This paper discusses the problems involved in comparing the rms roughness values obtained from a plane of incidence differential scatterometer to those obtained from a TIS system. The theoretical justification for making the comparison and the importance of spatial frequency bandwidths are reviewed first. The two types of instruments are briefly described and referenced. Several practical problems relating to the types of samples that allow the comparison are presented. The comparison is independent of the surface height distribution (i.e., a Gaussian astribution is not required); however, because the data is taken in one plane only by the differential scatterometer only isotropic samples, one dimensional samples (grating like) or a combination of the two may be used to make the comparison. Then two less obvious mechanisms associated with TIS devices are discussed. These are the small angle scatter assumption and the angle of incidence variation at the detector. Both of these effects act to discriminate against high frequency roughness in the calculation of rms roughness from TIS data. Finally, some experimental comparisons of several samples are presented.

Surface irregularities range in lateral dimensions from those usually associated with optical figure error through values associated with zonal errors to those usually described as microroughness and extending to submicron dimensions. Typically, the irregularities are a small fraction of a wavelength A in height so that physical, not geometrical, optics must be used to calculate their contribution to optical performance. When the ratio of the root mean square irregularity height to the wavelength is small, first-order perturbation methods can be used to predict the scattering characteristics of such surfaces. No upper limiting value on the lateral dimensions of the surface irregularities appears in such a calculation, although, for normal incidence, much of the scattering becomes virtual when the lateral dimension 2. of the irregularities becomes less than X. Lateral dimensions associated with figure and zonal errors yield scattering in the near-specular region. These errors reduce image contrast and cause the optical system to be sensitive to stray light from extraneous near-axis sources. Lateral dimensions associated with microroughness yield scattering at large angles from the specular direction. Large-angle scattering may also be important but becomes of particular concern for an imaging system such as a telescope where light may enter the optical system from large off-axis angles, strike the optical component, and be scattered into the focal plane. A calculation of the effect of surface errors having a range of autocovariance lengths on the performance of a typical mirror telescope will be given to demonstrate the possible degra-dation effects of both near- and large-angle scattering. Vignetting effects that occur when the mirror is illuminated at off-axis angles are also considered. It should be pointed out that we are discussing scattering of light into the optical path by the optical components themselves and that no arrangement of baffles following the component itself will have any effect on this type of scattering intensity.

Last year at this conference our lab presented some data which strongly supports the use of the vector perturbation relationship between light scattered from smooth surfaces and the surface power spectral density (PSD). Last year's data showed that a consistent answer was obtained for the PSD for measurements taken with S and P polarization, with incident angles up to 45', for positive and negative sweeps, and for one and two dimensional samples. If in fact the theoretical relationship is correct then this must be the case, as the PSD is determined by a combination of surface topography and the scattering situation (geometry, polarization, wavelength, etc.). It appears, however that for large scatter angles (>700) and for large angles of incidence (>60°) there is some deviation in the calculated PSD. In addition, the high angle scatter region is an area where a scatterometer with a semicircular detector sweep loses the one-to-one relationship between detector position and surface spatial frequency. Or, in other words, light diffracted to high scatter angles from single spatial frequencies appears over a small band on the observation semicircle instead of a diffraction limited point. Fata supporting these two deviations is presented. Although they are not regarded as serious violations of the vector perturbation theory they do impose a limit on the useful range over which the PSD may be calculated.

Nature obliges us to use a statistical description of the scattering of light from randomly-rough surfaces. This paper discusses the role of statistical models and fluctuation phenomena in the measurement and characterization of such surfaces using area and profiling techniques.

The Effect of Oxygen Interaction with Materials experiment flown on Shuttle flight STS-8 examined the effects of atomic oxygen on a variety of coatings, composites and polymeric films. The ultraviolet, visible, and near, middle, and far IR spectra of Martin Black and Infrablack samples used in this experiment are described. A second test of this experiment included tubes coated on the inside with Martin Black. Mirrors were placed on one end of each tube to simulate a telescope. No degradation of the Martin Black was observed. A plasma etching chamber was used to simulate exposure of Martin Black, Enhanced Martin Black, and Chemglaze Z-306 to atomic oxygen. Martin Black and Enhanced Martin Black changed very little, but the changes in Chemglaze Z-306 were significant.

BRDF (bidirectional reflectance distribution function) is the accepted expression for describing directional reflectance. BRDF is the ratio of outgoing radiance (watts per square centimeter per sterradian) in one direction divided by incoming irradiance (watts per square centimeter) from another direction, and the units of BRDF are sterradians to the minus one power. BRDF is an unusual scientific quantity, and it is not simple to visualize, to measure or to use. Its unusual character gives rise to certain intrinsic considerations for measurement equipment, and this paper addresses those intrinsic design considerations.

It has been incorrectly argued that the surfaces of a plane parallel transmissive optical element do not contribute equally to the total far-field scatter from the element even though the micro-roughness characteristics of the two surfaces are the same. Three seemingly logical arguments that predict very different results are presented. The correct one predicts that the two surfaces contribute equally to the total scatter.

