The new military requirements that call for the use of eyesafe lasers for range finding capabilities on aircraft, ships, tanks, and other vehicles has put an extra burden on the transmission of conductive coatings. Originally, the window coatings have low resistivity, less than 7.0 ohm/square, for EMI control were required to transmit up to 1.06 micron. With the new eyesafe lasers for range finding, the requirements of near infrared transmission have been pushed toward longer wavelengths. The windows are now required to have good transmission at 1.543 microns (eyesafe laser wavelength). The traditional indium-tin oxide coating has poor transmission varying from 20% to 50% due to free carrier absorption. A new conductive coating developed at HDOS has good transmission in photopic and xenon tracker regions and it has good transmission at 1.543 micron. This coating also has good environmental durability. The spectral performance, environmental durability, rain erosion resistance, acid resistance, and sand abrasion resistance of this coating are also significant.

Thorium fluoride ThF₄ has been used over the last several decades as a low index coating material for many optical applications such as in anti-reflection (AR) coatings, high reflective coating, beam splitters, coating dichroics and optical filters. ThF₄ has excellent optical and mechanical properties but unfortunately is slightly radioactive. New guidelines from many government agencies require that ThF₄ shall not be used as coating material for safety reasons. Also new stringent requirements on the disposal of radioactive material in many states within the USA have made it economically difficult to use. This paper therefore deals with AR coatings which do not have radioactive materials in them. Three types of coatings based on applications have been discussed in this paper. These are namely (1) AR coatings for long wavelength infrared region (LWIR), (2) AR coatings for 1.06 micrometer and long wavelength infrared regions (LWIR), and (3) AR coatings for multiple wavelength regions namely VIS/NIR/MWIR (medium wavelength infrared region) and VIS/NIR/MWIR and LWIR. The spectral performance, mechanical and environmental durability, and rain-erosion test data is presented in this paper.

Optical properties and rain and sand erosion resistance of the following infrared window materials were measured: ((1) Barr & Stroud boron phosphide coating on multispectral zinc sulfide, (2) Barr & Stroud gallium phosphide coating (with a thin outer layer of boron phosphide) on multispectral zinc sulfide, (3) Raytheon zinc sulfide coatings on multispectral zinc sulfide, (4) Texas Instruments bulk gallium phosphide, (5) polycrystalline magnesium fluoride, and (6) single-crystal silicon. ZnS-coated ZnS has low optical emission for operation at 500 degrees Celsius in both the 3 - 5 and 8 - 10 micrometer regions. Bulk GaP and bare MgF₂ have low emission only in the 3 - 5 micrometer region. BP/ZnS and BP/GaP/ZnS have prohibitive optical emission at 500 degrees Celsius in both the 3 - 5 and 8 - 10 micrometer regions. In whirling arm rain erosion experiments, none of the coated materials was as durable as bare MgF₂. BP/ZnS is more durable than ZnS/ZnS, but subsurface damage preceded damage to the BP coating in BP/ZnS. GaP fractured easily on orthogonal crystal planes upon raindrop impact. In sand erosion experiments, BP and BP/GaP/ZnS were best and MgF₂ was second most durable. A procedure is proposed for conducting comparative rain and sand erosion tests of inclined windows.

Carbon nitride films were deposited on a variety of substrates by electron cyclotron resonance (ECR) chemical vapor deposition (CVD) using halogenated hydrocarbon precursors in a high-density-nitrogen plasma. With the proper deposition conditions, the chlorine-doped carbon nitride films are conformal, extremely smooth and uniform over large areas. Deposition rates in the 5-kilowatt ECR-CVD chamber of up to ten microns per hour over a four-inch-diameter substrate have been demonstrated. Low-melting substrates like plastics and low-melting glasses have been coated because the deposition temperatures can be kept below 100 degrees Celsius in the streaming plasma of the ECR-CVD. Film microstructure and chemical composition were studied using atomic force microscopy (AFM), auger spectroscopy, electron spectroscopy for chemical analysis (ESCA) and Fourier transform infrared (FTIR) spectroscopy. Preliminary results indicate that the carbon nitride coatings deposited using halogenated- hydrocarbon precursors contain less than ten atomic percent chlorine and very little carbon-hydrogen bonding. One set of experimental conditions resulted in the deposition of 100- micron-size crystallites on a glass substrate.

