Today's optical systems are sometimes exposed to environments in which traditional optical materials rapidly degrade. In contrast, polycrystalline MgA1204 Spinel, transparent from three hundred nanometers to five and a half microns, with a unique combination of optical and physical properties provides a superior alternative in respect to stability to solar radiation, rain and particle erosion, high temperatures, and humidity, and is resistant to attack in strong acid and alkali solutions. Comparative data prior to and following exposure to a variety of environments will be reported. In addition, current and projected size and shape capability will be reviewed.

Yttria (yttrium sesquioxide, Y203) and lanthana-doped yttria (typically with 9 mole-percent La2O3) are being developed as infrared window materials. This paper presents a comprehensive review of mechanical, thermal, and optical properties.

Aluminum oxynitride (AlON) ceramic is an attractive material to replace magnesium fluoride on next generation missile domes. An original process has been developed to manufacture and sinter transparent AlON ceramic. First AlON was synthesized by carbothermal reduction of aluminum oxide in nitrogen. The process consists in a two step thermal treatment of an homogeneous mixture of fine alumina and carbon powders under nitrogen. The crushed powder was thus nearly full densified by hot pressing at 1700-1750°C. Significant visible and infrared transparency was achieved by an appropriate grain growth annealing at 1940°C for several hours. Yttria-doped AlON powder could also be sintered close to theoretical density under normal pressure. However translucent but not transparent samples could be obtained. This result could be related to the sample microstructure.

The properties of sapphire make it an ideal choice for future high speed missile applications compared to other existing or emerging materials. High fabrication cost and low material utilization from a sapphire boule cause high costs for sapphire domes. Significant improvements have been made by growing sapphire boules with (0001) orientation and contour slicing. A more desirable approach is to produce near net-shaped sapphire blanks directly from the melt. The feasibility of producing crack-free near net-shaped blanks with controlled inside and outside curvature from the melt in (0001) orientation has been demonstrated using the Heat Exchanger Method (HEM).

ALON (aluminum oxynitride) and sapphire windows measuring approximately 10 x 4 inches have been produced. Transmittance and mechanical durability in an extreme operational environment are essential for these windows. ALON is produced by ceramic powder processing techniques, while sapphire is grown as a single crystal. The ALON blank fabrication has been scaled up at Raytheon to produce several windows. Near net shape blanks were successfully polished to demanding optical and dimensional specifications at Zygo. Wavefront distortion and an atypical scattering phenomenon in large ALON windows are discussed. Sapphire windows were fabricated from plates cut from 10-inch diameter single crystal boules, using optical fabrication procedures similar to those developed for the ALON windows. The relative cost of optical fabrication of the two materials is discussed. Optical and mechanical properties for ALON and sapphire windows are presented.

Recently there has been a growing interest in using fluoride glass rather than crystalline materials for mid-infrared bulk optics. Applications in infrared bulk materials include high-power and low-power laser optics which involve windows, domes and lenses and infrared spectrophotometry which also requires windows as well as fluoride glass light pipes or conduits. Two types of fluoride glass, namely ZrF4- based and A1F3-based glass, are generally used for mid-infrared bulk optics. Although both glasses exhibit high optical transparency in the wavelength region from 0.3 microns to 7.0 microns, zirconium fluoride glass differs to a greater extent from aluminum fluoride glass in terms of its chemical durability, mechanical strength and glass forming ability. Thus, in selecting the type of fluoride glass, each criterion should be carefully weighed for each bulk optics application.

Lanthana-strengthened yttria (LSY) is a material of great interest for infrared window and dome applications. The ability to join LSY to structural materials such as metals is essential in the construction of advanced systems. A study was undertaken to evaluate joining LSY to metal by brazing. Brazing is often the method of choice in dissimilar materials joining because of the resultant hermeticity and because of the ability of a ductile metallic braze to accommodate differential thermal expansion. The thermodynamics of the braze-LSY interface was studied, and potential chemical reactions and the adhesion mechanism were examined. Based on this, a candidate list of reactive materials for use as either coatings or alloying additions was generated. Wetting behavior and reactivity were measured using the sessile drop test. A series of brazed samples were made using coupon specimens to obtain shear test mechanical strength data for the proposed material systems. Finally, the practices developed were demonstrated with the successful brazing of a 2.8 in. diameter LSY dome to a metal ring.

