Teachers cross our paths in life. Some teachers have names, others leave their marks anonymously. Among my teachers at the Optical Sciences Center at the University of Arizona were James Palmer, Eustace Dereniak, and Jack Gaskill. They freely shared their knowledge with their students. Some teachers teach through the pages of their books, and here I have to thank Bill Wolfe, George Zissis, and many more. Many years ago, R. Barry Johnson presented a short course which influenced my career most decisively.

The intent with this book is to now share some of my experience, accumulated through years of practical radiometry: design, measurements, modeling, and simulation of electro-optical systems. The material presented here builds upon the foundation laid at the Optical Sciences Center. I had the opportunity to share this material in an academic environment at graduate level in an engineering school, thereby clarifying key concepts. Beyond the mathematics and dry theory lies a rich world full of subtle insights, which I try to elucidate. May this book help you, the reader, grow in insight and share with others.

The reductionist approach holds the view that an arbitrarily complex system can be understood by reducing the system to many, smaller systems that can be understood. This view is based on the premise that the complex system is considered to be the sum of its parts, and that by understanding the parts, the sum can be understood. While the reductionist approach certainly has weaknesses, this approach works well for the class of problems considered in this book. The methodology followed here is to develop the theory concisely for simple cases, developing a toolset and a clear understanding of the fundamentals.

The real world does not comprise loose parts and simple systems. Once the preliminaries are out the way, we proceed to consider more complex concepts such as sensors, signatures, and simple systems comprising sources, a medium, and a receiver. Using these concepts and the tools developed in this book, the reader should be able to design a system of any complexity. Two concurrent themes appear throughout the book: fragmenting a complex problem into simple building blocks, and synthesizing (designing) complex systems from smaller elements. In any design process, these two actions take place interactively, mutually supporting each other. In this whirlpool of analysis and synthesis, uncontrolled external factors (e.g., the atmosphere, noise) influence the final outcome. This is where the academic theory finds engineering application in the real world. This book aims to demonstrate how to proceed along this road.

Toward the end of the book, the focus shifts from a component-level view to an integrated-system view, where the 'system' comprises a (simple or composite) source, an intervening medium, and a sensor. Many real-world electro-optical applications require analysis and design at this integrated-system level. Analysis and design, as a creative synthesis of something new, cannot be easily taught other than by example. For this purpose several case studies are presented. The case studies are brief and only focus on single aspects of the various designs. Any real design process would require a much more detailed process, beyond the scope of this book.

The purpose with this book is to enable the reader to find solutions to real-world problems. The focus is on the application of radiometry in various analysis and design scenarios. It is essential, however, to build on the foundation of solid theoretical understanding, and gain insight beyond graphs, tables and equations. Therefore, this book does not attempt to provide an extensive set of ready-to-use equations and data, but rather strives to provide insight into hidden subtleties in the field. The atmosphere provides opportunity for a particularly rich set of intriguing observations.

The strict dictionary definition of 'radiometry' is the measurement of optical flux. In this book, the term 'radiometry' is used in its wider context to specifically cover the calculation of flux as well. This wider definition is commonly used by practitioners in the field to cover all forms of manipulation, including creation, measurement, calculation, modeling, and simulation of optical flux. The focus of this book is not on radiometric measurement but on the analysis and modeling of measured data, and the design of electro-optical systems.

Antoine de Saint-Exupèry once wrote, "You know you've achieved perfection in design, not when you have nothing more to add, but when you have nothing more to take away." The painful aspect of writing a book is to decide what not to include. This book could contain more content on radiometric measurement, emissivity measurement, properties of different types of infrared detectors, or reference information on optical material properties; however, these topics are already well covered by other excellent books, much better than can be achieved in the limited scope of this book.

The book provides a number of problems, some with worked solutions. The scope of problems in the early chapters tend to be smaller, whereas the problems in later chapters tend to be wider in scope. The more-advanced problems require numerical solutions. Although it is certainly possible to read the book without doing the advanced problems, the reader is urged to spend time mastering the skills to do these calculations. This investment will pay off handsomely in the future. Some of the problems require data not readily found in book format. The data packages are identified (e.g., DP01) and are obtainable from the pyradi website (see Section D.3.4).

To the uninitiated, the broader field of radiometry is dangerous territory, with high potential for errors and not-so-obvious pitfalls. Our work in the design labs, on field measurement trials, and in the academic environment led to the development of a set of best practices, called the 'Golden Rules,' which strives to minimize the risk error. Some of these principles come from James Palmer's class, while most were stripes hard earned in battle. The readers are urged to study, use, and expand these best practices in their daily work. Any feedback, on the golden rules or any other aspect of the book, would be appreciated.

A book is seldom the work of one mind only; it is the result of a road traveled with companions. Along this road are many contributors, both direct and inadvertent. My sincere thanks to all who made their precious time and resources available in this endeavor. My sincere thanks goes to Riana Willers for patience and support, as co-worker on our many projects -- her light footprints fall densely on every single page in this book: advising, scrutinizing every detail, debating symbols and sentences, editing text and graphics, compiling the nomenclature and index, and finally, acting as chapter contributor. Riana is indeed the ghost writer of this book! Fiona Ewan Rowett for permission to use her exquisite "Karoo Summer" on the front cover. The painting beautifully expresses not only the hot, semi-arid Karoo plateau in South Africa, but also expresses radiated light and vibrant thermal energy, the subject of this book. My teachers at the Optical Sciences Center who laid the early foundation for this work. Ricardo Santos and Fábio Alves for contributing to the chapter on infrared detector theory and modeling. The pyradi team for contributing their time toward building a toolkit of immense value to readers of this book. Derek Griffith for the visual and near-infrared reflectance measurements. Hannes Calitz for the spectral measurements, and Azwitamisi Mudau for the imaging infrared measurements. Dr Munir Eldesouki from KACST for permission to use the Bunsen flame measured data in the book. The many colleagues, co-workers, and students at Kentron (now Denel Dynamics), the CSIR, KACST, and the University of Pretoria for influencing some aspect of the book. Scott McNeill and Tim Lamkins for patience and guiding me through the publication process. Scott's untiring patience in detailed correction deserves special mention. Eustace Dereniak for encouraging me to submit the book for publication. Barbara Grant, Eustace Dereniak and an anonymous reviewer for greatly influencing the book in its final form. Finally, Dirk Bezuidenhout, and the CSIR for supporting the project so generously in the final crucial months before publication.

Mark Twain wrote that he did not allow his schooling to get in the way of his education. It is my wish that you, my esteemed reader, will delve beyond these written words into the deeper insights. Someone else said that the art of teaching is the art of assisting in discovery. May you discover many rich insights through these pages.

Nelis Willers
Hartenbos
March 2013

1) The pyradi library source files for use with the book are at https://code.google.com/p/pyradi
2) The PyPI distribution of the library is at https://pypi.python.org/pypi/pyradi
3) The online library documentation for the library is at https://pyradi.googlecode.com/svn//trunk/pyradi/doc/_build/html/index.html
4) Some examples and tutorials are at https://github.com/NelisW/ComputationalRadiometry#computational-optical-radiometry-with-pyradi