The preceding paper in these proceedings provided a detailed discussion of colloidal and crystalline-phase chlorine-doped carbon nitride (CN_x:Cl) films synthesized by electron cyclotron resonance (ECR) chemical vapor deposition (CVD). This paper focuses on diamond-like carbon (DLC) and hydrogenated carbon-nitride (CN_x:H) films deposited by ECR-CVD in either a nitrogen- or an argon-high-density plasma using one of the following hydrocarbon precursors; trichloroethylene, tetrachloroethylene, neopentane or ethylene. For clarity, results and discussions for the CN_x:Cl deposits are presented again. Electron spectroscopy for chemical analysis (ESCA) and Fourier transform infrared (FTIR) spectroscopy show that the ECR-CVD experiments so far have resulted in four distinctly different chemical compositions; CN_x:Cl, CN_x:H, hydrogenated amorphous carbon (a-C:H) and chlorine-doped amorphous carbon (a-C:Cl). The a-C:H films deposited from neopentane and nitrogen have the lowest refractive index and the highest bandgap. The a-C:Cl films exhibit a strong infrared peak at about 1560 cm^-1 indicating that they may contain a large amount of microcrystalline graphite.

The degradation of various aerospace window materials by the action of solid particle impact is investigated. Laboratory simulations of high velocity sand, dust and hail impacts have been carried out, and the damage assessed in terms of the reductions in optical and mechanical performance. A new erosion rig has been designed to cover the broad size and velocity range of airborne particulates encountered by moving craft, and an ice-firing gas-gun was used to simulate hail impacts. IR-transmitting materials studied include CVD diamond, sapphire and coated zinc sulphide. A radar-dome composite material was compared with polymethylmethacylate (PMMA) for hail impact. Nylon spheres were assessed as a convenient simulation for ice and were found to cause very similar damage over parts of the velocity range studied. The different damage mechanisms observed in these materials are discussed and the extent of degradation by multi-particle erosion related to the particle size and impact velocity.

Accurate and reliable testing is paramount to the development of LWIR window materials. Without appropriate characterization and testing, improvements to existing technologies are impossible to document with certainty. Reliable and repeatable testing provides the data needed to measure advancements and identify improvements in any technology. No single test can be completely definitive, and the continuous evaluation of emerging technologies using different test methods under varying conditions is critical when evaluating a new materials' capability. The environmental testing of infrared (IR) window materials has traditionally consisted of rain erosion testing, single impact water jet testing, and sand erosion testing. While these three tests provide the materials engineer with significant insight into the durability of a window material, these tests have generally ignored the combined effect of rain and sand. This paper looks at the combined effect of rain and sand erosion on a standard LWIR window material, zinc sulfide (ZnS).

An experimental study has been made of the deformation and fracture produced around scratches in brittle solids such as soda-lime glass, sapphire, and poly-crystalline spinel. In the scratch experiments, a Vickers indenter was translated against a surface at velocities of a few mm/min with the normal load being kept in the range of 10 g to 700 g. The contact region was observed and photographed in-situ using a high resolution, optical microscope and video imaging system, while forces were measured with a piezoelectric force sensor attached to the indenter. The experiments have enabled several unique observations to be made of the contact damage such as the formation of plastic scratches, slip lines, and ribbon-like wear particles; the loads at which cracks form and their evolution during loading and unloading; and a new type of crack system around scratches in soda-lime glass which forms during unloading of the indenter. The implications of these results to contact damage and wear in brittle solids are discussed.