Zinc sulfide powders with submicron particle size were prepared by homogeneous precipitation from acidic zinc solutions by aging with thioacetamide at elevated temperatures. Monosize, polycrystalline particles having 0.35 μm diameter were obtained. Sulfide ion generation rate in solution depended on temperature, pH and initial thioacetamide concentration. The rate of sulfide ion generation, in turn, influenced precipitation kinetics and particle morphology. The types of supporting anions in solution also influenced the morphology of the particles produced. Monosize powders were hot pressed for various time, temperature and pressure combinations. Compacts aehieved near-theoretical density in less than 30 minutes at temperatures 850-900°C and applied pressures of about 70-105 MPa. The dominant densification mechanisms were identified at various stages of the process.

Pressure-induced sintering of ZnS compacts was shown to require much lower temperatures than those used in conventional sintering processes. The hardness of the ZnS compacts was found to be directly proportional to the initial densification pressure. Micrographs of the ZnS compacts showed that trapped air causes pores and produces spring-back effects hindering densification and leading to cracking. Initial studies were carried out in a miniature diamond anvil high pressure cell. More recent work involved the preparation of much larger samples utilizing large volume hydraulic presses. Hardness and toughness were measured by the microindentation technique. Fracture toughness, as well as hardness, was shown in some cases to be superior to those obtained by conventional sintering of ZnS. It was also shown that the addition of NiS to the ZnS markedly improved the fracture toughness of the material. Pore pressure is a critical factor in pressure-induced sintering.

Although zinc sulfide has good infrared properties, it has poor fracture toughness. Thus, processing or particle-impact induced flaws can make zinc sulfide windows and domes susceptible to cracking under stresses arising from thermal or mechanical loads. An attempt was made in this program to increase the fracture toughness of zinc sulfide by adding zirconium sulfide, which is also transparent in the infrared. By adding different quantities of zirconium sulfide, it was observed that the resultant composite had a toughness that was more than double that of pure zinc sulfide processed in the same manner. More significantly, the toughness improvement was not accompanied by any strength reduction, although the strength of pure zirconium sulfide was only one-fourth that of zinc sulfide. Microstructural examination indicated toughening by crack-bridging by zirconium sulfide, that was stretched into stringers by the processing. The data and details on toughening mechanisms are presented here.

Germanium is extensively used in fabricating infrared optical elements. Its excellent mechanical, optical and electrical properties, along with its moderate cost and availability in large sizes, make it attractive in many aerospace applications. Its primary limitation in these applications is free carrier absorption, which limits its utility to thermal environments where the temperature does not exceed 200°F. Free carrier absorption can be controlled to a limited extent by doping of the material, allowing optimization for a given thermal environment. This paper presents the results of a recent effort to characterize the infrared absorption of low resistivity germanium over temperatures ranging from 0°F to 250°F. A computational model of germanium absorption in the long-wave infrared (LWIR) is compared with measured data.

Transmission measurements have been made on a set of germanium samples having a range of resistivity values, at temperature increments up to 150°C. Values of absorption coefficient have been derived from these measurements and plotted against temperature on a logarithmic scale, covering two orders of magnitude. The curves allow the optimum resistivity to be selected for a given, application, accorilling to the component thickness and the operating temperature range.

A number of chalcogenides and phosphides crystallize with the tetrahedral structure in which the cations and anions are in tetrahedral positions. The II-VI compounds such as ZnS, ZnSe, CdS and CdSe are known to transmit in the far infrared. However, they are soft and decompose in air at relatively low temperatures. Efforts have been made at Brown University to improve the properties of these compounds, either by alloying them with III-V semiconductors, or by substituting transition metals such as nickel for the divalent specie.

