Bradley W. Kimmel

A novel approach for simulating light interaction with particulate materials: application to the modeling of sand spectral properties

In this paper, we present a new spectral light transport model for sand. The model employs a novel approach to simulate light interaction with particulate materials which yields both the spectral and spatial (bi-directional reflectance distribution function, or BRDF) responses of sand. Furthermore, the parameters specifying the model are based on the physical and mineralogical properties of sand. The model is evaluated quantitatively, through comparisons with measured data. Good spectral reconstructions were achieved for the reflectances of several real sand samples. The model was also evaluated qualitatively, and compares well with descriptions found in the literature. Its potential applications include, but are not limited to, applied optics, remote sensing and image synthesis.

Hyperspectral Modeling of Skin Appearance

Exploration of the hyperspectral domain offers a host of new research and application possibilities involving material appearance modeling. In this article, we address these prospects with respect to human skin, one of the most ubiquitous materials portrayed in synthetic imaging. We present the first hyperspectral model designed for the predictive rendering of skin appearance attributes in the ultraviolet, visible, and infrared domains. The proposed model incorporates the intrinsic bio-optical properties of human skin affecting light transport in these spectral regions, including the particle nature and distribution patterns of the main light attenuation agents found within the cutaneous tissues. Accordingly, it accounts for phenomena that significantly affect skin spectral signatures, both within and outside the visible domain, such as detour and sieve effects, that are overlooked by existing skin appearance models. Using a first-principles approach, the proposed model computes the surface and subsurface scattering components of skin reflectance taking into account not only the wavelength and the illumination geometry, but also the positional dependence of the reflected light. Hence, the spectral and spatial distributions of light interacting with human skin can be comprehensively represented in terms of hyperspectral reflectance and BSSRDF, respectively.

On the noninvasive optical monitoring and differentiation of methemoglobinemia and sulfhemoglobinemia

Abstract. There are several pathologies whose study and diagnosis is impaired by a relatively small number of documented cases. A practical approach to overcome this obstacle and advance the research in this area consists in employing computer simulations to perform controlled in silico experiments. The results of these experiments, in turn, may be incorporated in the design of differential protocols for these pathologies. Accordingly, in this paper, we investigate the spectral responses of human skin affected by the presence of abnormal amounts of two dysfunctional hemoglobins, methemoglobin and sulfhemoglobin, which are associated with two life-threatening medical conditions, methemoglobinemia and sulfhemoglobinemia, respectively. We analyze the results of our in silico experiments and discuss their potential applications to the development of more effective noninvasive monitoring and differentiation procedures for these medical conditions.

Spectral appearance changes induced by light exposure

The fading of materials due to light exposure over time is a major contributor to the overall aged appearance of man-made objects. Although much attention has been devoted to the modeling of aging and weathering phenomena over the last decade, comparatively little attention has been paid to fading effects. In this article, we present a theoretical framework for the physically based simulation of time-dependent spectral changes induced by absorbed radiation. This framework relies on the general volumetric radiative transfer theory, and it employs a physicochemical approach to account for variations in the absorptive properties of colorants. Employing this framework, a layered fading model that can be readily integrated into existing rendering systems is developed using the Kubelka-Munk theory. We evaluate its correctness through comparisons of measured and simulated fading results. Finally, we demonstrate the effectiveness of this model through renderings depicting typical fading scenarios.

Increasing the predictability of tissue subsurface scattering simulations

Models of light interaction with matter usually rely on subsurface scattering approximations based on the use of phase functions – notably, the Henyey-Greenstein phase function and its variations. In this paper, we challenge the generalized use of these approximations, especially for organic materials, and propose the application of a data-oriented approach whenever reliable measured data is available. Our research is supported by comparisons involving the original measured data that motivated the use of phase functions in algorithmic simulations of tissue subsurface scattering. We hope that this investigation will help strengthen the biophysical basis required for the predictable rendering of organic materials.

An investigation on the use of data-driven scattering profiles in Monte Carlo simulations of ultraviolet light propagation in skin tissues

Ultraviolet light can affect the appearance and medical condition of the human skin by triggering biophysical processes such as erythema, melanogenesis, photoaging and carcinogenesis. The evolution of these processes is related to the amount of ultraviolet light absorbed by skin pigments. This amount may vary with the wavelength and path length of the radiation that is propagated within the skin tissues. For many years, biomedical researchers have been investigating the propagation of ultraviolet light in skin tissues through Monte Carlo simulations. The scattering of the incident radiation by tissue internal structures, a key component in this process, is usually approximated by functions without a plausible connection with the underlying physical phenomena. In this paper, we examine the origins of such an approach, and question its generalized use with respect to wavelengths and biological materials for which there is no supporting data available. Furthermore, we perform comparisons to demonstrate that the accuracy and predictability of Monte Carlo simulations of ultraviolet propagation in skin tissues can be improved by using a data-driven approach to represent the scattering profile of these tissues.

