Infrared Optics and Zoom Lenses

Excerpt

This tutorial is an outgrowth of my SPIE short course entitled “Infrared Optics and Zoom Lenses.” The title was selected to reflect the scope of the subject matter, and this has been carried over to the tutorial. The first three chapters present an introduction to the principles of optics and the unique aspects of the infrared region of the wavelength spectrum. This foundation makes it possible for those readers who are not optical engineers to acquire the background information needed for a treatise on infrared zoom lenses.

Chapter 1 presents overall system considerations involved in establishing the requirements for an application that includes an optical system as one of its elements. Chapter 2 sets forth the basic fundamentals of optics involved in the design and analysis of optical systems. Chapter 3 presents the optics features that are unique to the infrared region of the spectrum. Chapter 4 discusses some of the optical design techniques that may be utilized in the optical design of infrared systems. These four chapters could serve as an introduction to any treatise on infrared optical systems. Further discussion of these topics may be found in the tutorial text on this subject by Max J. Riedl.

Chapters 5 through 8 present the subject matter that is unique to the subject of zoom lenses in the infrared. Chapter 5 sets forth the basic types of zoom lenses and the establishing of specifications to meet the requirements of a particular application. Chapters 6 and 7 present numerous examples of refractive and reflective infrared zoom systems; the optical design techniques from Chapter 4 are employed in designing these representative infrared (IR) zoom lenses to illustrate the utilization of these techniques. Companies identified in Chapters 6 and 7 are the names in existence at the time the reference papers were published; some of them have since merged with other companies and lost their separate identity. Chapter 8 presents a brief discussion of future trends in this subject area. Chapter 9 presents a summary of infrared zoom lens applications.

Appendix A contains three landmark IR zoom lens patents in their entirety as published. This appendix is included not only for the insights contained therein, but also to provide lens prescription data to serve as potential starting points for future design activity. Appendix B presents computer analysis that I have performed on these patents and on one additional patent described in Sec. 7.2.1. A definition of the analysis categories is to be found in Chapter 2. Appendix C gives the answers to self-test problems presented in Sec. 2.9.

The infrared zoom lens literature consists primarily of patents and of papers presented at conferences or published in journals and proceedings. In 1993 SPIE published in its Milestone Series of Selected Reprints a volume on zoom lenses which included a number of infrared papers and patents. To my knowledge, this tutorial is the first publication to be devoted exclusively to IR zoom lenses. It should serve as an introduction to the subject for the uninitiated and as an aid to the engineer who has an infrared zoom lens application to pursue. It is not intended to be a step-by-step instruction manual for this complex optical design activity.

Additions to Infrared Optics and Zoom Lenses are included in the second edition of this tutorial. The additions are based on an expanded short course that I recently presented. There are substantive additions to the topics in the table of contents. They are discussed below. Also, 18 new refractive and reflective systems have been added to the 23 zoom systems in the first edition, bringing the total to 41 optical systems. The 18 new systems were published in the reference literature since publication of the first edition, in the time interval from the year 2000 to 2007. These additional systems are in part the result of adding a new category—focal plane arrays—to the chapter on refractive infrared zoom lenses. In part these additions are a result of including dual field-of-view infrared optical systems in this tutorial. The 18 new zoom systems are intended to bring the technology and the list of refractive and reflective zoom infrared systems up to date. There are 24 additional references.

One of the themes that will be presented is the gradual shift in recent years from the 8- to 12-micrometer (μm) region to the 3- to 5-μm region of the wavelength spectrum. This shift is discussed in Secs. 2.7, 3.1, 3.2, 6.5, and 6.6.

The rationale for the substantive additions is presented below:

2.8 Principal planes: The location of the principal planes is important in order to calculate accurately the separation between lens elements when going from a thin-lens solution to a thick-lens solution. The location also affects the overall length of the zoom system.

2.9 Self-test problems: A problem set is included in order to ensure a clear understanding of optics fundamentals before discussing the infrared spectrum and infrared zoom systems.

3.5 Glass substitution: Glass substitution is a powerful technique for performing computer optimization and athermalization simultaneously by passive substitution of infrared optical materials. I have done this glass substitution successfully, and I present a detailed example with a reference to the paper I wrote on this subject.

4.14 Global search: Global search has been demonstrated in recent years to be a viable computer optimization tool. An example is presented of designing zoom lenses by means of global search without designer intervention. The computer program flowchart of the decision-making process is included in this discussion.

5.3 Extenders: Extenders are a practical means of extending the focal length range of zoom lens systems. It is important to understand the optical limitations of extenders.

6.6 Focal plane arrays: The use of focal plane arrays (FPA) to eliminate scanning is an important development in infrared optical systems. Techniques for overcoming the limitations of resolution of FPAs at higher spatial frequencies are discussed in this section.

7.3 Special reflective systems: Due to the increase in the number of reflective infrared zoom systems, it is important to understand techniques for dual-channel detector arrays and for designing compact reflective systems through the use of folding mirrors and the Mangin mirror.

Chapter 9 Summary of applications: It is useful to summarize the scope and variety of infrared zoom lens applications. The discussion includes a reference to each of the zoom systems presented in this tutorial. This overview makes this chapter a fitting conclusion.

I would like to thank the reviewers for their helpful comments and suggestions. Acknowledgment is also due to Gwen Weerts of SPIE for her editorial assistance in the publication of this second edition of Infrared Optics and Zoom Lenses.

Allen Mann

January 2009

1. Riedl, M. J., Optical Design Fundamentals for Infrared Systems, Second Edition, SPIE Press, Bellingham, WA (2001).

2. Mann, A., Ed., Selected Papers on Zoom Lenses, SPIE Press, Bellingham, WA (1993).