III-V compound semiconductors, specifically gallium arsenide and gallium phosphide, have potential application as high-speed windows and domes for broadband infrared transmission, especially the 8- to 12-μm region. Their application as very high speed domes is made possible by enhancing their mechanical properties, particularly fracture toughness. This work describes the process used to hot press GaAs and GaP powders to near theoretical densities. High resistivity materials were used to form the powders in simple milling techniques. Near theoretical densities were achieved at pressures of 30,000 to 40,000 psi and temperatures approximately two-thirds of the material melting point. Morphological studies using Nomarski microscopy, SEM, and TEM of fracture surfaces and polished and etched surfaces reveal significant dislocation loops, subgrain boundaries and texturing consistent with an increased fracture toughness in the hot pressed powders. The morphology and force versus displacement data verify the mechanism of densification to be plastic deformation. Sample disks of the hot pressed powders were fabricated to a 1-inch diameter and 1/8-inch thickness. The hot pressed GaAs showed 8- to 12-μm transmission to 16 percent limited by light scattering, probably caused by residual porosity. The hot pressed GaP has not shown any transmission, probably because of light scattering from residual porosity and absorption caused by a Ga-rich phase from decomposition of the powder surfaces during pressing.

Theoretical and experimental studies have been made on the effect of thin hard coatings on the stress fields generated by indentation and impact onto a flat half space. The theoretical work uses finite element techniques and shows that a thin hard coating can have a significant effect on the maximum tensile stress generated in the substrate providing there is a good bond at the coating/substrate interface. As it is technically difficult to deposit layers of thickness greater than a few microns without residual stresses causing debonding and peeling; double layer and multilayer systems have also been examined. Hertzian indentation on hard carbon coated germanium has shown that such coatings do offer protection against indentation damage. High speed liquid impact on coated germanium and zinc sulphide has shown, however, that although the quasi-static performance of the material may be improved the impact resistance of the coated material is not significantly affected. Reasons for this are discussed. In conclusion, although it is still to be hope that coatings that do improve the impact performance of materials can be manufactured, current coatings offer better protection against handling damage rather than against impact.

Ultra-hard multilayer anti-reflection coatings possessing a wide optical band-width have been developed for the common IR transmitting substrate materials. Coatings have been produced using RF plasma assisted chemical vapour deposition from a variety of feedstocks. All the coatings possess excellent abrasion resistance - on a par with that of diamond-like carbon, when assessed in a wiper test using a sand/water slurry. The coatings all exhibit excellent resistance to the effects of rain erosion. The best of these coatings on zinc sulphide substrates for 8-12μm, when tested in the whirling arm erosion rig at RAE, Farnborough, exhibited a higher level of protection than any other coating or substrate material ever tested there.

Sophisticated coatings are increasingly being required for use in a wide variety of laser applications. This work is concerned with a study of the problems influencing the fabrication of such structures using molecular beam techniques. Many of the issues involved are concerned with the achievement of stable structures that do not shift under temperature cycling or laser irradiation. These centre around the fundamental properties of the coating materials selected, the degree of perfection of the films, and the control of microstructure and interface interdiffusion. Results have been obtained, for example, which show the effect of varying the thickness of the reflecting interfaces on the bandwidth and intensity of the fundamental reflection band of Distributed Bragg Reflectors. The degree of interface perfection in such structures has been examined using cross-sectional transmission electron microscopy, and correlated with the results of depth profiling X-ray photoelectron spectroscopy studies.

Layers of amorphous germanium/carbon with low absorption loss and good abrasion resistance have been deposited on infrared substrates by RF plasma deposition of germanium in carbon containing atmospheres (from feedstocks of CH4, C4H10 etc). The optical properties of the layers (up to 100 μm thick) have been assessed using a FTIR spectrometer and found to be strongly dependent on deposition parameters and layer composition. Layers can show high hydrogen content and absorptions caused by C-H and Ge-H bonding, but, by suitable choice of starting materials and deposition conditions, layers free of such absorption bands can be prepared with good transmission from 3μm to 12μm. Application of this novel material to the development of high durability antireflecting coatings for zinc sulphide and germanium is discussed.

