The accepted usefulness of detailed stray light analyses and BRDF data, has grown steadily and significantly since 1980. Today the usefulness of these analyses is clearly understood in most areas of the optical space research community. The Bidirectional Reflectance Distribution Function (BRDF), which defmes the scatter characteristics of surfaces, is a term familiar enough to the community that an ASTM standard is in the fmal stages of approval. The published literature now has significant amounts of BRDF data on an assortment of useful materials. Important space based systems have been analyzed for stray light, and then measured in operation. The conelation between the measured and analysis results has been very good. The software tools are more numerous, and they have more capability. There is much more theory documented in the literature. Some of the theory is still being argued about, to different degrees of disagreement, but the basic concepts are well established. Probably one of the most significant advances has been a series of contracts out of Rome Air Development Center (RADC) on the Detection, Removal, and Prevention of Contamination. These works hold great promise for the future.

Scattering from mirror surfaces arises from topographic and non-topographic sources. This paper considers the nontopographic scattering of beryllium mirrors modelled as a collection of randomly oriented bireflective grains. Simple scattering theory shows that this type of scattering scales as r2, rather than as 2" for topographic scattering, which means that it is relatively more important at long radiation wavelengths. Estimates of the intensity based an available short-wavelength values of the anisotropic optical constants of beryllium indicate that this type of scattering could dominate the topographic scattering from smooth surfaces at CO2 wavelengths.

The scattering from surfaces whose profile has a center of symmetry is considered. It has been reported that the diffuse component displays a very narrow peak in the specular direction. We show that if the portion of the surface illuminated has a length L. the width of this peak is of order X I L .andthat the enhancement doubles the diffuse intensity in the specular direction. We also show that, by phase-shifting one half of the pupil, it is possible to destroy the effect and deplete the intensityin this direction. These effects are difficult to detect experimentally and, to alleviate this situation, we have also considered the scattering from diffusers which are symmetric in sections. They have similar scattering characteristics to those in the previously considered case, but the enhancement is easier to detect experimentally. Some random surfaces with symmetries have been prepared, and we present experimental results to support our conclusions.

The scattering by small particules of an incident light beam of a prescribed polarisation is an important scientific problem. This study demonstratres that the distribution of microprecipitates included in semiconductor wafers, can be revealed and studied by scattering phenomena. Two kinds of scattering experiments have been carried out : Laser Scanning Tomography (LST) and Angular Scattering Analysis (ASA). ASA is based on scattering of an infra-red laser beam by microprecipitates contained in bulk material. The scattered light is collected by a camera and the corresponding signal sent to a computer. ASA is concerned with measuring the scattered intensity as a function of the scattering angle. LST also consists of analysing light scattered by microdefects. This technique has already been described in details in references 1,2 and 3. Scattering is a wide and complex mathematical problem, depending on a large number of parameters. The theoretical section of this paper refers to the Mie theory of single scattering for different particle shapes. Investigations have been performed on InP material, manufactured by the Liquid Encapsulated Czochralsky (LEC) technique. The samples were pulled in the <111> or <100> directions and were undoped, doped (Sn,S,Fe) or co-doped (Cds, S-Ge). This presentation gives results in terms of the evolution of the manufacturing quality, according to density, size of microprecipitates, and homogeneity of their distribution.

The enhanced transmission effect, manifested as a narrow peak in the angular distribution of the intensity of diffuse scattered light in the antispecular direction, has been experimentally investigated when a p-polarized laser beam passes through a metal film deposited on a piece of glass substrate. This peak is an analogue of enhanced backscattering in the transmission of light through a randomly rough metal surface. A fully automatic bidirectional reflectometer was used to measure the enhanced transmission of gold and silver surfaces under illumination of He-Ne laser. The results are compared with a recent perturbation theory of the localization of surface polaritons.

