A framework for systematically treating various aspects of optical propagation in ceramics and similar composite materials. The point of view is a compromise which attempts to deal with some of the difficulties of a completely rigorous treatment of the subject while providing a convenient means for evaluating the optical properties of real materials and designing processing strategies for new materials.

The scattering of laser radiation in window materials gives rise to several effects which have considerable practical importance. The angular distribution of the radiation scattered within the window can be interpreted to aid in understanding the homogeneity and microstructure of the material. The distribution of scattered radiation which escapes from the window may have important consequences for applications of the material in a laser system. The absorption of scattered radiation within the window will confound the measurement of the linear absorption coefficient when a calorimetric absorption technique is employed. Experiments designed to measure the internal scattering distribution, the external scattering distribution and the influence of absorbed scattering on calorimetric absorption are described. Data for CVD ZnSe are presented.

Polarized Raman scattering spectroscopy is a useful tool for investigating fundamental vibrational properties, structure and bonding, origins of IR edge absorption, and dispersion of the IR refractive index. In this paper we describe the application of Raman spectroscopy to halide glasses and, in particular, heavy metal fluoride glasses. The spectra of the latter differ substantially from those of simple oxide, halide or chalcogenide glasses and, moreover, display a wide range of vibrational characteristics depending on composition. In combination with infrared spectroscopy, one is able to develop useful guidelines for tailoring glass compositions for specific applications.

In this report, we present the first study of resonantly stimulated desorption of pyridine from silver surfaces excited by polarized CO₂ laser radiation. The desorption behavior has been investigated with surface enhanced Raman scattering (SERS), mass spectrometry, conventional thermal desorption and X-ray photoemission spectroscopy. It is observed that when the ring mode vibration of pyridine molecules adsorbed on silver island films at either 15K or 95K is excited by CO₂ laser pulses in 1025~1042 cm^-1 frequency region, pyridine desorption from the metal surfaces is readily detected by these surface techniques. The desorption yield per laser pulse increases nonlinearly with the laser intensity and is strongly dependent on the polarization of the incident radiation. Namely, the p-polarized light is much more effective than the s-polarized light in inducing the desorption. The results clearly show the important role played by the vibrational excitation of adsorbed species in the observed desorption phenomenon.

Raman scattering measures vibrational frequencies in materials, and is used to identify and characterize species and structures. Generally, scattering of laser radiation from a surface upon which molecules are deposited leads only to very weak Raman signals because the number of molecules which are able to interact with the beam is very small. We have recently used integrated optics, i.e., in the form of light propagation in dielectric waveguides, and plasmon surface polaritons (surface electromagnetic waves) on metals, to increase the number of interacting molecules, in the first case, and to greatly increase the strength of the optical field, in the second. The details of the techniques will be described and illustrated with spectra from interfacial molecules. First, the optical properties of a film are determined from measurements of the angular dependence of the reflectivity or transmission of light. Analysis of these results can give the film thickness and refractive index. Second, the scattered light is focused on the slit of a double monochromator and the Raman spectrum recorded as a spectral shift from the exciting light. Frequency shifts, intensity changes and polarization variations are observed in the spectra of surface molecules compared to bulk molecules. Scattering from the substrate and from defects in the films are also observed. Measurements of this type aid in the characterization of surfaces and our ability to engineer its properties for specific uses.

The vector theory of scattering from slightly rough surfaces is applied to metallic surfaces. BRDF (Bidirectional Reflectance Distribution Function) curves are calculated for several wavelengths and angles of incidence. These are compared with experimental data for the same condition.

