A review of the development of infrared optics is presented in the context of its relationship with the overall evolution of infrared technology. Significant events in optics, detectors, materials, systems, etc. are summarized in a chart that spans the time period from 1829 to the present. Numerous references and examples of optical systems are included.
Thermal, or infrared imaging systems have continued to increase in importance over the years. This is due to several factors:
The applications of IR systems have grown dynamically in importance both in the military as well as in industrial applications
IR detector technology has matured substantially, to the point where many IR detectors are available and economically producible
This increased importance of infrared systems, combined with continually better performing and more cost effective IR detectors has put an ever increasing demand on the optical system and it's design. Although many of the classical optical design and engineering related derivations, guidelines, tradeoffs, and other considerations developed for visible systems can be applied directly to infrared systems, there are many important considerations, some of them quite subtle, that are considerably different in the infrared, and which can cause devastating problems if not properly taken into account. In fact, if one were to design the optics for an infrared scanning system using totally the guidelines derived from visible system engineering principles, there is a good chance that the system would perform poorly, if at all.
In this paper we will review the many aspects of optical system design for the infrared. Although we intend to cover primarily those areas which are unique to IR systems, one will quickly realize that this will in reality cause us to cover most aspects of optical design technology.
This session is a critical review of the technological field called IR Optical Design and Fabrication. When Warren Smith called me and asked for a paper that would look into the future, looking for trends, I pointed out that everyone here knew much more about IR optics than I—the list of papers and speakers attests to that! The years spent in the field of IR technology since I first began at The University of Michigan in October 1955 seemed always to focus on the underlying physics of such technology, always driven strongly by the applications needs. This led me to a strong interest in IR systems defined, as was so well stated by Hudson in his book , by the functions to be performed by such systems. To paraphrase his statements, an IR system is defined by what it is supposed to do. Thus the user defines the system-the application determines the system design. As I told Warren, I have developed some rather strong biases concerning the pacing elements of IR components in this context. Unfortunately he said to go ahead and here we are.
Modem infrared technology can now offer the interceptor or air superiority fighter pilot the dual advantage of a "passive radar" for target search and track, combined in the same conformal package with an imaging FLIR for navigation and landing aid. This paper describes the key design issues and traces the steps in the design process of a multimode combined IRST/FLIR sensor for airborne use.
Narcissus has been understood by FLIR (Forward Looking Infrared) designers for many years. The purpose of this paper is not to elevate the engineering state-of-the-art, but rather, to give an overview of the problem starting with early FLIRs, look at what is being faced by current designers, and to extrapolate future trends.
Following the trend in Broadcast TV and Video, more and more infra-red systems require zoom lenses. The purpose of this paper is to point out two key technologies, aspherics and athermalization, and to address the most difficult industrial aspect of these technologies : measurement.
Diffractive optical imaging elements have been proposed in numerous papers1-17 over the past decade. Few have been produced in quantities. The primary method of fabricating such diffractive elements has been reactive ion etching of a multi-level surface relief grating on one side of a lens. This approximation is known as Binary Optics1.
Recent experiments have shown that single point diamond turning can be very effective in generating continuous diffraction phase profiles. Combined with the long established method of aspherizing, this machining process is especially suitable for applications in the infrared spectrum. It provides a means of reducing the number of lens elements otherwise required for an objective to correct existing aberrations.
An update on diamond machinable materials is presented with emphasis on A201 cast aluminum and electroless nickel plating. Surface figure is discussed for spherical and aspherical surfaces, including base radius tolerances, irregularity, clear aperture and slope. A review of recent work on surface finish and scatter is summarized. Current machine tool capabilities are presented with considerations for post polish, where machine produced accuracies do not suffice.
The index of refraction is a measure of the speed of light in matter. The dispersion, or change of the refractive index with wavelength, can be represented mathematically many different ways for use in optical design. What is covered in this paper is not what the index of refraction is, but how it is represented and how well it is represented over the transmission band of a material. More specifically, the emphasis of this paper is on how the dispersion characterization of a material (particularly in the infrared) effects the chromatic performance of a lens design and the development of a refractive-index interpolation fit criterion based on dispersion.
Diamond exhibits an unusually favorable combination of properties in terms of mechanical strength, thermal conductivity, and optical transmission, which makes this material highly attractive for infrared (IR) applications that involve severe environmental conditions. Until recently, diamond has been available only in the form of relatively small crystals, but this situation is evolving rapidly as a result of major advances in the art of growing diamond by low-pressure chemical vapor deposition (CVDI techniques. Success in producing large, free-standing deposits with properties that match those of natural diamond has stimulated enormous interest and has given rise to much speculation about CVD diamond as an "ideal" optical material for a wide range of engineering uses in the IR: there are. however, limiting factors that must be taken into consideration. The objective of this Critical Review is to provide an initial assessment of some of the issues that arise in connection with the use of diamond artifacts for two highly demanding tasks: domes for high-speed missiles and windows for high-power lasers.
Monolithic diamond deposits lack the degree of transparency required for operation as transmissive optics elements in the mid-IR: at longer wavelengths (LWIR), it can be reasonably expected that singlephonon absorptions will be tolerable in the sense that self-emission phenomena at elevated temperatures should not be catastrophic. Besides surface hardness, the features that confer diamond its advantage are the high thermal conductivity and the low thermal expansion, which indicate that diamond has orders of magnitude more thermal shock resistance than some of the best competing materials. There is little doubt that the "new diamond" technology will provide a credible, if not outstanding LWIR dome material for tactical missiles operating at speeds up to but not beyond Mach 4 in the atmosphere.
Defect-free, single-crystal diamond also emerges as a promising candidate material for high-average-power laser window applications in the near-IR. The power-handling capability (<1 MW independently of the window size! will be limited by la) thermally induced optical distortion, which can only be eliminated if absorption-free anti-reflection coatings become available, and (b) the edge heat-transfer coefficient, which must be greatly enhanced to take advantage of the exceptional thermal conductivity of diamond at low temperatures. Pressure-induced and thermally-induced stresses are of no consequence, but peak laser intensities in excess of, say 1 GW/cm2 should be avoided because of unfavorable non-linear refractive index characteristics.
In recent years, metal mirrors have grown in popularity. Many short courses and papers have discussed the mechanical properties and calculated rigidity and thermal performance of mirrors. This presentation will briefly touch on these but give more emphasize on metal mirror fabrication procedures, material comparisons, and cost. Fabrication procedures for aluminum, metal matrix composite (MMC), beryllium (five classes) and glass mirrors are given. Procedures should be used only as a starting point since they vary from part to part.
A Computer Controlled Optical Surfacing (CCOS) process has been developed that is in routine use for fabricating off-axis and centered aspheric mirrors. An industrial robot effects surface removal by moving a relatively small tool over the mirror surface in a path covering the entire surface. The removal is computed by the convolution of the tool work function with the path of the tool over the mirror surface. The combination of CCOS with microgrinding (grinding with fine diamond powders that produce a specular surface) allows interferometric testing at an early stage of the process. Removal rates and tool conformance to the mirror surface are enhanced by vacuum applied to the grinding and polishing tools. Surface figure accuracies better than 0.02 μm rms and finishes better than 10 Å are currently achieved.
Infrared optics defocus with temperature change due to the nature of the optical materials employed in their design. Methods of eliminating this defocus - mechanical, electro-mechanical and optical - are discussed and evaluated in detail.