Two of the materials most often used as optical windows due to their high transmittance at infrared wavelengths are zinc selenide (ZnSe) and zinc sulfide (ZnS). However, these materials are soft and often degrade when subjected to a particle-impacting environment. Diamondlike carbon (DLC) films have the potential to protect optical windows, such as ZnSe and ZnS, from rain and particle erosion as well as chemical attack. Diamondlike carbon films were deposited on ZnSe and ZnS, and have been evaluated as protective coatings for the optical windows exposed to particle and rain erosion. The DLC films were deposited on the windows using three different ion beam methods. One method was sputter deposition, while the other two methods used a 30 cm hollow cathode ion source with hydrocarbon/argon gas to directly deposit the DLC films. In an attempt to improve the adherence of the DLC films on ZnSe and ZnS, techniques such as ion beam cleaning, ion implantation using helium and neon ions, and thin ion beam sputter deposited intermediate coatings were employed prior to deposition of the film and were also evaluated. The protection the DLC films afforded the windows was quantitatively determined by exposing the surfaces to 27-μm-diam A1203 particles in a microsandblaster. A Perkin-Elmer IR spectrophotometer was then utilized to indicated the change in specular transmittance between 2.5 and 50 μm as a result of the erosion. The DLC coated windows were also subjected to water droplets at 400 mph for exposure times up to 15 minutes. These samples were qualitatively evaluated by optically viewing the surfaces. The DLC films were also evaluated for adherence, intrinsic stress, and infrared transmittance.

Relatively thick free standing diamond films were grown using a microwave plasma technique and the IR optical absorption coefficient and refractive index determined over the wavelength range from 2.5 to 25 μm. The diamond layers were grown on arsenic doped silicon wafers and the silicon subsequently etched away to leave a free standing diamond layer. Both interference and absorption effects were seen. Interference was observed due to the film thickness of 22.5 ± 1.5 μm and absorptions were seen, characteristic of Type Ilb material, which are believed due to an acceptor impurity, probably aluminum. The absorption coefficient was found to range from 212 cm-1 to 20 cm-1, varying with wavelength with the highest effective energy loss at the high energy end of the spectrum. The effective refractive index also varied from a low of 2.36 to a high of 2.73. This relatively high measured "absorptivity" reflects energy losses arising from all sources including scattering from the relatively rough surface of the as grown film which is suspected to be a primary source of energy loss with unpolished CVD diamond coatings. It is shown here that CVD diamond, unlike most natural diamond, contains substantial amounts of non-diamond carbon and has a complex microstructure which can vary over the thickness of a given coating. Both can contribute to energy losses, particularly if the scale of the microstructure is of the same order as the wavelength of interest.

Polycrystalline diamond coatings and free-standing layers up to 70um thick have been grown by microwave plasma assisted chemical vapour deposition. A measurement of infra-red forward scatter indicates a low absorption loss.

An amorphic diamond-like film can be deposited from the plasma ablated by a laser from pure carbon feedstock. In this technique the output from Nd-YAG laser is focused on a graphite target placed in a UHV environment. The gross effect of the laser beam is to eject a plume of carbon vapor and then to ionize it. The resulting plasma traverses a drift space to the substrate to be coated. The deposited material generally appears smooth and transparent having an index of refraction of 2.2 to 2.5 and displays bright interference colors. Unrecognized variation of process variables sometimes produces a brownish coloration to the film. On Si(100), Ge, quartz, and glass electrical resistivity is high and the materials seem to fall into the same class of "dehydrogenated" materials as are traditionally produced by the ion beam methods. However, growth rates are much higher with this laser plasma source, routinely reaching 0.5 μm/hr. No seeding or heating of the substrate is needed and substrate temperatures seem to remain at ambient room values during processing. Progress in the characterization of this material will be reported.