The Rayleigh Rice vector perturbation theory of diffraction is used to relate surface topography to scatter. In fact, this relationship allows the calculation of common surface roughness parameters (power spectrum, rms roughness, average surface wavelength, etc.) from the BRDF under the conditions that the surface is smooth, clean and front surface reflective. The theory can also be used to predict situations where the topography of such a surface will produce no scatter. That is, surface scatter can be eliminated under certain combinations ofviewing parameters. Under these conditions, the measured BRDF is the result of contaminants and subsurface defects. This paper presents previously unpublished material, discusses the theoretical basis for the effect, and gives experimental results.

This paper presents details of the optical and mechanical layout of a cryogenically cooled IR-sensor named CRISTA (CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere). The instrument is designed to analyze dynamical processes in the middle atmosphere. For this purpose, it measures IR emissions of several trace gases at the limb using three independent telescopes with high off axis rejection performance. CRISTA, integrated in the free-flyer ASTRO-SPAS, will be launched in 1993 by the Space Shuttle for a 9 day mission.

The CRISTA experiment is designed to detect and analyze short term upper atmospheric waves and turbulence of the middle atmosphere. This paper presents two of the more intriguing stray light characteristics of the CRISTA instrument as revealed through a much more extensive stray light analysis. The two topics are the diffraction propagation from a series of edges, and the thermal loading characteristics of the outer baffles by the earth's radiation. The interesting parameters that play very complex roles relative to each other are: CRISTA's three different telescopes peer through a common aperture; the Center Telescope has an image plane shared by two spectrometers offset above or below the axis by 0.358 deg; the point source angles walk away from one slit but across the other; the wavelength bands vary from 4 microns to 70 microns; all of the imaging mirrors are simple spherical surfaces; the major source of stray light is the earth, which is only .5 deg from the optical axis; and the intermediate field stop is oversized.

This paper will discuss the problem of the decorrelation of speckle. When a target isirradiated with a coherent laser source, speckle leads to signal variations which is signal noise when the signal can't be adequately signal averaged. Movement of a diffuse incoherent scattering target decorrelates the spatial coherence but a temporal correlation is introduced.

For an infrared spectrophotometer with the convergent beam in the sample compartment the stray radiation is estimated. The efficient method with different filters used to test the stray radiation is discussed. The sample-induced interreflection sources are analyzed. The method to eliminate the intereflection is introduced. It avoids the oblique errors and did not need complicated calculation. The stray radiation ratio estimations from 2.Ln to 15.tfl wavelength are less than O.4 transmittance unit. The interreflection correction improves the transmittance measurement accuracy more than ten times.

The APART/PADE program is used to examine whether the SIRTF aperture stop can be moved back to the primary mirror without jeopardizing the requirement for natural background-limited observations. The straylight performance of the baseline design, which has the aperture stop at the secondary mirror, is compared to that of a revised design with the stop at the primary mirror. Both designs include reimaging optics and a LYOT stop, as well as a chopping secondary mirror. The straylight due to off-axis sources and telescope thermal emission is investigated at 50 and 550 microns. It is found that in SIRTF, where every instrument will include reimaging optics and a LYOT stop, the aperture stop may be moved to the primary mirror with modest degradation of the straylight performance.

A computer program has been written to rapidly calculate stray light in optical systems using the IBM PC. The program calculates the Point Source Transmittance by either of two methods. The first method uses formulas requiring only firstorder system parameters such as F-number, aperture diameter, primary Bidirectional Scattering Distribution Function, and baffle tube length. In the second method, paraxial rays are traced through the optical system and the program evaluates scattering from all of the optical surfaces. Stray irradiance at the focal plane of an optical system from an extended source such as the sun or the earth can be calculated by integrating the incident irradiance over the source, weighted by the Point Source Transmittance. The result is then multiplied by the integral of the system spectral transmittance function weighted by the blackbody function. The program also computes veiling glare, ghost images, narcissus, spectrally weighted transmittance, and illuminance and spectral luminance using the photopic and scotopic spectral efficacy functions. Blackbody-related functions including radiance contrast are also computed. Good agreement is obtained between GUERAP III and SOAR for Point Source Transmittance calculations.

