Influence of probe pressure on human skin diffuse reflectance spectroscopy measurements

In a routine procedure for collecting human skin spectra using fiber optic diffuse reflectance spectroscopy, the fiber optic probe is put gently in contact with the skin surface. The purpose of this work is to investigate the influence of probe pressure on in-vivo diffuse reflectance spectroscopy measurements performed by a non-trained operator. In addition, we include simulation using diffusion theory that explains the behavior of diffuse reflectance spectra with the increase of probe pressure. The article is published in the original.     

中医面诊中面部肤色特征基函数确定

为满足中医面诊中对面色精确描述的要求,用光谱反射率来表述颜色.基于有限维模型,面部的光谱反射率可以用一组基函数的线性组合表示.因此可采用少量的图像通道数据求解像素位置基函数系数,复原面部各像素点的光谱反射率.而基函数的确定是光谱反射率复原的基础,针对面部肤色的特点,基于标准色系及真实肤色研究了不同面部光谱反射率样本集条件下,确定的面部光谱反射率基函数对复原精度的影响,并确定了适用于面部光谱反射率复原的优选样本集及基函数组合.结果表明,优选样本集使用较少的基函数就可以达到良好的复原效果,将为基于中医面诊光谱反射率复原提供依据,进而促进中医面诊的客观化发展.     

A dorsiventral leaf radiative transfer model: Development, validation and improved model inversion techniques

A Dorsiventral Leaf Model (DLM) is presented to simulate leaf radiative transfer. DLM was conceived as a plate model with a stochastic distribution of different groups of layers. Leaf asymmetry was modeled by assigning non-uniform distributions of pigments, water and dry matter to palisade and mesophyll layers and by simulating different amounts of light diffusion for adaxially and abaxially incident light. Surface reflections are based on micro-facets theory enabling the simulation of directional-hemispherical reflectance and a range of bidirectional reflectance factors. Adaxial and abaxial optical properties could be accurately simulated for a variety of leaf types with an overall error in reflectance and transmittance below 1.3%.Sensitivity analysis focused on optimizing model inversion schemes improves parameter estimation accuracy. Different inversion schemes were compared for two independent datasets. Results underpin most of the propositions of the sensitivity analysis: (i) masking the near-infrared wavelengths (band weighting) to account for variability in the dry matter composition consistently increased predicted accuracies for dry matter content, (ii) white reflectance measurements (reflectance with a 100% diffusely reflecting background) provided results superior to other optical measurements, making it a valuable and fast alternative and (iii) combining reflectance and transmittance into absorptance however did not result in improvements. Comparisons of DLM with the PROSPECT 5 model indicate an almost equal performance in content estimations. Improvements were thus not related to differences in model structure but to techniques that reduce the impact of leaf structure and compensate for sampling errors and variations in specific absorption spectra. DLM has important potential in the study of leaf radiative transfer and in the integration with canopy radiative transfer models.     

植物花色模式仿真

利用反应-扩散系统与颜色的频谱表达体系,将色素分布仿真与色素颜色表达相结合,并统一在同一个系统框架内,以对植物花色模式进行仿真.首先使用经植物脉络信息修正后的反应一扩散系统对色素分布进行仿真,然后用与传统的基于手绘或照片等不同的基于色素吸收光谱的方法来获得数据.该系统可通过调整参数实现对现实世界中大量不同植物花色模式的仿真,而且可嵌入其他系统,以改进它们的渲染效果.     

Effects of lichens on the reflectance spectra of granitic rock surfaces

The objective of this study was to determine the effect of lichen cover on the spectral reflectance characteristics of granitic rocks. Rock samples collected from bedrock exposures and talus slopes were granodiorite, tonalite, granite, and quartz-diorite. The lichens found on these rocks varied in color, i.e., black, dark brown, medium gray, olive-green, yellow-green, green, orange, and orange-red. The visible and near infrared reflectance spectra (400–1100 nm) of the rock and lichen samples show that lichens can affect the rock's spectra. These changes are dependent on the reflectance contrast between the rock surface and the lichen covering.     