In silico assessment of environmental factors affecting the spectral signature of C4 plants in the visible domain

Monocotyledonous (C 4) plants, such as maize and sugarcane, have a central role in the economy and ecology of our planet. In many regions, the main food sources are based on C 4 crops. These crops are also major suppliers of raw materials used in the production of biofuel. Due to their increasing global demand, it becomes essential not only to monitor and analyse the effects of abiotic stress factors, such as limited water and nutrient supplies, on their productivity, but also to determine their ecological impact (e.g. related to their irrigation needs). Computer simulations, or in silico experiments, are being routinely employed in remote-sensing investigations aimed at these goals. Besides these applications, in silico experiments paired with measured data can also contribute to expand the existing knowledge about the biophysical mechanisms responsible for the remarkable tolerance of C 4 plants to adverse environmental conditions. In this article, we evaluate the applicability of a computer model (ABM-U) to the assessment of biophysical responses of C 4 plants in the visible (photosynthetic) domain when subjected to abiotic stress factors. Initially, we verify the accuracy of model readings obtained in this spectral domain. This verification is performed through quantitative and qualitative comparisons of modelled results with measured data. We then proceed to investigate apparently conflicting reflectance profiles resulting from experiments involving maize specimens under moderate water stress, which is usually associated with unfavourable climate changes. The results of our simulations indicate that ABM-U can reliably predict spectral signature variations caused by abiotic stress factors affecting the photosynthetic apparatus of these plants, which, in turn, have a direct impact on their agricultural yield. Furthermore, our in silico experiments suggest that the decrease in the amount of light reflected by (in vivo) water-stressed specimens may result from changes in the internal arrangement of the main components of their photosynthetic apparatus, namely the chloroplasts. We close the article with a discussion of putative physiological processes responsible for such changes.

Simulating the appearance of sandy landscapes

Sand is one of the most complex materials found in nature. Undeniably the correct modelling of its appearance attributes (such as hue, lightness, and glossiness) is essential to the realistic image synthesis of a wide range of outdoor scenes. Despite this central role, to date, few simulation efforts have been specifically directed to this ubiquitous material. In this paper, we present a modular framework for simulating the appearance of sandy landscapes. It is based on the use of a comprehensive light transport model specifically designed for granular materials like sand, and robust numerical reconstruction methods. While the former provides the physical basis for the generation of predictive results, the latter add efficiency to entire simulation process by enabling the use of analytical formulae to represent the spectral and spatial (scattering related) appearance attributes of sand. The fidelity and usefulness of the proposed framework are demonstrated through several image sequences depicting sand appearance variations resulting from changes of mineralogical characteristics and environmental conditions.

Effects of sand grain shape on the spectral signature of sandy landscapes in the visible domain

In this paper, we investigate the effects of sand grain shape on the reflectance of sandy landscapes within the visible domain. Our investigation is supported by computer simulations performed using SPLITS (Spectral Light TransportModel for Sand) and taking into account actual sand characterization data. Our findings indicate that the spectral effects of grain shape may vary considerably depending on the distribution patterns of iron oxides present in sand-textured soils. These patterns and grain shape properties, namely roundness and sphericity, are largely determined by the formation processes of these soils. Hence, we believe that their interplay should be carefully taken into account in the retrieval of information about the mineralogy and morphology of sandy terrains.

Influence of Sand-Grain Morphology and Iron-Oxide Distribution Patterns on the Visible and Near-Infrared Reflectance of Sand-Textured Soils

The overall shape of a sand grain can be defined by two morphological properties, namely sphericity and roundness, and it is largely determined by soil-formation and weathering processes. In this paper, we investigate the effects of these properties on the visible and near-infrared reflectance of sand-textured soils characterized by the presence of iron oxides. Our investigation is supported by computer simulations performed using the SPLITS (Spectral Light Transport Model for Sand) model and considering actual sand characterization data. Our findings indicate that the influence of grain morphology may vary considerably depending on the distribution patterns of iron oxides present in sand-textured soils. These minerals may occur as pure particles, as contaminants mixed with the grain parent material, or as coatings. Since these distribution patterns are also significantly affected by soil-formation and weathering processes, we believe that the combined influence of sand-grain shape and iron-oxide distribution patterns on the reflectance of sandy landscapes should be carefully taken into account in the retrieval of information about their mineralogy and environmental history.