This paper reviews design goals for an infrared scatterometer being built to measure cryogenically cooled samples. The instrument, which will be installed in the Optical Characterization Facility at Oak Ridge National Laboratory, will be capable of operation at both 3.39 and 1 0.6 micrometers for both reflective and transmissive samples up to six inches in diameter. BSDF is measured through the specular beam and out to grazing scatter angles. The paper gives simulated instrument signatures obtained in TMA's measurement facility.

Nationwide BRDF round robin results in the visible and infrared showed the need for standardized scatter measurement and reporting procedures within the optical community. ASTM Subcommittee E12.09 was formed in 1988 to address this need. This presentation reviews the round robin results, the work of E12.09 to promulgate a BRDF Standard Practice, and work on infrared BRDF reference samples.

This paper reviews the design of a rather unique portable scatterometer. The instrument consists of a measurement head connected by cable to a microprocessor based controller. Several different measurement heads can be used with the same controller. One measurement head is a hand held unit designed initially to measure rain erosion on aircraft sensor windows. The controller stores the scatter by aircraft tail numbers and downloads them later to a PC. A second head, a little larger than a shoe box, is designed to take either reflective or transmissive measurements from optics. The objective is to make inexpensive, fast, accurate BSDF measurements available in a variety of situations where location, or expense, has prevented it in the past. The paper gives specifications for the two heads and compares measurements to those taken on a TMA CAS1.

A design review of a five wavelength, laser-based, full reflection hemispherical BRDF instrument is given. A source box design which provides full polarization control of incident radiation is described. The receiver design uses a periscope mirror which allows near (< 1 .5°) retroreflection measurements with full received polarization control. Replacement of the periscope mirror with a beam splitter pellicle allows 0° retroreflection measurements. The goniometer design shown allows the measurement of full reflection hemisphere scatter with transmission measurements possible. A new software control approach which allows a user to define scans in scatter space without being concerned with mechanical axes motion is described.

In addition to the ambient condition tape test, five mechanical tests of the IR-black coating, Ames 24E2, were performed at either liquid helium or liquid nitrogen teperatures. Tensile strain in the coating at liquid nitrogen temperature was measured up to values of 4E-3, both before and after the coating was cycled down to liquid helium temperature. When applied to an aluminum substrate which was then bent in liquid nitrogen, the aluminum substrate always failed (permanently deformed) well before the coating failed. Sinusoidal accelerations up to 45 Gs in liquid nitrogen and 25 Gs in liquid helium did not crack or otherwise visibly damage the coating. Both sinusoidal and random acceleration at about 90 K of a representative baffle vane structure, at frequencies from 10 to 2000 Hz and up to 15 Gs, did not damage the coating, even at the intersection of a baffle with the telescope tube. Thus on a macroscopic level, cryogenic cooling and various levels of acceleration and strain did not affect this coating. However, on a microscopic scale, some loose particles were found associated with several tests. Outgassing data are also given.

The reflectance and the ellipsometric parameters for two black samples were measured at 5-j.tm wavelength and at multiple incident angles using an ellipsometer. Different models were used to reduce the ellipsometric data and to calculate the reference specular reflectance. With the correct model, the measured reflectance and the near-angle scattering with respect to the reference specular reflectance can agree with Beckmann's scattering theory. The roughness reduced from reflectometry is independent of ellipsometric models and is used to select the correct set of solutions. A three-phase model in which the complex dielectric constant is computed from Bruggeman's effective medium theory can provide consistent solutions between roughness from reflectometry and effective thickness from ellipsometry. A combination of ellipsometry, reflectometry, and scatterometry can predict accurately the complex index of refraction, roughness, and other optical properties of black samples.

BSDF measurements are made on a large variety of materials at TMA’s measurement facility. Typical BSDF for many of these materials is not available in literature. This paper gives scatter data at several wavelengths for samples that we believe will be of interest to the scatter community and outlines the techniques and considerations required for some of the more difficult measurements. Included are some very low scatter optics, an off axis parabola and several different window materials.

Based upon a robust Bidirectional Scatter Distribution Function (BSDF) data reporting format, a full featured BSDF database has been developed. The database uses a relational database schema and provides a rich query ability through the support of Structured Query Language (SQL) statements. The database can be searched and reports generated for such items as sample type, BSDF range, measurement and laboratory environment. The database was developed to provide support functionality for a light scatter modeling program. It represents a step toward convenient use of existing BSDF data and opens the door to a BSDF data library that could be accessed by many facilities.