Heeding a warning by Nicodemus (1965), we examine the problem of reflectance measurements of rough surfaces made on different reflectometers. Our analysis indicates that specular reflectance measurements in the infrared should be normalized by the projected detector solid angle before they are compared. This normalization effects a primitive deconvolution of the instrument function and produces a quantity commonly called the bidirectional reflectance distribution function (BRDF). Direct comparison of the measured specular reflectance from rough surfaces fails at far-infrared wavelengths because the diffuse component of the measurement is larger than the specular. The diffuse component is larger than would otherwise be expected because it is related to the strength of the instrument function, which increases with wavelength. These analytical findings are confirmed by comparison of specular BRDF measurements of optically black coatings made at three different laboratories. Measurements at a fourth laboratory were inconclusive. We also find that nonspecular BRDF measurements calibrated by a standard diffusely reflecting surface are identical to those calibrated by an image of the source upon the detector. The rms difference between BRDF measurements of identical or very similar samples made on three different instruments was about 13% in both the specular and large-angle nonspecular cases. However, large differences in BRDF measurements occur at angles near the specular direction because there the diffuse reflectivity of the surface is tightly convolved with the instrument function.

A black paint has been developed and tested for cryogenic applications involving wave-lengths beyond. 10μm. The paint has been used extensively for liquid helium cooled applications in a variety of instruments operating between 10 and 120pm. The material is applied by brush over a spray or brush applied primer coat. A final covering is applied by either brush. or spray. The composition and preparation of the paint are described, as are data about its outgassing properties. Infrared reflection and scattering data obtained by other groups which compare this material to other commercially available surface preparations are presented.

Far-infrared specular reflectance spectra of six optically black coatings near normal incidence are presented. The spectra were obtained using nine bandpass transmission filters in the wavelength range between 12 and 300 μm. Data on the construction, thickness, and rms surface roughness of the coatings are also presented. The chemical composition of two coatings can be distinguished from that of the others by a strong absorption feature between 20 and 40 μm which is attributed to amorphous silicate material. Inverse relationships between these spectra and coating roughness and thickness are noted and lead to development of a reflecting-layer model for the measured reflectance. The model is applied to the spectra of several coatings whose construction falls within its constraints.

An electrically conductive flat black paint has been developed for use on the Galileo spacecraft which will orbit Jupiter in the late 1980s. The paint, designed for equipment operating in high-energy radiation fields, has multipurpose functions. Its electrical conductivity keeps differential charging of the spacecraft external surfaces and equipment to a minimum, preventing the buildup of electrostatic fields and arcing. Its flat black aspect minimizes the effects of stray light and unwanted reflectances, when used in optical instruments and on sunshades. Its blackness is suitable, also, for thermal control, when the paint is put on spacecraft surfaces. The paint has good adherence properties, as measured by tape tests, when applied properly to a surface. The electrically conductive paint which was developed has the following characteristics: an electrical resistivity of 5 x 10⁷ ohms per square; a visual light total reflectance of approximately 5 percent; an infrared reflectance of 0.13 measured over a spectrum from 10^-5.⁵ to 10^-3 meter; a solar absorptivity, α_s, of 0.93, and a thermal emissivity, ∈, of 0.87, resulting in an α_s/∈ of 1.07. The formula for making the paint and the process for applying it are described.

The general term for optical surfaces better than the lowest level of the scratch-dig standards has become the word "supersmooth". It is seen in the technical literature as well as in advertising pieces. A performance number, such as BRDF, which can be measured from the uncoated optical surface by equipment such as the Variable Angle Scatterometer at the Air Force Wright Aeronautical Laboratories at Wright-Patterson Air Force Base is proposed as a method of generating better optical surface specifications. Data shows that surfaces of average BRDF near 10 parts per billion per steradian (0.010 PPM/Sr) are now possible and measurable.

A computer program for the evaluation of surface ripple effects on an optical system is discussed. The program consists of a modification of conventional ray tracing so that the effects of all roughness frequency scales on image quality can be analyzed. The code is designed to deal with image quality problems arising in connection with x-ray telescopes and near normal incidence systems used in the infrared, visible, and near UV parts of the spectrum. The emphasis of the program analysis is on narrow-angle scattering and its effect on image quality. The intent is to provide the optical engineer with a measure of the sensitivity of image quality to various surface characteristics. The paper will highlight some of the unique modeling features of the code and illustrate some system examples.

The manufacture of scratch standards for use with MIL-0-13830A has been hampered by the lack of an objective measurement technique. The U.S. National Bureau of Standards has therefore undertaken a comprehensive program to provide quantitative measurements of the light scattered by the scratches and to correlate them with assessments made by trained observers. In this paper, I apply scalar diffraction theory to developing design criteria for a polar scanning apparatus, describe an apparatus that includes a novel optical system, and show scans from one full set of secondary standards. Comparing these scans with the visual assessments is not straightforward.