The exceptional properties of diamond have stimulated a considerable research effort into the low pressure synthesis of diamond thin films for a diverse range of applications including:- tribological coatings, semiconductor heat sinks and (as in this work) protective optical coatings. Numerous deposition techniques have been reported in the literature including Microwave Plasma Assisted CVD (MPACVD). This paper briefly describes the MPACVD deposition system used at Plessey Research Caswell Ltd and outlines the effects of important deposition parameters on the growth morphology of diamond crystallites and thin films. Techniques including SEM, TEM and X-ray diffraction have been used to study the growth mechanisms of MPACVD diamond. IR and Raman spectroscopy have been used to characterise the deposited films and an IR reflection technique is described for studying the infrared properties of the layers. The effect of deposition parameters on the properties of diamond thin films is discussed with regard to the use of these films for protecting IR windows and domes.

Diamond has many properties that make it a promising candidate for demanding optical applications. Recently, it has become possible to grow polycrystalline diamond films at relatively low temperature and pressure using chemical vapor deposition (CVD) techniques with growth rates exceeding 50 μm per hour. This development may make diamond optical elements economically feasible. Unfortunately, CVD diamond films are rough on optical length scales, which causes considerable surface scattering. The surface roughness must be eliminated before meaningful optical measurements can be performed.

This paper presents the details of an experimental evaluation of the film cooling effectiveness of gaseous nitrogen and argon when injected near the nose tip of a hemispherical dome. A series of tests were conducted in the Johns Hopkins University/Applied Physics Laboratory Infrared (IR) Seeker Aerothermal Test Facility to determine the amount of coolant flow required for thermal and structural survival of an IR dome when exposed to stressing missile flight environments. The tests measured the temperature distribution over a simulated IR dome for two representative altitudes at a nominal Mach number of 5 and pitch angles of zero and 10 degrees. The key experimental result was that the entire dome can be cooled with the predicted coolant flow requirements. Reduced levels of heating into the dome was still achieved at 10% of the predicted flow. Coolant flow correlations based on a modified flat plate model were validated.

Normal spectral and normal total emittance values were obtained for sapphire and translucent alumina using Fourier-transform infrared spectroscopy. Values were derived from radiance measurements in the spectral range of 5000 cm-1 to 400 cm-1, and in the temperature range of 900 K to 1450 K. Samples were dropped from a furnace into the nitrogen ambient of the spectrometer to the position where the internal globar source is normally focused. Temperatures were determined from radiance values at 1027 cm-1 where alumina has zero reflectivity and transmittance. Standard deviations of the total normal emittances, expressed as percentages of the average of three drops, were ≤3% of the averages.

Electrically conductive (EC) coatings on optically transparent windows can be used to protect internal system's components against electromagnetic interference effects. The purpose of this paper is to derive simple but correct formulas for assessing the far-field shielding effectiveness of a thin metallic film or multilayer stack deposited on top of a dielectric slab. Using conventional transmission-line theory in conjunction with proper expressions for the transmittance of a perfect dielectric, the author demonstrates that, for coatings much thinner than the skin depth, the following holds. CO In a half-wave geometry, the shielding effectiveness (in decibel) is a function of the sheet resistance only: (SEλ/2 = 20log(1+188.5/Rs) if Rs is in ohm per square. And (b), in a quarter-wave geometry, the shielding effectiveness is best approximated by means of a semiempirical expression, (SE)λ/4 = 201og(1+Er)/(2√Er) + 188.5/(√ErRs), where Er refers to the dielectric constant of the substrate. These formulas provide upper and lower limits for the effective shielding performance of an EC-coated window, depending upon the phase thickness; they accurately predict measured microwave attenuations for a variety of visible- and infrared-light transmitting domes as well as for standard conductive-glass windows.

A series of environmental tests is being undertaken on various combinations of Infra Red coating and substrate materials, namely polycrystalline germanium, monocrystalline germanium and zinc sulphide. The purpose of this programme is to evaluate and select a suitable material combination for an aircraft borne Forward Looking Infra Red (FUR) system and to produce a material data base. The function of the window under evaluation was to act as both an environmental and aerodynamic protection to the front of the FLIR system. In this role it is required to be very durable and resilient. The window is expected to resist all environmental conditions that the aircraft may encounter and still maintain very low transmission losses. This paper details the environmental and optical aspects of the evaluation programme.