Current spectrometers are unsuitable for measuring transmission of large thick electro-optical windows because the optical path through the instrument is changed by the optic. Customization of a standard FTIR spectrometer capable of making normal and non-normal incidence angle transmission and reflection measurements is presented which addresses this and other problems associated with high precision spectroradiometry. Emphasis is on design features to ensure ordinate accuracy of these measurements. These techniques can replace witness sample testing commonly made in the industry.

A variety of nondestructive characterization techniques has been used to detect and measure subsurface damage in single- crystal sapphire to develop methods suitable to inspect high performance optics for sub-surface damage. These techniques include polarized light microscopy, x-ray diffraction topography, transmission electron microscopy (TEM) and Raman spectroscopy. TEM examination shows that for ground surfaces damage can extend up to 6 - 7 micrometers into the bulk and includes cracks, twins and dislocations, while under polished surfaces only dislocations are seen. X-ray diffraction topography can image defects such as long-range strain, dislocations, residual surface scratches (not visible optically) and low-angle grain boundaries (lineage). Polarized light is also sensitive to strain and provides a relatively easy method for detecting defects such as cracks and lineage. Of all of the techniques Raman spectroscopy offers the best potential for quantifying strain in terms of stress.

The sudden exposure to a supersonic flight environment subjects a missile window, or missile dome, to intense convective heat loads stemming from the rise in temperature of the boundary layer. The thermal response of the window then results in temperature gradients through the thickness, which generate transient stresses that may exceed the tensile strength of the material, thus causing thermal shock induced fracture. Since most of the materials that possess favorable optical properties in the infrared (IR) are relatively weak brittle solids, the problem of selecting window/dome materials and assessing their performance on a fly-out trajectory requires a careful evaluation of the window's ability to withstand thermally induced shocks. In this context, it is essential to keep in mind that the transient stress intensity depends on the nature of the heat flow as characterized by the Biot number (Bi). The allowable heat flux depends not only on intrinsic material properties but also on the heat-transfer coefficient if the condition Bi greater than 1 holds, or the thickness of the window if the condition Bi less than 1 applies. In a first approximation, the thermal shock performance of a 'thick' window will be controlled by the figure of merit (FoM)_{Bi greater than 1} equals R_H, i.e., the Hasselman parameter for strong shocks; in a thermally thin regime, however, the appropriate figure of merit is (FoM)_{Bi less than 1} equals sigma_fⁿR'_H with n equals 1/2 for flat plates and n equals 2/3 for hemispherical shells, and not the Hasselman parameter R'_H for mild shocks. Judging from the results of thermal shock testing performed elsewhere, we conclude that in a laminar flow environment the allowable heat flux on a thermally thin IR dome can be expressed as follows: Q_lim equals 2R'_H/L, where L is the dome thickness. This expression provides a direct means of obtaining the Mach altitude failure line for a dome of given thickness and given radius, if the initial wall temperature is known. Furthermore, it then becomes straightforward to assess the thermal shock resistance capability of a thickness-optimized IR dome either in terms of the allowable heat load or, more simply, the allowable stagnation temperature.

With increasing speed of missiles, rain erosion becomes a more and more worrying problem for seekers and particularly for infrared domes and windows, since infrared materials are relatively brittle. In the last twenty years some efforts have been done to measure the rain erosion resistance of these materials and to rank them. These measurements were performed at room temperature. In fact, the increase in velocity is followed by an increase in temperature of the infrared materials. Consequently, the real rain erosion resistance can be different of the one determined at room temperature. ONERA made in 1989 a first attempt to measure the influence of temperature by using a special holder in the SAAB-SCANIA rotating arm. The samples were heated at the extremity of the arm during the rain erosion experiments. Preliminary results were obtained but the temperature domain was limited to about 100 degrees Celsius by the high value of the convection transfert coefficient due to the arm rotation. More recently, such experiments were performed in our lab using a water jet generator. In this case there is no displacement of the sample, thus its temperature can be higher than in rotating arm experiments and well known. Rain erosion measurements were performed up to 200 degrees Celsius. The strong influence of temperature on the rain erosion resistance was confirmed but one can wonder if the decrease of rain erosion resistance with temperature is due to a decrease of the mechanical characteristics of the material or a thermal shock effect. Some basic experiments were done to answer this question and computations were performed to determine if, during a typical flight, there is a risk of thermal shock due to the difference of temperature between droplets and the window material.

