Tenn F. Chen

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.

Bulk scattering approximations for HeNe laser transmitted through paper

An experimental set-up was devised to record the transmission of red and green HeNe lasers through different types of paper. The measured data was compared with data obtained using the Henyey-Greenstein function (often employed in paper optics models to represent the bulk scattering of material samples) and data obtained using an alternative exponentiated cosine function. The comparisons are used to qualitatively assess the degree of fidelity of the bulk scattering approximations provided by both functions.

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.

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.

On the high-fidelity monitoring of C3 and C4 crops under nutrient and water stress

The different photosynthetic and morphological characteristics of C3 and C4 plants may lead to distinct physiological responses of C3 and C4 crops to stress factors. These responses are strongly correlated with the red edge of these plants, the s-shaped curve in the 680-800nm region of their reflectance spectra. We performed controlled in silico experiments to investigate the patterns of the red edge displacements resulting from C3 and C4 specimens subjected to the same stress conditions. Our findings indicate these patterns need to be taken into account in the development of effective monitoring procedures for C3 and C4 crops.

On the decomposition of foliar hyperspectral signatures for the high-fidelity discrimination and monitoring of crops

Hyperspectral technologies are being increasingly employed in precision agriculture. By separating the surface and subsurface components of foliar hyperspectral signatures using polarization optics, it is possible to enhance the remote discrimination of different plant species and optimize the assessment of different factors associated with the crops’ health status such as chlorophyll levels and water content. These initiatives, in turn, can lead to higher crop yield and lower environmental impact through a more effective use of freshwater supplies and fertilizers (reducing the risk of nitrogen leaching). It is important to consider, however, that the main varieties of crops, represented by C3 (e.g., soy) and C4 (e.g., maize) plants, have markedly distinct morphological characteristics. Accordingly, the influence of these characteristics on their interactions with impinging light may affect the selection of optimal probe wavelengths for specific applications making use of combined hyperspectral and polarization measurements. In this work, we compare the sensitivity of the surface and subsurface reflectance responses of C3 and C4 plants to different spectral and geometrical light incidence conditions. In our comparisons, we also consider intra- species variability with respect to specimen characterization data. This investigation is supported by measured biophysical data and predictive light transport simulations. The results of our comparisons indicate that the surface and subsurface reflectance responses of C3 and C4 plants depict well-defined patterns of sensitivity to varying illumination conditions. We believe that these patterns should be considered in the design of new high-fidelity crop discrimination and monitoring procedures.

On the identification and interpretation of human skin spectral responses under adverse environmental conditions.

The identification and interpretation of skin spectral responses play a central role in a wide range of biomedical engineering applications, from the noninvasive assessment of human health parameters to the location of individuals in distress during search and rescue operations. In this paper, we investigate the sensitivity of these responses to physiological changes triggered by adverse environmental conditions. Our findings, which are supported by predictive computer simulations and experimental observations reported in the scientific literature, indicate that the resulting variations of skin reflectance can be substantial. Accordingly, if not properly taken into account, they may considerably impair the efficacy of systems designed for the detection and analysis of skin signatures within and outside the visible spectral region.

Melanosome distribution patterns affecting skin reflectance: Implications for the in vivo estimation of epidermal melanin content.

Several techniques employed in the in vivo estimation of epidermal melanin content rely on the assumption that the effects of different distribution patterns of aggregated melanin (clustered within the melanosomes) on skin spectral responses, particularly across the 600-1350 nm range, can be ignored. Accordingly, for all practical purposes, only the non-aggregated (colloidal) form of melanin is taken into account by these techniques. In this paper, however, we demonstrate through predictive computer simulations that these responses are directly influenced by the occurrence of both forms of melanin. Our in silico findings, in turn, indicate that such an assumption may lead to inaccurate estimations of epidermal melanin content.