Measuring oxygen saturation of skin with advancing age using iterative optical path-length matrix method

We measured the oxygen saturation of skin at various ages by iterating our previously proposed method that can rapidly simulate skin spectral reflectance with high accuracy. Oxygen saturation is commonly measured by a pulse oximeter to evaluate oxygen delivery for monitoring the functions of heart and lungs at a specific time. On the other hand, oxygen saturation of skin is expected to assess peripheral conditions. Our previously proposed method, the optical path-length matrix method (OPLM), is based on a Monte Carlo model of steady-state light transport in multi-layered tissue (MCML), but can simulate skin spectral reflectance 27,000 times faster than MCML. In this study, we implemented an iterative OPLM with a nonlinear optimization technique such that this method can also be used for estimating hemoglobin concentration and oxygen saturation from the measured skin spectral reflectance. The iterative OPLM was over 25,000 times faster than the MCML with 95 % accuracy. Before measurements in clinic, stability of oxygen saturation among measuring pressure was confirmed. Then, the skin reflectance spectra of 72 outpatients aged between 20 and 86 years were measured by a spectrophotometer. Three points were measured for each subject: the forearm, the thenar eminence, and the intermediate phalanx. The result showed that the oxygen saturation of skin remained constant at each point as the age varied.     

A novel restoration approach for vegetation reflectance spectra at noisy bands using the principal component analysis method

ABSTRACT

Due to the signal-to-noise ratio (SNR) of sensors, as well as atmospheric absorption and illumination conditions, etc., hyperspectral data at some bands are of poor quality. Data restoration for noisy bands is important for many remote sensing applications. In this paper, we present a novel data-driven Principal Component Analysis (PCA) approach for restoring leaf reflectance spectra at noisy bands using the spectra at effective bands. The technique decomposes the leaf reflectance spectra into their principal components (PCs), selects the leading PCs that describe the most variance in the data, and restores the data from these components. First, the first 10 PCs were determined from a training dataset simulated by the leaf optical properties model (PROSPECT-5) that contained 99.998% of the total information in the 3636 training samples. Then, the performance of the PCA method for restoration of the reflectance at noisy bands was investigated using the ANGERS leaf optical properties dataset; the results showed the spectral root mean squared error (RMSE) is in the range 6.46 × 10^?4 to 6.44 × 10^?2, which is about 3 ? 34 times more accurate than the stepwise regression method and partial least squares method (PLSR) for the ANGERS dataset. The results also showed that if the noisy bands are far away from the effective bands, the accuracy of the restored leaf reflectance spectra will decrease. Thirdly, the reliability of the restored reflectance spectra for retrieving leaf biochemical contents was assessed using the ANGERS dataset and leaf optical properties dataset established by the Beijing Academy of Agriculture and Forestry Sciences (BAAFS). Three water-sensitive vegetation indices ? normalized difference water index (NDWI), normalized difference infrared index (NDII) and Datt water index (DWI), derived from the restored leaf spectra ? were employed to retrieve the equivalent water thickness (EWT). The results showed that the leaf water content can be accurately retrieved from the restored leaf reflectance spectra. In addition, the PCA method to restore vegetation spectral reflectance can be applied on canopy level as well. The results showed that the spectral root mean squared error (RMSE) is in the range 8.22 × 10^?4 to 1.87 × 10^?2. The performance of the restored canopy spectra was investigated according to the results of retrieving canopy equivalent water thickness (CEWT) using the five spectral indices NDWI, NDWI₁₃₇₀, NDWI₁₈₉₀, NDII and DWI. The results indicated that the restored canopy spectra can be used for retrieving CEWT reliably and improve the accuracy of retrieval compared to the results using original canopy reflectance spectra.     

Scientific information repository assisting reflectance spectrometry in legal medicine

Reflectance spectrometry is a fast and reliable method for the characterization of human skin if the spectra are analyzed with respect to a physical model describing the optical properties of human skin. For a field study performed at the Institute of Legal Medicine and the Freiburg Materials Research Center of the University of Freiburg, a scientific information repository has been developed, which is a variant of an electronic laboratory notebook and assists in the acquisition, management, and high-throughput analysis of reflectance spectra in heterogeneous research environments. At the core of the repository is a database management system hosting the master data. It is filled with primary data via a graphical user interface (GUI) programmed in Java, which also enables the user to browse the database and access the results of data analysis. The latter is carried out via Matlab, Python, and C programs, which retrieve the primary data from the scientific information repository, perform the analysis, and store the results in the database for further usage.     