This paper describes polished CVD polycrystalline diamond components, many in excess of 1 mm in thickness, being produced in the sizes and geometries required for many infrared window and dome applications. For instance, 100 mm diameter flats can now be fabricated routinely and 70 mm diameter hemispherical shells are being developed. It is essential that the properties of the material are suitable for such applications and this paper describes the characterization of these properties. Data on the material's optical transmission properties, absorption coefficients (at a wavelength of 10.6 micrometers) and imaging properties is presented. The dielectric properties, such as loss tangent and permittivity, have been assessed at microwave and millimeter wave frequencies and the suitability of CVD diamond for multispectral and multimode window applications is discussed. The strength of the material is also of considerable importance and this parameter has been evaluated as a function of sample thickness and grain size and the specific problem of assessing the strength of curved samples is also considered. Numerous curved and several hundred planar samples have been strength tested using a three point bend geometry and a statistical analysis (Weibull) has been applied to the results. The average strength of the CVD diamond growth surface, for samples of thickness 0.4 to 1.4 mm, has been determined to be 470 to 390 MPa respectively, with a Weibull modulus of 23 (when systematic variations in sample strength with thickness are taken into account), while the nucleation surface was found to have a higher average strength of 1000 to 770 MPa (at 0.4 and 1.4 mm thickness respectively) but a lower modulus of 11. The cause of this difference in strength is discussed in detail.

Microwave plasma deposition techniques for preparing optical quality, uniformly thick diamond plates and domes continue to be actively developed. Thicknesses greater than two millimeters have been demonstrated for optical quality dome blanks, and excellent optical quality has been obtained for five inch diameter plates. Presently achievable CVD diamond is the strongest LWIR-transparent material and is impervious to thermal shock. It can be used in its present form for prototype window preparation. Nevertheless, further strength improvement is required for CVD diamond for applications requiring resistance to particle (e.g. raindrop) impacts. Fracture strength improvement is a primary objective of diamond technology development. Continued development of optical diamond technology must also emphasize increased deposition rates for the highest quality material and optimized polishing processes to assure its place as a cost effective optical material for high performance applications.

The high velocity environment endured by infrared (IR) domes often results in strength and transmission loss due, in part, to rain erosion. The multiple impact jet apparatus (MIJA) has proven, over a number of years, to be an accurate and rapid means of simulating this erosion phenomenon on many conventional IR materials in the laboratory and obtaining quantitative data on damage threshold velocities and transmission loss. Recent years have seen a rapid development of good optical grade CVD diamond with the eventual achievement of full hemispherical IR domes. Samples of CVD diamond from different sources and of differing thicknesses have been rigorously investigated on MIJA. Due to the low shock wave attenuation and the high shock wave velocity (approximately equals 18 mm microsecond(s) ^-1) a number of damaging mechanisms are additional to other than the typical liquid impact pattern, with the relative importance of the damage mechanisms depending upon the thickness of the sample, the surface (i.e. growth or nucleation) and location of the impact. The results in this paper include initial tests on full hemispherical diamond domes, comparison with natural diamond and high temperature and high pressure (HTHP) diamond and discussion of the minimum thickness of a proposed CVD diamond dome. These data are compared with those obtained on other IR transmitting materials.

