

Unresolved point sources

Courtesy Austin Roorda

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Any scene is a collection of points. Any scene is a collection of points. The image of a scene is The image of a scene is

represented by the point spread represented by the point spread function of every point in the function of every point in the

scene. So what we learn about a scene. So what we learn about a point image can be used to point image can be used to

simulate the image of an object.simulate the image of an object.

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Defocus = 0 D; RMS WFE = 0 µmAiry disc diameter = 2.8 µm

PSF

20/40

20/205 arc min.5 arc min.

20/12

Pupil Diameter = 8.00 mm© RAA

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20/40


20/12


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20/40


20/12


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20/40


20/12


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20/40


20/12


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20/40


20/12


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Clinical Implications of Clinical Implications of Diffraction Include:Diffraction Include:

Diffraction fundamentally defines the Diffraction fundamentally defines the upper limits of retinal image quality.upper limits of retinal image quality.

Diffraction effects increase as pupil size Diffraction effects increase as pupil size decreases making the quality of the decreases making the quality of the retinal image poorer and poorer.retinal image poorer and poorer.

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Simple Myopia© RAA

Simple Myopia with Optical Correction

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In reality it is not so In reality it is not so simple.simple.

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The eye has higher order The eye has higher order wave aberrations that wave aberrations that become increasingly become increasingly manifest as the pupil manifest as the pupil diameter increases.diameter increases.

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1mm 2mm 3mm 4mm

Diffraction-Limited System

Normal Eye with

Typical Aberrations

5mm 6mm 7mm 8mm© RAA

Diffraction-Limited System

Normal Eye with

Typical Aberrations

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For many clinical eyes (ie. For many clinical eyes (ie. keratoconics), it is important to keratoconics), it is important to

correct the higher order correct the higher order aberrations. aberrations.

For the normal eye, the gains For the normal eye, the gains obtained by correcting higher obtained by correcting higher

order aberrations are primarily for order aberrations are primarily for large pupil sizes and diminish as large pupil sizes and diminish as

the pupil size gets small.the pupil size gets small.

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In the past, a patient’s eye In the past, a patient’s eye which could not be corrected which could not be corrected

with conventional spherowith conventional sphero--cylindrical corrections, was cylindrical corrections, was

often dismissed with a diagnosis often dismissed with a diagnosis of irregular astigmatism. of irregular astigmatism.

We are now in a position to We are now in a position to attempt to correct these eyes. attempt to correct these eyes.

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Clinical Implications of Wave Clinical Implications of Wave Aberrations Include:Aberrations Include:

The adverse effects on retinal image quality of The adverse effects on retinal image quality of wave aberrations in the normal healthy eye wave aberrations in the normal healthy eye increase with pupil diameter.increase with pupil diameter.Retinal image quality has the highest fidelity for Retinal image quality has the highest fidelity for pupil diameters around 3mm.pupil diameters around 3mm.The effects of diffraction cause most eyes to see The effects of diffraction cause most eyes to see the same for pupil diameters < 2mm.the same for pupil diameters < 2mm.Correlations between visual performance and Correlations between visual performance and wavefront aberrations must be made for the same wavefront aberrations must be made for the same pupil size.pupil size.

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Clinical Implications of Scatter Clinical Implications of Scatter Include:Include:

Despite the availability of a surgical cure, Despite the availability of a surgical cure, scatter resulting from cataract is the leading scatter resulting from cataract is the leading cause of legal blindness in the world.cause of legal blindness in the world.Scatter decreases image quality by “washing Scatter decreases image quality by “washing out” spatial detail in the retinal image.out” spatial detail in the retinal image.

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The speed of light in in the eye varies with the wavelength of light. The

shorter the wavelength the slower the speed of light. Since the index of

refraction is the ratio of the speed of light in a vacuum to the speed of light

in the new medium, the index of refraction is greater for short

wavelengths than it is for longerwavelengths.

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As light enters the eye, the As light enters the eye, the higher the refractive index the higher the refractive index the greater the angle of refraction greater the angle of refraction

as dictated by Snell’s law. as dictated by Snell’s law.

i

'in 'n 'sin'sin inin =

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1D

The difference in longitudinal chromatic aberration between 486 and 656 nm is just over 1 D.

Adapted from Bennet & Rabbetts, 1989© RAA

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Clinical Implications of Clinical Implications of Chromatic Aberration Include:Chromatic Aberration Include:

Chromatic aberration degrades the retinal image.Chromatic aberration degrades the retinal image.The degradation in image quality is partially offset The degradation in image quality is partially offset by the spectral sensitivities of the receptors.by the spectral sensitivities of the receptors.Chromatic aberration is capitalized on by the Chromatic aberration is capitalized on by the duochromeduochrome test.test.The adverse effects of low to moderate levels of The adverse effects of low to moderate levels of mononchromaticmononchromatic aberrations are partially offset by aberrations are partially offset by chromatic aberration.chromatic aberration.

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In summary, In summary, diffraction, diffraction,

wave aberrations, and wave aberrations, and scatter along with scatter along with

pupil diameter affect the pupil diameter affect the optical quality of the optical quality of the

retinal image. retinal image.

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Basic Module 2 will introduce:Basic Module 2 will introduce:

Wavefront error.Wavefront error.The specification of wavefront error.The specification of wavefront error.Why wavefront error is important in the Why wavefront error is important in the design of ideal corrections.design of ideal corrections.The advantage of specification of optical The advantage of specification of optical errors in terms of wavefront error as opposed errors in terms of wavefront error as opposed to dioptric error.to dioptric error.

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This module was written and produced by:This module was written and produced by:

Raymond A. Applegate, OD, PhDRaymond A. Applegate, OD, PhD

The Visual Optics InstituteThe Visual Optics InstituteCollege of OptometryCollege of OptometryUniversity of HoustonUniversity of Houston

HoustonHouston, TX, TX

Dr. Applegate gratefully acknowledges Larry N. Thibos, PhD, School of Optometry, Indiana University for reviewing and offering constructive criticism on the contents of the module.

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AcknowledgementsAcknowledgements

Supported by:Supported by:–– Alcon, Inc.Alcon, Inc.–– Visual Optics Institute, College of Optometry, Visual Optics Institute, College of Optometry,

University of HoustonUniversity of HoustonThanks to:Thanks to:–– Rachel ApplegateRachel Applegate–– Lisa BoggsLisa Boggs–– Ilona KurandaIlona Kuranda–– Dean BurnsDean Burns

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