The optical properties of dielectric window materials are represented by the complex index of refraction, n(v,T), as a function of wavenumber and temperature. Recent experimental and theoretical efforts at JHU/APL have resulted in the development of models of intrinsic n(v,T) over a wide range of wavenumbers (from microwaves to the ultraviolet) and temperatures (from absolute zero to the melting temperature), on the basis of experimental data for a variety of materials. These efforts are now represented by a computer code that calculates the complex index of refraction for 19 optical materials. This capability represents a significant improvement over conventional handbook tables and figures. Because of the theoretical basis of the models, extrapolation and interpolation of experimental data are valid. The code greatly extends the current representation of optical materials and provides information not previously available.

It is well known that many of the thermomechanical properties of crystals are due to anhannonic effects in the motion of the atoms. The anhannonic terms in the potential energy which lead to this behavior depend, in part, on the symmetry of the crystal lattice. Thus the thermomechanical properties of the crystal, including the coefficient of thermal expansion, depend on the point group of the lattice. In order to study this symmetry dependence the path integral representation of the density matrix in the adiabatic approximation is used here to evaluate the thermal expectation value of the atomic positions. From this result the approximate scaling behavior of the atomic displacement with temperature, mass, bond strength (harmonic coupling), and anharmonic coupling is determined. The path integral calculation is fully quantum mechanical and can be used, in principle, to derive all the thermomechanical properties of the crystal within the adiabatic approximation.

The water drop impact conditions prevailing in several rotating arm facilities are analyzed. Indirect measurements of the water drop impacts on polymethylmethacrylate specimens were made when each facility was operating in order to evaluate the effects of the aerodynamic interactions on the actual water drop impact conditions in each facility. These results are contrasted with the static measurements of the water drop dimensions in order to demonstrate the consistency between the static evaluations and the dynamic operating conditions unique to each facility. It is found that the stated monodispersed distribution of 2.0 mm spherical water drops in these facilities is actually deviating significantly from this idealized condition when they collide with the specimen.

A comparison between G.S. Springer's theory, that predicts rain optical and rain mass degradations for different kinds of infrared materials, and the experimental results obtained by ONERA on the SAAB rain rotating arm on IR homogeneous samples is presented. The agreement between theory and experiment is rather poor. A very simple correlation between optical degradation and V, C, p (respectively : droplet impact velocity, longitudinal wave velocity and density of infrared materials) is proposed, which is in good agreement with all data obtained in the range of 200 to 300 m/s for the 9 tested materials.

This paper reviews recent liquid impact work in this laboratory. The research has included a theoretical study of the early stages of a liquid/solid collision where compressible effects in the liquid dominate the behaviour; experiments with two-dimensional gelatine shapes which combined with high-speed schlieren photography have allowed shock structures and jetting to be recorded; the development of techniques such as the jet method for producing controlled liquid/solid collisions in the laboratory, and studies of damage production in a wide range of aerospace materials. Quantitative studies of damage using post impact ("residual strength") measurement are described. Of particular interest are the "threshold velocities" for strength loss and the extent of strength loss following impact. Research on zinc sulphide has included a study of the effect of grain size on the hardness and Kic values of the material. Data are presented on a range of glasses, zinc sulphide, sapphire and germanium.

The advantages of jet methods over other forms of impact, for single impact studies, have been well documented. The recent development of a multiple jet impact apparatus (MIJA) at the Cavendish Laboratory now allows us to study multiple impact in a controlled way. Using the same nozzle design as the previous single impact devices, MIJA produces jets by pneumatically accelerating a titanium shaft into the rear water chamber of the nozzle. The shock-accelerated jets are of the high quality (spherically fronted, symmetrical and with a coherent core) necessary for simulating 2 to 10 mm diameter drop impact. Impact velocities in the range up to ca. 250 m s-1 have been achieved. A major advantage of the equipment is that the velocity and position of impact of each jet can be controlled and repetition rates of ~10 min-1 can be maintained for extended periods of time. Computer control is used to ensure that the chosen impact velocity is maintained; to control the specimen position (single site or linear and random arrays are possible) and to maintain an accurate impact record. The first part of the paper describes the design, construction and performance of MIJA. The final sections present experimental data, for a range of "window" materials, on threshold velocities and damage development.