The measurement of scattered light has become more important during the past few years because of the increased ability to use this information in evaluating low scatter optics such as space optics, ring laser gyro and high energy laser optics, and most particularly, in the evaluation of electronic materials. Single point measurements are also no longer sufficient for characterizing surfaces, since it has been found that point to point variations in scatter can be of considerable magnitude. The instrument described in this paper is a highly accurate scatter measuring system which is capable of taking more than 150 million data points in a 125 x 125mm (5" x 5") square area. This data can be used to analyze both the surface and subsurface characteristics of a test article and the information used as a feedback mechanism to change the production process for improved quality.

Rejection of off-axis stray radiation by efficient baffling continues to be a major problem in designing low background, wide field of view infrared sensors. For an efficiently baffled sensor, direct scattering from baffle vane edges becomes the dominant contributor to stray radiation. Existing computer programs for stray radiation analysis assume ideal baffle vane edges and do not account for the scattering effects of actual baffle vane edges due to the uncontrollable edge geometry. We propose a baffle vane edge model based on empirical data; this model is described by an edge scatter distribution function (ESDF) which quantifies the scattering effect of actual baffle vane edges in the same manner as a bidirectional reflectance distribution function (BRDF) quantifies scattering surfaces. The ESDF consists of three physical processes: 1) scattering of the beveled surface of a baffle vane edge, 2) diffraction of an ideal knife edge, and 3) diffused reflectance of an actual baffle vane edge. This empirical edge model has been incorporated into an existing stray radiation analysis computer program. Theory and measured data are compared.

The stray light performance of NASA's Diffuse Infrared Background Experiment (DIRBE) has been calculated using the APART/PADE code. That code has been upgraded to handle off-axis optical systems such as DIRBE. Under observing conditions, sunlight is attenuated by 21 to 29 orders-of-magnitude; uniform diffuse illumination, by 6 to 9 orders of magnitude. The sunlight is attenuated by multiple diffraction at an external sun shield and by a forebaffle at the entrance to the DIRBE. Stray light is nearly a linear function of the Bidirectional Reflectance Distribution Function (BRDF) of the primary mirror at all wavelengths - or it can be made to be so by reducing the size of the field defining stop from 0.866 to 0.7 degrees square. For uniform diffuse illumination, half of the stray radiation comes from within 5 degrees of the center of the field-of-view. Particulate contamination of the primary mirror is expected to be a problem and special polishing and cleaning procedures are recommended - before mirror overcoating and again before flight.

The Shuttle Infrared Telescope Facility (SIRTF) will operate over wavelengths from 2 to 1000 micrometers with background limited performance. This performance can only be achieved with good stray light suppression which places great emphasis on the absorption and scattering properties of optical black coatings. The stray light suppression performance of SIRTF was analyzed at 10.6 and 118 micrometers for two different optical black coatings by using Arizona's Paraxial Analysis of Radiation Transfer (APART) computer program. The coatings are Martin Black, and Infrablack. The results are presented as the Point Source Normalized Irradiance Transmittance (PSNIT) as a function of off-axis source angle. The dominant stray light paths in SIRTF are discussed with emphasis placed upon the scattering characteristics of the black coatings. The Bidirectional Reflectance Distribution Function (BRDF) of each coating is shown at 10.6 and 118 micrometers for incidence angles of 100 and 600. General conclusions are made about the propagation of stray light in SIRTF so that these results will be instructive for other infrared telescopes.

For many scanning IR systems a constant flux of stray radiation can be treated as a part of the background and ignored in subsequent processing. The optical system can, therefore, be less complex, and often less than 100% cold sheild efficiency is permitted. On the other hand, variations in stray radiation detected through scan can be a major concern. The evaluation of these fluctuations is often tedious, especially when many combinations of parameters must be considered (such as scan mirror position and detector element location). As an aid in analyzing these variations, a computer program has been developed which combines both ray-tracing and radiometry to provide quick evaluation of various mechanisms of stray light change over scan. This type of evaluation is especially useful in the initial design stages, when the system configuration is being selected and optimized, so that stray light control becomes part of the lens design process. An example will be used to describe the capabilities of the program, to illustrate some of the mechanisms which cause this unwanted scan-dependent noise, and to suggest methods for reducing these fluctuations while maintaining a simple design.

The fundamental validity of using an interferometric process to reduce scattered light in optical systems has been examined from a physical optics viewpoint. An elementary scatter nulling interferometer was considered in the context of a simple telescope system as it imaged a distant point source, whose image had been degraded by a single on-axis point scattering source. With the interferometer inserted in the optical train of the telescope, the diffraction image of the point source, and a wavefront model of the scatter source at the focal plane were studied in detail. An expression for gain in signal-to-noise ratio was evaluated for typical cases. An experiment to illustrate the basic concepts of a particular scheme was conducted to demonstrate the viability of the technique.