Durable coatings are used to improve the erosion resistance of high performance optical materials such ZnS. Diamond is the hardest and stiffest of all LWIR transparent materials and would make an excellent protective coating for ZnS. Direct deposition of diamond on ZnS by microwave plasma CVD has proved to be very difficult. Atomic hydrogen used in the diamond deposition process attacks and destroys ZnS very rapidly. In order to protect ZnS during the diamond deposition process protective IR transparent interlayers were developed. These layers encapsulate the ZnS and provide a nucleating surface for diamond deposition. Two different methods of nucleating diamond on these interlayers were developed to produce fully dense diamond films several microns thick. The sand erosion resistance of diamond coated ZnS was found to improve when the diamond was deposited on patterned ZnS substrates.

Forward looking infrared windows on aircraft suffer damage from liquid and solid particle impact. Diamond is a suitable material for protective coatings on the germanium and zinc sulphide windows used for the 8 - 14 micrometer waveband. The coating of small samples with diamond using microwave plasma assisted chemical vapor deposition has been reported previously, and that technology is being developed to coat flats and domes to over 150 mm in diameter. As a demonstrator, germanium windows for the Harrier GR7 and AV8-B FLIR systems have been diamond coated, and data on these windows is presented. This data includes optical performance in the 8 - 14 micrometer wave band, and the issues of reduction of reflection and scattering losses are discussed. The results of dust erosion and water jet impact testing are presented. The relation of the dust testing parameters to the conditions that military aircraft might be expected to experience in desert condition is discussed. At present, most dust/sand erosion tests appear too severe in terms of dust and sand particle size distributions at given velocities and altitudes.

Most commercial diamond synthesis processes involve some form of chemical vapor deposition (CVD) which results in heterogeneous nucleation on the surface of window or dome materials. Generally, these processes have relatively long deposition times driven by the slow CVD kinetics. An alternate method called DIACER^TM uses an aqueous seed crystal dispersion applied to the window substrates prior to CVD. These seed crystals reduce nucleation times and speed CVD deposition rates. Thicker coatings can be produced by repeating the seeding/CVD cycle until the required thickness is achieved. This paper reviews DIACER^TM coating results on silicon substrates. Scanning electron microscopy and atomic force microscopy images of images of the coatings are presented. IR transmission results are presented both before and after sand and rain erosion exposures. The results of this testing will show DICER^TM coatings to durable for the protection of silicon substrates after exposure to severe sand environments.

State of the art optical sensing systems performing target acquisition/tracking and surveillance functions are being designed to incorporate a number of sensors into one package. These include visual and MWIR cameras, FLIRs, and laser range finders. These combined systems are being configured to view through a common aperture window. Typical window diameters are to eleven inches, but some surveillance applications have windows approaching twenty inches in diameter. These sensor windows typically operate in hostile environments including very high pressure differentials, large thermal gradients, and severe rain and sand abrasion. EMI/EMC protection and de-icing capabilities are also commonly required. For airborne applications and to minimize thermal gradients, thinner, lightweight, high strength windows are also necessary. Sapphire is an ideal window material to satisfy these requirements due to its high strength, UV-MWIR bandpass, minimal optical scatter, excellent index of refraction homogeneity and very high scratch/impact resistance. Associated optical fabrication, grid lithography and optical coating processes have been developed at Hughes Danbury for sapphire windows. This paper addresses the development of a family of large aperture, broadband sapphire windows which also provide EMI/EMC protection and de-icing capabilities. The resulting design configuration and performance characteristics are also addressed. Future technology development requirements are also discussed.

Sapphire's loss of strength between 20 degrees and 1000 degrees Celsius depends on orientation and state of stress. The critical weakness of sapphire occurs in compression along the c-axis of the crystal. In flexure tests of sapphire that is not subject to c-axis compression, the strength actually increases between 20 degrees and 1000 degrees Celsius. Compression on the c-axis causes twinning on rhombohedral crystal planes. When twins on different planes intersect, a crack forms and the specimen is then subject to tensile failure. Doping with Mg²⁺, Ti⁴⁺, or introduction of a TiO₂ second phase each doubled the c-axis compressive strength of sapphire at 600 degrees Celsius, probably by inhibiting twin propagation. X-ray topography was employed to investigate the relationship between surface and bulk defects and mechanical strength in sapphire. Low angle grain boundaries were not associated with mechanical weakness. Wide, transverse scratches that are evident to x-rays, but not obvious in optical microscopy, can weaken sapphire. Topography demonstrated that annealing reduces long range strain in polished sapphire.

