Polarization modulation using wave plates to enhance foveal fixation detection in retinal birefringence scanning for pediatric vision screening purposes [Elektronische Ressource] / put forward by Kristina Irsch

Kristina Irsch

Sujets

• Gesundheit

Informations

• Publié par: ruprecht-karls-universitat_heidelberg
• Publié le: 01 janvier 2009
• Nombre de lectures: 26
• Langue: Deutsch
• Poids de l'ouvrage: 12 Mo

Extrait

Dissertation
submitted to the
Combined Faculties for the Natural Sciences and for Mathematics
of the Ruperto-Carola University of Heidelberg, Germany
for the degree of
Doctor of Natural Sciences






















Put forward by

Diplom-Physicist: Kristina Irsch
Born in: Merzig, Germany

Oral examination: December 17, 2008


Polarization Modulation Using Wave Plates
to Enhance Foveal Fixation Detection in
Retinal Birefringence Scanning for Pediatric
Vision Screening Purposes


















Referees: Prof. Dr. Josef Bille
Prof. Dr. Christoph Cremer










To

My Teachers
For Showing Me the Excitement and Joy of Ophthalmic Optics

My Parents
For Their Love and Abundant Support



















Zusammenfassung

Um die beidäugige foveale Fixationserkennung mit Hilfe der binokularen „Retinal
Birefringence Scanning“ (RBS)-Methode zu Seh-Screening Zwecken von Kleinkindern zu
verbessern, wurde ein neues Verfahren entwickelt, welches auf der Verwendung eines
rotierenden λ/2-Plättchens und eines festen Wellenplättchens beruht. Das rotierende λ/2-
Plättchen ermöglicht differenzielle polarisationsempfindliche Detektion des Fixationssignals
mit nur einem Detektor und überwindet damit Grenzen des vorherigen optisch-elektronischen
Aufbaus mit zwei Photodetektoren. Mit Hilfe der festen Verzögerungsplatte kann dieses durch
die doppelbrechende Eigenschaft der Henle-Faserschicht verursachte Fixationssignal quasi
unabhängig von der störenden kornealen Doppelbrechung, welche von einem Auge zum
anderen variiert, erfasst werden. Unter Zuhilfenahme gemessener Doppelbrechungswerte der
Hornhaut von 300 repräsentativen menschlichen Augen wurde unter MATLAB ein
Algorithmus und eine damit verbundene Computersoftware zur Optimierung der
Eigenschaften beider Wellenplättchen entwickelt. Das Optimierungsverfahren bestand in der
Maximierung des Fixationssignals bei gleichzeitiger Minimierung der inter- und intra-
individuellen Variabilität aufgrund verschiedener Hornhautwerte. Rotiert man das λ/2-
Plättchen mit 9/16 der Scan-Frequenz und verwendet man ein Wellenplättchen mit einer
Verzögerung von 45° und einer Orientierung von 90°, so wird das Fixationssignal optimiert.
Kombiniert mit der „Bull’s-Eye“-Methode zur Erkennung von Defokus eignet sich dieses
computeroptimierte RBS-basierte Verfahren als gerätegestützte objektive Methode zur
automatischen Erkennung eines Amblyopierisikos bei Kleinkindern, die Hauptursache des
Sehverlustes im Kindesalter.

Abstract

To enhance foveal fixation detection while bypassing the deleterious effects of corneal
birefringence in binocular retinal birefringence scanning (RBS) for pediatric vision screening
purposes, a new RBS design was developed incorporating a double-pass spinning half wave
plate (HWP) combined with a fixed double-pass retarder into the optical system. The spinning
HWP enables essential differential polarization detection with only one detector, easing
constraints on optical alignment and electronic balancing, and together with a fixed wave
plate, this differential RBS signal can be detected essentially independent of various corneal
retardances and azimuths. Utilizing the measured corneal birefringence from a dataset of 300
human eyes, an algorithm was developed in MATLAB for optimizing the properties of both
wave plates to statistically maximize the RBS signal, while having the greatest independence
from left and right eye corneal birefringence. Foveal fixation detection was optimized with the
HWP spun 9/16 as fast as the circular scan, with the fixed retarder having a retardance of 45
degrees and fast axis at 90 degrees. Combined with bull’s-eye focus detection, this computer-
optimized RBS design promises to provide an effective screening instrument for automatic
identification of infants at risk for amblyopia, the leading cause of vision loss in childhood.





























“Life is like riding a bicycle.
To keep your balance you must keep moving.”
Albert Einstein





Contents

1 Introduction.......................................................................................................................1
2 Vision Screening................................................................................................................4
2.1 Amblyopia ...................................................................................................................4
2.2 Traditional Screening Methods....................................................................................5
2.3 Newer Screening Modalities........................................................................................8
3 Polarization and the Eye ................................................................................................11
3.1 Polarization of Light ..................................................................................................11
3.1.1 Linearly Polarized Light .....................................................................................13
3.1.2 Circularly Polarized Light...................................................................................13
3.1.3 Elliptically Polarized Light .................................................................................14
3.2 Birefringence .............................................................................................................15
3.2.1 Wave Plates.........................................................................................................17
3.2.2 Form Birefringence.............................................................................................18
3.3 Müller- Stokes Matrix Calculus19
3.3.1 Stokes Vector Representation .............................................................................19
3.3.2 Müller Matrix Formalism....................................................................................23
3.3.3 Poincaré Sphere...................................................................................................23
3.4 Ocular Birefringence..................................................................................................25
3.4.1 Lenticular Birefringence .....................................................................................26
3.4.2 Corneal Birefringence.........................................................................................27
3.4.3 Retinal Birefringence ..........................................................................................29
4 Retinal Birefringence Scanning (RBS)..........................................................................32
4.1 Assessment of Foveal Fixation..................................................................................32
4.2 Pediatric Vision Screening Using Binocular RBS.....................................................34
4.2.1 Optical Design of RBS System...........................................................................35
4.2.2 Limitations and Problems ...................................................................................36
4.2.3 Hypotheses and Objectives .................................................................................39



5 Spinning Half Wave Plate Design for RBS...................................................................40
5.1 Phase-Shift Subtraction Technique............................................................................41
5.2 Modeling of RBS Using Wave Plates........................................................................43
5.2.1 Model of Ocular Birefringence...........................................................................43
5.2.2 Assessing the Influence of Corneal Birefringence on the RBS Signal...............53
5.2.3 Determination of Optimum Spinning Frequency of Double-Pass HWP ............56
5.2.4 Finding the Optimum Fixed Double-Pass Wave Plate .......................................63
5.2.5 Differential Polarization Subtraction with Optimized Spinning-HWP RBS
Design ..........................................................................................................................75
6 Validation of RBS Computer Model.............................................................................78
6.1 Experimental Setup....................................................................................................78
6.1.1 Intermediate Eye Fixation Monitor.....................................................................78
6.1.2 Method of Determining the Retardance and Fast Axis Orient