A study has been performed experimentally to determine the effects of surface roughness, damaged surface layer and chemical composition on the specular spectral reflectance of A1SI304 austenitic stainless steel from the machining point of view. Stainless steel samples are heat-treated under various conditions and polished by various methods. Flat samples are polished supersmoothly to a surface roughness of 20 angstroms Rz by terminating the damage-free polishing of the damaged surface layer at a point prior to the appearance of the grain boundary due to crystallographic anisotropy. The reflectance is strongly dependent on wavelength and surface roughness and increases as damage to the surface layer decreases. The combination of adequate annealing and special polishing is useful to produce higher reflectance on metal substrates. Surface chemical composition profiles are measured by ILIA and ALS, which show that chromium is depleted at the very topmost oxidized layer on the polished sample, while there is a chromium-rich oxide layer beneath the layer. The thickness of the oxide layer is about 30 angstroms on the supersmoothly polished stainless steel.

There are three major sources of scattering: bulk scattering, surface scattering, and scattering from subsurface damage. Bulk scattering is inherent in the material and is seldom, if ever, influenced by standard optical shop practices. Bulk scattering is measured by comparing the scattering from a thick sample with that of a thin one.

The amount of 1.06 μm laser light reflected or refracted into a detector from a bubble embedded on a laser glass/cladding glass interface is discussed. The theoretical reflectivity, in the geometric limit, is developed in order to establish realistic limits for the allowable bubble size and number density in an edge clad laser disc.

Relatively little is known about the extent of the optical degradation which takes place in window materials when exposed to realistic erosive environments even though the erosion resistance of optical windows is usually prescribed for aircraft and missile guidance systems. The operational requirements for high-performance aircraft and missiles are correspondingly placing more severe requirements on the window materials. It is therefore necessary to develop design relations which provide reliable correlations between laboratory measurements of a material's erosion resistance and the operational requirements which are to be satisfied. This paper attempts to illustrate how this may be accomplished while demonstrating the general lack of test data for this purpose.

Materials for infrared windows, selected on the basis of their transparency at selected wavelengths, are often subject to damage from exposure to rain at high velocities. This damage may result in a loss of transmittance, the magnitude of which depends on the velocity and the time spent in the rain, or it may result in a structural failure of the window. This latter may occur without significant prior loss of transmittance. It is necessary to know the effects of the anticipated rain environment on the performance of the various window materials, so operating limits can be defined. For this reason, a considerable amount of experimental and analytical work has been performed to determine the response of infrared-transparent materials to water drop impact. Additional objectives are to guide the development of materials with improved erosion resistance and to develop methods to protect the less erosion resistant materials. This paper describes some of this work which has been performed at Bell Aerospace Textron.

The surface quality of optics used in an extremely sensitive laser instrument, such as a Ring Laser Gyro (RLG), is critical. Experimental evidence is presented on the damage to this class of optics that results from the typical processing and handling. Also presented is a possible solution to this problem.

Chemical leaching processes used to produce antireflective surfaces on glass have been shown to exhibit extremely high laser damage thresholds. For evaluation of their utility in short wavelength operation, surface and bulk scattering effects in samples treated by two processes were investigated. The scattering effects observed in the two processes are compared with evaporated λ/4 interference films. The Neutral Solution process AR surface performs better at short wavelengths than the Minot etch/leached gradient AR surface produced on phase-separated glass because the light scattering associated with the Minot etch/ leached surface is high.

Time is a recognized enemy of both low scatter optical surfaces and metal mirror surface figure retention. Data has been accumulated over a four year period documenting the total integrated scatter (TIS), bidirectional reflectance distribution function (BRDF), and surface figure for the primary mirrors of the Space Infrared Experiment (SIRE). The scatter data shows that degradation of an order of magnitude in TIS can be expected to occur in roughly 19 months at a Los Angeles laboratory climate when using present storage techniques. The surface figure of the mirrors is shown to have little or no degradation with aging (less than or equal to 1/4 wave HeNe) but to have high dependency upon proper mounting and installation procedures. Cleaning and storage techniques are also presented.