The carbon-dioxide laser provides an effective, inexpensive method for simulating aerodynamic heating in sapphire seeker windows for thermostructural testing. It has been shown, for flat, side-mounted windows, that the window edges are design controlling for two reasons. Peak stresses occur in the window edge from aerodynamic heating, and the edge strength is degraded from subsurface machining damage during window fabrication. Window edge flight stresses can be approximately replicated in small laser-heated sapphire coupons that are geometrically similar to the seeker window edge. A 3-kW carbon-dioxide laser at The Aerospace Corporation Mechanics and Materials Technology Center has sufficient power to produce flight-like thermal stresses that occur in theater missile defense (TMD) interceptor seeker windows, with ample power to produce thermal stress fracture for statistical strength margin determination. The laser approach offers several advantages over alternative methods for thermostructural testing. These include its low test cost per sample, which permits many samples to be thermally fractured to obtain statistical strength data; its repeatable, well- defined thermal loading conditions; and the absence of contamination of the window surface.

Subjecting a-plane (90 degree) polished sapphire discs to a high temperature anneal resulted in a 31% improvement in the average strength, and a decrease in the strength variability. Implications on the design of sapphire pressure windows (thickness, weight, transmission) are reported.

Large sapphire boules, up to 34 cm diameter, 65 kg, are being grown by the heat exchanger method (HEM) and even larger sizes are sought to meet future requirements of advanced optical systems. These boules, especially in large sizes, exhibit lattice distortion and light scatter in a very narrow range. A qualitative grading system has been developed to characterize sapphire. Windows of five grades and different orientations were prepared and measured for refractive index homogeneity to evaluate transmitted wavefront distortion. The data showed that the refractive index homogeneity for all samples was in the 10^-7 (0.1 ppm) range. The fact that lattice distortion does not affect the transmitted wavefront allows fabrication of large sapphire windows in production mode at low cost.

Because the index of refraction is temperature dependent, a temperature gradient across a window causes image blur and boresight error. Accurate temperature and frequency dependent refractive index models can now be constructed from visible measurements of the refractive index, far-infrared reflectance measurements, thermo-optic coefficient measurements, and infrared measurements of the absorption coefficient. Visible measurements determine the contribution to the refractive index from electronic transitions. Measurements of dn/dT are reported on the ordinary ray of sapphire in the 4 micrometer region. Far-infrared measurements determine the contributions from fundamental lattice vibrations (phonons). Infrared absorption data are used to determine parameters in a multiphonon sum band model of the refractive index. Two- and three-phonon contributions to the refractive index are important for an accurate model that includes frequency and temperature dependence. Results for the ordinary- and extraordinary-rays are obtained.

Impact and thermal stresses can cause severe damage during use of single crystal electromagnetic window materials. In addition to cracks and plastic flow, deformation twinning can be activated at relatively low temperatures. However, because of the crystallographic nature of deformation twins, i.e., twins can only form on specific plans with a deformation shear in well defined crystallographic directions, proper choice of the crystalographic orientation can limit the amount of deformation twinning induced during use of a single crystal electromagnetic window or dome. The conditions to activate deformation twinning of several single crystal window materials are analyzed, and the implications on window design are discussed.