Optical window materials were investigated for infrared sensor systems used in observing ground targets from a hypersonic-glide vehicle. The equilibrium temperature of the window in the glide region depends on the emissivity and varied between 1,370 and 2,250 K. The high temperatures showed that a protective cover over the window is required during the entire glide region of the trajectory. Ejection of the window cover at 70-kft altitude in the terminal region was assumed, resulting in maximum window temperatures of 565 K and 592 K for magnesium oxide and diamond windows, respectively, both 0.8-in thick. The window temperatures for germanium and sapphire were also calculated. Thermal shock, thermal expansion, the effects of the window radiation on the infrared detectors and methods to reduce the hot window problem were examined.

A computer simulation of head-on infrared emission from missile noses has been performed. Both cruise missiles and tactical ballistic missiles were considered; results indicate a strong dependence of the emitted radiation on physical and geometrical properties of aerodynamic surfaces, flow character in the boundary layer and atmospheric characteristics. The model allows to give an order-of-magnitude estimate of radiant intensity in the 8-12 μm and 3-5 μm spectral bands and to determine their relative merits as far as the target detection is concerned.

A simple numerical method has been developed to predict the trajectories and impact characteristics of solid particulate material after traversal of the complex flow structure existing over a hypersonic forebody. Specifically, particle impact characteristics are examined for an aerodynamic structure incorporating a forebody window assembly which is shielded from aerodynamic heating via gas coolant flow. For calculation of the flow field properties inside the shock layer surrounding the forebody, a computational fluid dynamics (CFD) numerical solution of the hypersonic compressible flow field is employed. The nature of the aerodynamic drag acting on the particle is thoroughly investigated.

The Infrared Instrumentation System (IRIS) was developed and deployed by the U.S. Army Strategic Defense Command to gather signature data on objects reentering the earth's atmosphere. The system has subsequently gathered data on a variety of other IR emission sources of interest in the Strategic Defense community. Multiple calibrated IR imaging cameras make it possible to obtain data in two wavebands simultaneously. An airborne platform makes it possible to obtain data at high altitudes in close proximity to many targets. A large zinc sulfide window designed to provide good transmission in visible and IR wavebands has been installed in a removable hatch used in conjunction with the aircraft. Extensive experience in developing and using such a window have been gained as part of the IRIS Program. The IRIS and the window system and experience gained are described.

Renewed interest in hypersonic flight has generated a requirement to validate optical sensor performance in the hypersonic environment. In this paper, the flight aero-optic environment is examined and the mixing layer is determined to be the largest aero-optic contributor. The paper then examines how this mixing layer can be reproduced in a ground test facility. Teledyne Brown has developed and constructed a compact hypersonic aero-optic simulator which properly scales the mixing layer. This small simulator is designed to emulate the significant aero-optic features of the hypersonic window coolant mixing layer present in current interceptor designs. Through careful selection of gas supply conditions and species, this experiment is capable of reproducing the most significant aero-optic effects of the flight environment.

A novel experimental test setup has been assembled to simulate the primary aero-optical effects encountered by a cooled window on a hypersonic interceptor. Although the equipment was located in an ambient laboratory environment, emulation of the principal flight level aero-optical effects should be possible with some modification to the equipment. The test hardware and preliminary results from a proof-of-principle demonstration are presented in this paper.

Pure diamond has excellent mechanical and infra red properties and would make an ideal window material were it readily available. Synthetic diamond films can be grown with good infra red transmission and used as protective coatings. Many coating requirements can already be met using diamond-like carbon films.