Bidirectional reflectance distribution function (BRDF) measurements were made on single crystal, multicrystalline, and polycrystalline germanium (Ge). The multicrystalline substrate is a newly developed form of germanium that contains a few large crystals and has a structure that is more uniform than the polycrystalline material. Measurements were made with a helium neon laser operating at 3.39 microns with a 100 angle of incidence. The BRDF of the multicrystalline material more closely resembles that of the single crystal material than it does the polycrystalline material. The bulk material of the polycrystalline germanium scatters more than that of the single crystal germanium, and therefore has more effect on total sample BRDF.

The high-resolution (4 cm1) spectral characteristics of radiation specularly reflected and diffusely scattered from selected optical-black coatings in the infrared from 5 to 20 microns are reported. Various one-, two- and three-coat samples were measured for numerous combinations of incident and output angles. Many spectra show a surprising number of strong, sharp features, especially in the specular direction. The results for several commercially available and widely used black coatings are compared and interpreted.

Recent improvements in optical polishing techniques have made possible optical surfaces exhibiting random microroughness levels of less than one Angstrom rms. One such mirror, recently produced by the Photronics Corporation and tested at TRW, exhibited a measured microroughness of 0.22 A rms but had scatter behavior which was not commensurate with this value. The problem was discovered to be point defects in the surface which were not detected during profilometry, and which would not have been problematic in mirrors having microroughnesses of a few A rms. It is concluded that visual scatter inspection, at a minimum, should be used as an in-process polishing diagnostic, in addition to profilometry.

Fresnel lenses, when used as light condensors, generate many spurious ghost images due to multiple refraction and reflection of light between the facets and risers. In some applications, such as overhead projection, these images can be visible to the user. Using the technique of nonsequential ray tracing, the origin, direction, and intensity of these ghost images are analysed for two types of Fresnel lens doublet systems. Techniques for the reduction or elimination of these ghost images are also discussed.

Within the Aerospace community visible wavelength scatter data from a variety of space sensor materials is not widely available. The Scatter and Off Axis Rejection Laboratory has a comprehensive data base which shows typical and state of the art scatter performance of many typical sensor materials.

Within the Aerospace community visible wavelength scatter data from a variety of space sensor materials is not widely available. The Scatter and Off Axis Rejection Laboratory has a comprehensive data base which shows typical and state of the art scatter performance of many typical sensor materials.

The straylight analysis program GUERAP III requires a complicated. input dataset Which discourages its use. The GUFDIPP program package provides an environment for swiftly generating, cliecking and maintaining GUERAP datasets, so making GUERAP more accessible.

Fresnel lenses, when used as light condensors, generate many spurious ghost images due to multiple refraction and reflection of light between the facets and risers. In some applications, such as overhead projection, these images can be visible to the user. Using the technique of nonsequential ray tracing, the origin, direction, and intensity of these ghost images are analysed for two types of Fresnel lens doublet systems. Techniques for the reduction or elimination of these ghost images are also discussed.

Beryllium coatings with varying thicknesses and columnar grain sizes were deposited by low temperature magnetron sputtering and wet chemically etched to enhance diffuse absorption of light. After etching these coatings exhibited a matte black surface finish and low specular reflectance (below 2%) in the IR up to a critical wavelength dependent upon the original grain size of the coating. Extremely thick coatings (350 j.tm) with original grain sizes of 10 to 12 j.m were produced which exhibited specular reflectances below 0.5% up to 50 p.m wavelength and a Lambertian BRDF at 10.6 p.m averaging 4.3x103 ster1. Scanning electron micrographs are presented for etched and unetched beryllium coatings which showed the etching process produces roughness and porosity over several size scales simultaneously with the maximum size scale limited by the initial coating grain size and thickness. This technique for producing diffuse absorbing baffle materials has great versatility in choice of coating material and substrate and can be expected to provide optical system designers with a variety of material options for stray light management.