Aluminum oxynitride or ALON is a transparent polycrystalline ceramic material having high strength (380 MPa) and hardness (1950 kg/mm²). The transmission range of ALON extends from 0.2 micrometer in the UV through the visible to 6.0 micrometer in the infrared. This material is made by conventional powder processing and sintering a powder compact to full density and optical transparency. Powder compacts of near net shape and size are made by conventional dry pressing, by slip casting, and by injection molding methods. This gives the material great latitude in size and shape capabilities not afforded by materials formed by single crystal growth methods. Intrinsic transparency extending from ultraviolet wavelengths (UV) to mid-infrared wavelengths (MID-IR) and low levels of optical scatter have been achieved. In this paper recent measurements of the spectral dependence of forward optical scatter, the spectral emittance from room temperature to 1200 degrees Celsius, and the index of refraction (n) of ALON are presented. Literature values for the changes in refractive index with temperature (dn/dT) are compared.

The performance of an internally cooled silicon window in a high heat flux environment has been characterized in the laboratory. The article under test was convectively/radiatively heated with a large-area-flame oxy- acetylene torch, and cooled by circulating water through internal channels. Heating rates ranging from 5 to 120 W/cm² were achieved over the surface of the window and generated thermal gradients in the window that exceeded flight levels by an order of magnitude. This gave us a measure of window performance under stressing conditions. Thirty one heating tests were conducted to measure the thermal and optical efficiency of the windows. The degree of surface temperature uniformity was derived from midwave infrared images of the test window surface collected on a two- dimensional array, InSb camera. Optical wavefront distortion was measured with an infrared shearing interferometer. Data was collected on both a video tape recorder and a digital data acquisition system before, during, and after the period of window heating. Experimental data on two windows along with theoretical predictions are presented in this paper. The theoretical code took a given heat flux distribution into the window and predicted the surface temperature distribution, and the change in the window dimensions and index of refraction. Experimental data and theoretical predictions compared well.

The temperature and frequency dependent infrared properties of polycrystalline CVD (beta) -SiC have been measured. This was accomplished using both broadband and narrowband (laser) measurements as a function of temperature from room temperature up to 900 degrees Kelvin. Calculated multiphonon absorption shows good agreement with experiments. Furthermore the thermo-optic coefficient was measured in the 2.5 - 5 micrometer region and the BSDF for CVD (beta) -SiC was measured at 0.6328 micrometer for the first time.

The achievement of the Army's goal, 'Own the Night,' has evolved over the last few years to include mastery of the entire optical spectrum from 0.4 microns through 12 microns. This requires the integration of lasers, FLIR (forward looking infrared) sensors, CCD cameras and image intensifiers, and direct view optical assemblies, all on a single platform. The problem faced with such integration is to provide the functionality in a small, lightweight package. Common aperture optics may be a solution, but common apertures require the use of low-cost, hardened, multi-spectral windows. The general requirements for multi-spectral systems and lessons learned from the RAH-66 Comanche program are discussed.

At one time, arsenic trisulfide (As₂S₃) glass was the only IR optical material produced commercially for infrared optical systems. The glass was produced by the tons from the 50s into the 70s. However, as the emphasis shifted to the long wavelength 8 - 12 micrometer passive optical systems, the glass fell out of favor and production worldwide ceased. The production processes used were open systems which led to environmental concerns that also contributed to the decisions to cease production. In the 1990s, Amorphous Materials (AMI) became interested in the glass in part because of the reported ability of As₂S₃ glass fibers to transmit large amounts (greater than 100 watts) of laser power. A closed process which eliminated environmental concerns was developed to produce the glass. Major emphasis was in producing glass for IR fibers. Use for imaging systems was limited. Now, however, a trend has developed to produce imaging systems based on focal plane array technology which operate in the 3 - 5 micrometer wavelength region. A demand once again has been created for the glass. The method used at AMI to produce the glass is presented. Efforts to reduce absorption through purification of the elements are described. Properties of the glass are reviewed.

Optical properties of KRS-5 are measured from the far ultraviolet to the onset of infrared absorption near 50 micrometers. Measurements include temperature-dependent absorption at the visible and far-infrared edge of transparency, and temperature-dependence refractive index in the infrared, all from room temperature to over 473 K. These measurements, combined with data from the literature, form a comprehensive picture of the optical properties of KRS-5. Models are developed for some properties and compared to measurements.

