Wavefront Congress Symposium Feb, 2008

Unresolved Issues in Prediction of Subjective and Objective Refraction from

Wavefront Data

Larry N. Thibos School of Optometry, Indiana University,

Bloomington, IN 47405 thibos@indiana.edu www.opt.indiana.edu

Slideshow in public domain at http://research.opt.indiana.edu/

6 questions without answers

1)  Which subjective criterion is to be emulated: perceived or performance IQ?

2)  Do metrics need to include the neural system, or is Rx just optics?

3)  What are the sources of discrepancy between subjective and objective Rx?

4)  How close is close enough?

5)  Mono vs. poly chromatic IQ: does it matter?

6)  Is there a systematic error due to depth of retinal beacon? If so, does it depend on wavelength?

Traditional subjective refraction

Caveman Refractionist

Retinal Image (inaccessible)

Wow!

Vision (perceivable)

Patient decides which lens is better, #1 or #2

The subjective refraction paradigm

Visual Object Filter #1

Filter #2

Retinal image #1

Retinal image #2 Observer

task

Which optical filter (i.e. lens)

is better?

Objective refraction

Find the viewing distance (or correcting lens) for which retinal image quality is maximized.

Visual target

Adjust distance

Mini-clinician assesses

image quality

Objective wavefront refaction

object point

P′

Maxi-clinician inspects reflected wavefront

Δ Δ y

x

Measure wavefront aberration and compute the correcting lens needed to optimize an

external image of the fundus.

Aberrated rays

Lots of math

Wavefront refraction: finding the best correcting lens

Aberration map

Computed Image

IQ Fourier optics

Add sphero-cylindrical wavefront

Is IQ max?

Yes

No

6 questions without answers

1)  Which subjective criterion is to be emulated: perceived IQ or performance IQ?

2)  Do metrics need to include the neural system, or is Rx just optics?

3)  What are the sources of discrepancy between subjective and objective Rx?

4)  How close is close enough?

5)  Mono vs. poly chromatic IQ: does it matter?

6)  Is there a systematic error due to depth of retinal beacon? If so, does it depend on wavelength?

What does "better" mean?

The subjective refraction paradigm

Visual Object Filter #1

Filter #2

Retinal image #1

Retinal image #2 Observer

task

Task A: read the letters

Task B: judge perceived quality

Choice of metric for ranking optical filters depends on the task.

1)  Which subjective criterion is to be emulated: perceived or performance IQ?

2)  Do metrics need to include the neural system, or is refraction just optics?

3)  What are the sources of discrepancy between subjective and objective Rx?

4)  How close is close enough?

5)  Mono vs. poly chromatic IQ: does it matter?

6)  Is there a systematic error due to depth of retinal beacon? If so, does it depend on wavelength?

6 questions without answers

Objective prediction of perceived image quality

Area = SQRI

Barten (1987)

3 12

Area = SQF

Granger & Cupery (1972)

1)  Which subjective criterion is to be emulated: perceived or performance IQ?

2)  Do metrics need to include the neural system, or is Rx just optics?

3)  What are the sources of discrepancy between subjective and objective Rx?

4)  How close is close enough?

5)  Mono vs. poly chromatic IQ: does it matter?

6)  Is there a systematic error due to depth of retinal beacon? If so, does it depend on wavelength?

6 questions without answers

How large is the discrepancy?

Defocus (M) Astigmatism (J0 or J45)

Cheng et al., 2004 (J. Vision) •  computationally-blurred images, monochromatic light •  3 levels of 3rd & 4th order Zernike aberrations •  letter size & lens power (M, J0 or J45) varied to maximize VA

Subjective best lens (D)

Obj

ectiv

e be

st le

ns (D

)

R 0.998 0.999 0.998

R 0.973 0.965 0.970

Real-world clinical refractions

•  Subjective refraction uses polychromatic light …

Whereas, optical measurements of wavefront aberrations are monochromatic, typically with "invisible light" (infra-red)

•  Subjective hyperfocal refractions deliberately leave the eye myopic (i.e. undercorrected) …

Whereas, wavefront refraction aims to optimize image quality for infinity, not the hyperfocal distance.

How large is the discrepancy between subjective and objective refractions in the clinic?

1)  Which subjective criterion is to be emulated: perceived or performance IQ?

2)  Do metrics need to include the neural system, or is Rx just optics?

3)  What are the sources of discrepancy between subjective and objective Rx?

4)  How close is close enough?

5)  Mono vs. poly chromatic IQ: does it matter?

6)  Is there a systematic error due to depth of retinal beacon? If so, does it depend on wavelength?

6 questions without answers

Quantifying error of astigmatism predictions

Accuracy (systematic error) = distance of population mean from origin

Precision (random error) = mean radius of 95% confidence ellipse

J0

J45

J0

J45

+

Scatterplot of prediction errors

J0

J45

Subjective Objective

Prediction Error

J0 = 0.5*Cyl*cos(2*axis) = - 4√3 c2+2 /r2

J45 = 0.5*Cyl*sin(2*axis) = - 4√3 c2-2 /r2

Astigmatism results (Coe, WFC 2004)

Many metrics of WQ, IQ, and VQ make highly accurate predictions of subjective Rx (systematic error < 0.05D) Suggests errors in predicting astigmatism are random. For the most precise metrics, 95% of objective predictions are within 0.25 D of subjective measurements. This might be as good as we can expect, given variability in the “gold standard” of subjective refractions. It is hard to hit a moving target.

