<title>Variability of infrared BRDF for military ground targets</title>
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Two characteristics are critical in the understanding of target signatures, physical surface temperature and surface reflectance. An objects surface reflectance can be thought of as having two major components, the diffuse and specular components. The best way to understand these components is by examining the Bi-directional Reflectance Distribution Function (BRDF). The BRDF provides an understanding of the reflectance behavior of a surface from every incident angle and reflectance angle. With the BRDF one can provide an accurate computer model of how the material behaves. Databases of BRDF data are available for use in modeling and simulation of targets but are typically comprised of pristine samples that may not be representative of real world targets. This paper will provide methods, data and trends of the BRDF variability in the infrared regions. We will also explore appropriate data sets for use to represent typical fielded targets.Characterization of bidirectional reflectance distribution of individual leaf is essential in canopy radiative transfer modeling. Measuring bidirectional reflection over reflectance hemisphere is a very complex and time-consuming work. Considering that leaf's reflection is a linear combination of specular and diffuse component, where specular reflection is centered on mirror direction in the principal plane and diffuse reflection is uniform regardless of directions, we made an attempt to parameterize Bidirectional Reflectance Distribution Function (BRDF) over reflectance hemisphere from angular distribution of reflection in its principal plane. Angular reflection of maize (Zea mays L) leaves over reflectance hemisphere was measured with a custom-designed device. The leaves were illuminated with two wavelengths of 650 nm and 830 nm, at three incidence angles of 0deg, 30deg and 60deg. Reflection measured in the principal plane was used to model BRDF with a physically based BRDF model. They were used to predict the reflection in other directions outside the principal plane. The predicted reflection was in a good agreement with those measured. It indicates that reflection distribution in the principal plane carries sufficient information to characterize the BRDF over hemisphere.
Reflection
Diffuse reflection
Plane of incidence
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An effective image-based measurement for the BRDF of objects was proposed with specular reflection. Unlike traditional methods, using measurements to fit BRDF parameters directly, specular and diffuse parameters were fitted respectively. Firstly, the specular components and diffuse components of reflectance were reliably separated. Then, separated components were used to fit the two kinds of parameters independently. Experiments were conducted using the method as well as the traditional ones, and the results indicate that the method has a better performance for the BRDF acquisition of specular objects.
Diffuse reflection
Specular highlight
Reflection
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An efficient BRDF(Bidirectional Reflectance Distribution Function) measurement of glossy surface by photometric stereo was proposed. We assume that the surface reflectance can be approximated by the sum of a Lambertian and a specular component. We define the best window in one image and classify the patches into the diffuse points and the mix ones according to their radiance values. The diffuse parameter and surface normal are recovered by the diffuse points and the darkest three pixels of four pixels. The specular reflectance parameters are recovered by the specular components of mix points which normal and diffuse parameter are known. In the last, all reflection parameters are refined by minimizing the fitting error of the reflectance model. Experiments show that the reflectance of glossy object can be recovered in a good precision by the proposed method.
Diffuse reflection
Photometric Stereo
Reflection
Normal
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We analyze material properties underlying visual appearance, such as surface bidirectional reflection distribution function (BRDF) and texture. We perform gonioradiometric measurements on bricks and fit the data to sets of models of specular and diffuse reflectance on rough surfaces in order to describe the composite reflection mechanisms, of the surfaces under study. We also acquire images and perform image texture statistical discrimination techniques to determine the textural differences in the surface appearance, resulting from the variation of illumination and viewing.
Texture (cosmology)
Reflection
Bidirectional texture function
Specular highlight
Diffuse reflection
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For real diffuse surfaces, the bi-directional reflectance distribution function (BRDF) is non-Lambertian, and may require a more complex model in ray tracing simulations. The BRDF of a diffuse white surface is studied at multiple angles of incidence, and an additional reflectance component is observed, which becomes more specular as the angle of incidence increases. For angles of incidence >85°, the BRDF may be regarded as specular. In this article, a two-part model is proposed in which the BRDF of a diffuse surface consists of a Lambertian diffuse component and a Lorentzian pseudo-specular component — both of which vary with angle of incidence. This model may be used to reduce computation times for ray tracing simulations, as an alternative to large three-dimensional BRDF datasets.
Diffuse reflection
Component (thermodynamics)
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The bidirectional reflectance distribution function (BRDF) model developed by Torrance and Sparrow [J. Opt. Soc. Am. 57, 1105-1114 (1967)] is used to describe the specular reflection of rough surfaces. We compare this model with the BRDF measurements of four manmade surfaces with different roughnesses. The model can be used to describe the basic features of the measured BRDFs. We found that the width of the specular peak perpendicular to the principal plane decreases strongly with an increasing illumination zenith angle in the data as well as in the model. A model analysis shows that the width is approximately proportional to the cosine of the illumination angle theta(i), and the deviations are determined by the roughness of the surface. This relationship is accompanied by an increase in reflectance in the specular direction in the principal plane that is 1/cos theta(i) stronger than the increase for a perfectly smooth surface.
Zenith
Reflection
Specular highlight
Diffuse reflection
Specularity
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A ray-tracing method in conjunction with UTD (uniform theory of diffraction) has been widely used to characterize a wireless communication channel. Due to the complexity of the real situation, the ray-tracing method generally assumes buildings have homogeneous and smooth surfaces. However, in a real case, the building surface consists of many heterogeneous materials, whose shapes are also various. Hence, the scattering by the surface is much different from a homogeneous surface, which will affect the final accuracy of the ray-tracing method. In UTD, scattering mechanism by an object is mainly decomposed into two groups: reflection and diffraction. In general, the reflection in the specular direction is important, but the inhomogeneity may reduce the specular reflection drastically. In this paper, to consider the reduction of the specular reflection, we formulate the effective specular reflection based on PO (physical optics) approximation, and include the reflection in a 3D ray-tracing code. Then, we investigate the inhomogeneity effect on the final path-loss.
Reflection
Diffuse reflection
Geometrical optics
Ray
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Diffuse reflection
Reflection
Haze
Fresnel equations
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The BRDF model developed by Torrance and Sparrow describes the specular reflection of rough surfaces. We compared this model to BRDF measurements of 4 man-made surfaces with very different roughnesses. We found that the azimuthal width of the specular peak decreases strongly with increasing illumination zenith angle, in the data as well as in the model. We furthermore propose a simplification of the model by dropping one of the assumptions on the surface structure. This simplification is equivalent to ignoring masking and shadowing effects, but alters the resulting BRDF in most cases only negligibly, and provides several advantages like easier implementation and faster computing time.
Zenith
Reflection
Specular highlight
Specularity
Rough surface
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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.
Diffuse reflection
Specular highlight
Gloss (optics)
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