Optical windows employed in current and future airborne and ground based optical sensor systems are required to provide long service life under extreme environmental conditions including blowing sand and high speed rain. State of the art sensor systems are employing common aperture windows which must provide optical bandpasses from the TV to the LWIR. Operation Desert Storm experience indicates that current optical coatings provide limited environmental protection which adversely affects window life cycle cost. Most of these production coatings also have limited optical bandpasses (LWIR, MWIR, or TV-NIR). A family of optical coatings has been developed which provide a significant increase in rain and sand impact protection to current optical window materials. These coatings can also be tailored to provide either narrow optical bandwidth (e.g., LWIR) or broadband transmittance (TV- LWIR). They have been applied to a number of standard optical window materials. These coating have successfully completed airborne rain and sand abrasion test with minimal impact on optical window performance. Test results are presented. Low cost service life is anticipated as well as the ability to operate windows in even more taxing environments than currently feasible.

Tactical missile infrared (IR) windows are exposed to high heating conditions during free-flight trajectories. These high heating rates and resultant non-uniform temperature distributions result in degradation of optical performance as well as structural capability of the IR window. Optically, the wavefront and the signal-to-noise ratio (S/N) can be catastrophically degraded and is strongly dependent on the time varying magnitude and distribution of the heat on the window. Therefore, for each missile application, it is critical to be able to determine the optical effects of the heating on window performance. As analytical techniques for predicting optical heating effects are limited, test results are critical in providing conclusive measured data on performance as well as validating analytical predictions. Unfortunately, conventional techniques for measuring the free flight heating effects, e.g. shock tubes and wind tunnels, are expensive and require complex set-ups for measuring the optical performance. The high speed flow and resultant thermal and vibration environments require that the hardware be robust and nearly form factored. This increases the cost of the hardware and limits the available volume for instrumentation and optical measurement devices. In addition, because of the volume limitations, many times it is necessary to use a form factored, expensive IR seeker to measure the optical performance. A new test technique has been developed by Raytheon to measure the optical performance of a non-form factored IR window while simultaneously exposing it to tactical missile free flight heating levels. The high power, continuous wave carbon-dioxide laser, located at the Laser Hardened Materials Evaluation Laboratory (LHMEL) at Wright Patterson Air Force Base, was used to heat the exterior surface of a flat, internally cooled silicon window. During the application of the laser heating, the optical performance of the cooled window as measured using a 128 by 128 focal plane array (FPA) midwave IR detector. Several weeks of testing was conducted in which significant data on the wavefront distortion, thermal response, and overall optical performance of the internally cooled silicon window was captured. The test technique can be utilized for any IR window material, internally cooled or uncooled. The test set-up and technique are discussed in this paper. The low cost of performing the tests allowed significant data to be collected on the IR performance of the window. The ability to test non- form factored hardware resulted in significant cost savings as available test equipment was utilized in lieu of expensive form factored IR seekers. Also, the ability to measure non- form factored hardware early in the development process resulted in reductions in overall development cost and schedule.

Fourteen ZnSe samples of different quality were fabricated and tested to determine the effect of specific defects, such as haze, flowering and inclusions on ZnSe absorption at 10.6 micrometer and transmission in the wavelength range 0.5 to 22 micrometer. The results show that haze, large inclusions (diameter equals 1 - 2 mm) and the sample cleaning procedure can affect the ZnSe absorption at 10.6 micrometers significantly. However, the absorption is not very sensitive to variations in surface finish or the presence of flowering and small inclusions (diameter equals 0.05 - 0.3 mm). Further, the infrared transmission in the wavelength range 2.5 - 22 micrometers did not show any appreciable variation with material quality or the surface finish, but that visible transmission around 0.55 micrometers was significantly degraded by the presence of haze and flowering.