The good news:

Defocus results (Coe, WFC 2004)

Accurate and precise predictions of spherical defocus have proven elusive, even when using unbiased "infinity" refractions designed to make the retina optically conjugate to infinity, rather than the hyperfocal distance used clinically.

This discrepancy might be due to various features of ocular chromatic aberration.

The bad news:

1)  Which subjective criterion is to be emulated: perceived or performance IQ?

2)  Do metrics need to include the neural system, or is Rx just optics?

3)  What are the sources of discrepancy between subjective and objective Rx?

4)  How close is close enough?

5)  Mono vs. poly chromatic IQ: does it matter?

6)  Is there a systematic error due to depth of retinal beacon? If so, does it depend on wavelength?

6 questions without answers

Predicting defocus (M) is more challenging

Accurate and precise prediction of defocus (M) is hampered by ocular chromatic aberration for

several reasons: 1.  Wavefront aberrometers using infrared light are subject to

potential measurement bias. 2.  Wavefronts measured in infrared must be converted to a

visible wavelength using some model of ocular chromatic aberration (OCA). The model could be wrong.

3.  Although OCA models should include higher-order chromatic aberrations, HOCAs are probably less important than individual variation in lower-order OCA.

4.  Polychromatic calculations of defocus (M) require knowledge of wavelength-in-focus when white-light targets were optimally focused subjectively.

1)  Which subjective criterion is to be emulated: perceived or performance IQ?

2)  Do metrics need to include the neural system, or is Rx just optics?

3)  What are the sources of discrepancy between subjective and objective Rx?

4)  How close is close enough?

5)  Mono vs. poly chromatic IQ: does it matter?

6)  Is there a systematic error due to depth of the retinal beacon used to measure the eye? If so, does it depend on wavelength?

6 questions without answers

A possible explanation for myopic bias in IR

Classical, refractive CA

Reflective CA ?? IR light may reflect from deeper layers of fundus, making eye appear myopic.

Melanin and hemoglobin are more transparent for IR than for visible light.

If IR light of aberrometer penetrates into choroid, eye will appear longer, hence more myopic, than for visible light.

IR Visible

A possible artifact of aberrometry

Myopic bias of infrared aberrometry (Warren, 2006)

Cone apertures

0.25 ±0.16 D = 92 ±59 µm

850nm retinal beacon

Previous reports indicate fundus reflection is near cone apertures:

•  Williams et al. (1994) 0.01D @ 633nm

•  Lopez-Gil & Artal (1997) < 0.1D @ 543 & 780 nm

Our measurements at Indiana (N=30) indicate significant bias (-0.25D) @ 850nm, suggesting the retinal beacon is near Bruch's membrane.

RPE Bruch's memb.

Even more unanswered questions:

Computational • Is the dynamic range of the metrics large enough for Rx? • Are the metrics robust enough for iterative solutions to

converge to global maximum IQ? • Is wavefront reconstructiona and Fourier optics

calculations necessary? Or can we go straight from measured wavefront slopes to the prescription?

Is compensation for individual variation necessary? • Ocular chromatic aberration models used for

polychromatic analysis • Wavelength-in-focus for polychromatic objects • Measurement bias due to depth-of-penetration of fundus

beacon

Peripheral refractions • Do foveal methods of objective refraction work also for

peripheral vision?

• What is the best way to deal with elliptical entrance pupils that occur off-axis?

Clinical issues

• Might cyclopean metrics of visual quality prove useful for performing binocular wavefront refractions?

• Will wavefront refractions replace clinical trials for new designs of refractive therapy?

• Will objective wavefront refractions replace subjective refractions as the "gold standard" of clinical practice?

Even more unanswered questions:

The Visual Optics Group at Indiana University

Larry Thibos, PhD Arthur Bradley, PhD Steve Burns, PhD Ann Elsner, PhD Donald Miller, PhD Carolyn Begley, OD Rowan Candy, OD PhD Jacob Rubinstein, PhD Jayoung Nam, Phd Nikole Himebaugh, OD Charles Coe, OD Haixia Liu, MD Jingyun Wang, BS Pete Kollbaum, OD Sowmya Ravikumar, BS Toco Chui, MS Xin Wei, BS Danielle Warren, OD Weihua Gao, BS Kevin Haggerty, BS

Vision Research at

http://www.opt.indiana.edu

Benno Petrig, PhD Ravi Jonnal, MS Jungtae Rha, PhD Yan Zhang, PhD Barry Cense, PhD Zhangyi Zhong, BS Jie Shen, BS Hongxin Song, BS

Support: National Institutes of Health / NEI National Science Foundation Borish Center for Ophthalmic Research