The optical and semiconductor properties of lead telluride coatings are dependent on various factors contributing to its performance. In this paper, we will present the temperature dependent effects of single layer lead telluride coatings on the dispersion and absorption characteristics, absorption edge, and carrier concentration from 15 K to 436 K using both experimental and theoretical analysis.

Present research indicates that amorphous materials that exist in metastable equilibrium impart environmental ruggedness to multilayered infrared (IR) optical components. This study throws new light on the 'microstructure engineering' of IR optical layers for improved performance. Thin film designs in the IR band were experimentally realized with sputtered Germanium and plasma-deposited amorphous hydrogenated carbon (a-C:H or DLC) as the terminating layers. All the optical components survived very stressing environmental tests like thermal cycling/thermal shock and acid immersion. The grain boundary-free nature together with the inherent chemical inertness of the sealing diamondlike carbon layers imparts the environmental ruggedness to the IR optical components.

Boron Phosphide satisfies many of the material requirements for a coating suitable for use on high velocity IR windows. Such coatings have to be robust, resistant to abrasion and to rain erosion, and also capable of surviving thermal shock. Films of BP of several tens of microns in thickness have been produced by plasma assisted chemical vapour deposition on a variety of feedstocks. Growth has been achieved on a wide range of substrate materials, with no apparent limitation in film thickness, suggesting low levels of stress. The degree of optical transparency is high, covering 0.8 μm to 12 μm and beyond, with absorption levels being an order of magnitude lower than found for typical diamond-like carbon (DLC) films. The ability to survive severe erosive conditions is also significantly improved compared with DLC in wiper tests, water jet impact experiments are in whirling arm rain erosion tests.

Thin films deposited by standard electron beam evaporation have a columnar microstructure and consequential water adsorption which is not prevalent in fully dense ion plated optical coatings. In electron beam evaporated films the water becomes an intrinsic part of the film since the packing density is less than unity, and causes absorption at and around 3 μm. In contrast, the absence of water in ion plated films makes a number of oxide coatings suitable for use in the first atmospheric window (3-5 μm). In this study, the spectral transmission curves of single layer and broadband multilayer coatings in the 4000 - 2000 cm-1 range show clearly that ion plated coatings do not contain any water, while electron beam evaporated thin films have strong absorption around 3400 cm-1. The refractive indices of several oxide, silicon nitride, and pure silicon films are shown to be consistently higher for ion plating than for electron beam evaporation, also indicating a higher packing density.

Rugate filters have come under considerable investigation over the last several years. Their attraction stems from two possibilities, 1) the creation of almost any kind of filter and 2) the graded index nature of these films should make them more robust. The index - thickness profiles necessary to make these filters can be quite complex. The rather sophisticated feedback control system required to fabricate these films has made progress difficult. Most rugate filters have been designed and fabricated for use in the visible and near infrared portion of the spectrum. Filters designed for regions beyond 1.4μ become very thick, where stress in the films becomes severe and the films delaminate. We report here the fabrication of rugate filters at wavelengths from 1.0 μ to 11μ. By varying the refractive index with film thickness, a reflection or transmission band may result at a specific wavelength. With a well chosen index versus film thickness profile, the filter can be tailored to cause narrow band reflection spikes at any wavelength and yet allow transmission of broadband radiation. The principle of the rugate filter is similar to the principle of multilayer dielectric stack filters such as the quarter-wave stack. Major differences exist, however, in the way they are physically realized. In the rugate filter, the discontinuous material interfaces of dielectric stacks are replaced by a controlled, gradually changing refractive index profile in the film. This index profile is achieved by changing the stoichiometry of a material (re.g., ZnSxSey ) as the film is deposited.

The high dynamic range of naturally occurring atmospheric emissions can present a serious challenge to infrared instrumentation designers. The requisite optical filters have out-of-band rejection requirements that are beyond the capability of the standard measuring techniques. While the filter manufacturers can predict their filter performance to more than seven orders of magnitude, their measurement capability is limited to about four orders. The Space Dynamics Laboratory has recently tested a new measurement method which employs a cascaded test filter and a Michelson interferometer spectrometer.