Switchable Retroreflector Films for Enhanced Visible and Infrared Conspicuity: Naked eye response switchable retroreflectors with integration into an optical interrogation and response system A dissertation submitted to the Division of Research and Advanced Studies of the University of Cincinnati in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY (Ph.D.) In the Department of Electronic and Computing Systems of the College of Engineering and Applied Science 2015 by Phillip Jordan Schultz B.S., University of Cincinnati, 2009 Dr. Jason C. Heikenfeld, Committee Chair Abstract Retroreflectors are common optical devices used in several commercial conspicuity applications. Since the 1980's, there have been several investigations aimed at switching the retroreflected signal. Most of the developed switching technologies are based on MEMS actuation and quantum mechanics, among others. These devices were aimed at the free-space optical communications markets, lending them to have high operating speeds. However, they are limited by single devices/small areas, spectral range (short wave infrared), low contrast, physical rigidity, and high fabrication costs. For naked eye conspicuity applications, these limitations are not suitable. The approach presented in this dissertation does not provide the high switching speeds of the previous MEMS and MQW methods, but provides superior performance in nearly all other metrics of interest to visual conspicuity applications, most notably safety. These metrics include high contrast of >2000:1 at 635 nm, and >400:1 at 850 nm, large area of 75 cm2 +, visible and infrared operational spectral range from 400 - 1600 nm, input angles of ±38o, optical efficiencies of 25%, low power, thin and flexible construction at <0.6 mm thick, and switching speeds sufficient for rapid visualization (<100 ms). This dissertation investigates several novel types of switchable retroreflectors, and compares them to a set of desired metrics for naked eye applications. It determines that the use of a polymer dispersed liquid crystal (PDLC) based switchable retroreflector would be the ideal choice for such applications. This dissertation further achieves a complete system level integration and demonstration, including development of an encoded laser interrogation system. It provides optical models for both day and night operation. It also provides day and night field demonstration results using visible (2.5 - i 10 mW - 635nm) and infrared (1.5 mW - 850 nm) light sources (with night vision) out to 400 meters to validate the technology’s viability and potential use. Overall, the goal of this dissertation is to introduce and demonstrate an improved switchable retroreflector and electronic system satisfying all of the ideal requirements for enhanced naked-eye optical conspicuity. ii Copyright Page iii Acknowledgements I would not have been able to complete, much less pursue, this dissertation without the help and support of several individuals throughout my personal and professional life. I would like to thank some of them here. First, I would like to thank some of those closest to me. I would like to thank my wonderful wife, Carrie, for being there when I needed her on several levels. Though there were ups, and downs, she hung in there and allowed me to finish without worries. I would like to thank my son, Sullivan, for always being there and putting a smile on my face when I needed it most. Next, I would like to thank my parents for emphasizing the importance of education and doing everything to the best of your ability. I would also like to thank my twin brother, Alex, for always giving me that on-the-spot competition that you cannot find anywhere else. I would like to thank my advisor and mentor, Dr. Jason Heikenfeld. His support, and seemingly non-human levels of energy, helped push me to work harder to achieve what I wanted on a daily basis. His advice, encouragement, and guidance, allowed me to complete this dissertation knowing I would be proud of the work I had accomplished. I would also like to thank all of my committee members for their encouragement and acceptance throughout my Ph.D. candidacy. I would like to thank my financial, material, and equipment support from , Xetron, and Reflexite. Without their help, none of this would have been possible. I would also like to thank everyone from NDL. Their encouragement, friendship, training, and assistance, helped get me through the occasional grind and allowed me to understand what a cohesive work environment can do. I would especially like to thank iv Brad Cumby, who took time out of his day to help me collect data for my various field demonstrations. Life would have been a lot more difficult without that. I still owe him. I would finally like to thank all of the faculty and staff of the University of Cincinnati. Their assistance, support, and guidance allowed me to pursue my aspirations for the last 9+ years with relative ease. I am proud of my undergraduate and graduate tenure here. I will encourage others to follow the same path. v Table of Contents Abstract .......................................................................................................................... i Copyright Page ............................................................................................................. iii Acknowledgements ...................................................................................................... iv Table of Contents ......................................................................................................... vi List of Figures .............................................................................................................. xi List of Tables .............................................................................................................. xiv Chapter 1: Introduction ................................................................................................. 1 1.1 Introduction to this Chapter ............................................................................... 1 1.2 Conspicuity ....................................................................................................... 1 1.3 Retroreflectors: Theory in Brief ......................................................................... 2 1.4 Research Aims and Outline ............................................................................... 5 1.5 References........................................................................................................ 7 Chapter 2: Background and Prior Art ......................................................................... 8 2.1 Introduction ....................................................................................................... 8 2.2 Prior Art ............................................................................................................. 8 2.2.1 Micro-Electro-Mechanical Systems (MEMS) Retroreflectors ........................... 9 2.2.2 Multiple Quantum Well (MQW) Retroreflector ............................................... 13 2.3 Display Technologies ...................................................................................... 16 2.3.1 Electrowetting ............................................................................................... 16 2.3.2 Liquid Crystal ............................................................................................... 19 vi 2.4 Summary ........................................................................................................ 23 2.5 References...................................................................................................... 24 Chapter 3 - Comparison of 5 Types of Switchable Retroreflectors ......................... 29 3.1 Introduction ..................................................................................................... 29 3.2 Background ..................................................................................................... 29 3.3 Choosing the Retroreflecting Optical Film ....................................................... 30 3.4 Selection of Electrical Modulation Methods ..................................................... 33 3.4.1 Electrowetting ............................................................................................... 34 3.4.2 Liquid Crystal ............................................................................................... 34 3.5 Optical Efficiency Model .................................................................................. 35 3.6 Electrowetting Lenslet Scattering .................................................................... 36 3.6.1 Fabrication and Construction ........................................................................ 36 3.6.2 Electrical Switching ...................................................................................... 36 3.6.3 Retroreflection Results ................................................................................. 38 3.6.4 Discussion .................................................................................................... 39 3.7 External Electrowetting Light Valve ................................................................. 40 3.7.1 Fabrication and Construction ........................................................................ 40 3.7.2 Electrical Switching ...................................................................................... 40 3.7.3 Preliminary Results .....................................................................................