Let the wavefront normal (ray) entering a Michelson interferometer make an angle with respect to the perpendiculars to the movable and stationary mirrors of the interferometer. If the movable mirror is displaced along its perpendicular a geometrical distance x, an optical path difference of 2 x cosθ will be generated. A monochromatic line of wavenumber νθ will then be modulated as if it had a wavenumber νθcosθ. When used in this fashion, the interferometer codes the angular data into wavenumber data. Multiplexing for many types of scattering measurements is thus possible. An apparatus to accomplish these measurements is described, and results for a mirror contaminated with a single spherical particle are presented.

The problem of light scatter from optical surfaces is amplified to a critical level for the optics used in the Ring Laser Gyro (RLG). This has led to the development of a scatterometer at the RLG Lab, Wright-Patterson AFB, which can detect low level light scatter from the high quality optics used in RLG's, without first overcoating with metals. A helium-neon laser is used to illuminate a 0.5 mm diameter spot on the surface of the test part. The incident angle can be varied from 0 to 90 degrees, and the test piece can be maneuvered with five degrees of freedom, four of which are computer controlled. Scatter measurements are made with a photomultiplier detection system which can easily measure down to 10 parts per billion per steradian. The computer can analyze and plot the results very quickly and accurately for further interpretation.

Design considerations for three scatterometers are presented. The first instrument, a total integrated scatter device, is currently used to make scatter measurements on flat reflectors in an optical finishing lab. The second scatterometer is a small research oriented system, nearly complete, which measures scattering intensity from relatively small diameter reflectors (a few inches) as a function of angle in the incident plane. The third system, which is still undergoing final design, also measures scattering intensity as a function of angle but will be able to do so in a nearly complete hemisphere in front of the sample. In addition, this third system will have variable angle of incidence capability and be able to measure scatter from any spot on flat reflectors up to 14 inches in diameter. Each of these systems is controlled by a small digital computer which, after initial system alignment, directs the measurement procedure, records data and analyzes results. This paper examines the problems associated with the mechanical motions of the systems, sample holding and orientation, system alignment, data compression and measurement of optical intensities that vary by as much as eight orders of magnitude. A short review of some applications for the data generated by these types of scatterometers is also given.

The methods, capabilities and limitations of a multiwavelength scatterometer are described.

New technology in directed energy systems introduces requirements for optical mirrors that are uniquely met by electroplated metals. Thermal conductivity, optical efficiency, and economy make electroplated metals the first choice in many mirror applications. However, control of electroplating process variables becomes more important as surface finish and optical figure specifications become more stringent. The morphology and mechanical properties of electrodeposits are being recognized for their effect upon the overall optical quality of mirrors.

Experimental methods have been developed to determine the millimeter wave dielectric properties of materials at temperatures up to 1600°C. Accuracies of the order of ±1.5% for dielectric constant measurements and ±5% with a detection limit of ±0.0001 for loss tangent measurement over the entire temperature range have been demonstrated.

When a beam of light strikes a piece of paper or similar substrate, a portion of its energy penetrates into the bulk and, due to multiple scattering, may re-emerge at some distance from the point of entry. We refer to this phenomenon as substrate scattering. In this paper we describe a general model for scattering substrates and, using linear systems theory, we investigate its implications for bar-code scanning. We show that the effects of substrate scattering can be represented as a modified reflectance distribution associated with the original printed reflectance pattern. This effective distribution is shown to be independent of the details of scanning system configuration. We show that under a broad range of conditions substrate scattering will decrease modulation and will cause scanners to overestimate barwidths. A specific diffusion model of the scattering process is developed, providing a family of functions which can be used in empirical studies. We conclude that the scattering distributions will not even be approximately Gaussian. Experimental results are presented which are consistent with this conclusion and inconsistent with a Gaussian model. Further experimental results are presented which show that, for typical substrates, depth of modulation may be decreased by 20 percent or more and perceived bar-width increases will be on the order of one mil.