全球典型植被叶片光谱特征及其对LAI反演的影响分析

在全球范围长时间序列LAI遥感产品反演算法中,植被冠层反射率模型仅使用少量叶片光谱特征代表全球植被全年的典型植被光谱特征,叶片光谱的不确定性导致LAI遥感产品存在一定的误差。目前全球已经构建了多个典型植被叶片波谱数据集,这些数据集包含多个植被物种、不同空间地域及多时相叶片光谱数据,为定量分析叶片光谱特征提供了数据支持。主要利用LOPEX’93、ANGERS’03、中国典型地物波谱数据库和野外实测的叶片光谱数据,以黄边参数、红边参数和叶片光谱指数作为分析指标,探讨不同植被物种、不同气候区和不同物候期的叶片光谱特征差异,及其对植被冠层反射率、LAI反演的影响,为发展考虑现实叶片光谱差异的LAI反演算法提供研究基础。结果表明:植被叶片光谱存在多样性,叶片光谱特征差异主要影响MODIS传感器近红外波段和绿波段反射率值,其中,绿波段反射率值对叶片光谱变化最为敏感;在LAI反演算法中,如果只考虑植被类型而不考虑物种叶片光谱差异,可能会给LAI反演带来大于3的误差。     

Two ways of assessing absorbance of fresh leaves from near-infrared reflectance spectroscopy

In the standard way of measuring fresh leaf absorbance (logl/R) using laboratory equipment, the leaf generally needs to be put on a reflective background. Leaf transmissivity can be deduced from modelling the propagation of light within this bi-component sample and its subsequent reflectance or absorbance. The contribution through the leaf of the energy emanating from this background is highest near 1000 nm. A model of radiation transfers inside a fresh leaf put upon a reflective background, provides a simulation of the subsequently measured reflectance and allows an index of transmissivity T to be calculated. This index takes into account possible optical differences between the two leaf surfaces. Tests carried out using near-infrared reflectance spectroscopy at 1000 nm are presented in relation to a structural (leaf mass per area) and a chemical (chlorophyll content) variable: the index of transmissivity and the related absorbance (logl/T) may provide an improvement in the predictive character of standard leaf absorbance (log l/R) spectra. Some problems involved in fresh leaves spectroscopy are examined.     

太湖水体叶绿素浓度与反射光谱特征关系的研究

根据2004年10月在太湖实测的水体反射光谱及实验室分析得到的叶绿素浓度数据,对太湖水体反射光谱特征与叶绿素浓度之间的关系进行探讨与分析。研究结果表明:水体叶绿素浓度与各波长处的反射光谱相关性不大,但是反射光谱经过比值法和微分处理后,两者具有很好的相关性,而且叶绿素浓度与反射光谱700nm附近波峰几何形态特征(波峰位置、面积、净高度)相关性很好,建立太湖水体叶绿素浓度估算模型。     

Spectral mixture analysis of inland tropical waters

This letter presents results of experiments conducted to assess the use of linear mixing models to unmix the reflectance spectra of water. The model was tested with 11 reflectance spectra measured at Tucurui reservoir, Brazilian Amazon. The results showed that the fraction of chlorophyll reflectance accounting for the reflectance spectra of the various samples correlate significantly with the chlorophyll concentration in the 600 to 700 nm spectral region.     

松花湖水体叶绿素a含量与反射光谱特征关系初探

根据2008年7月在松花湖实测的水体反射光谱及实验室分析得到的叶绿素浓度数据,对松花湖水体反射光谱特征与叶绿素浓度之间的关系进行探讨与分析。研究结果表明:水体叶绿素浓度与各波长点处反射率相关性均较好,并选择700 nm处反射率建立单波段模型。而700 nm和677 nm波长处反射率比值、685 nm处光谱一阶微分、700 nm波长处波峰几何特征具有较好的相关性,给出了松花湖水体叶绿素浓度估算模型,为松花湖水体叶绿素浓度反演监测提供了一定的理论基础与参考。     

Surface Emissivity Image Simulationfor Atmospheric Absorption Bands based on Spectral Mixing

A simulation method based on spectral mixing is proposed for surface emissivity image generation in atmospheric absorption bands,in order to provide surface input data for the corresponding end to end image simulation.First,endmember selection is conducted on data source to acquire image endmember spectra.Then,substitute endmembers are selected from surface measured spectra by spectral matching with image endmembers,and used for abundance inversion.Finally,using emissivity spectra of substitute endmember in the absorption bands and abundance maps,emissivity images are simulated based on linear spectral mixing model.In the simulation experiment,Landsat8 OLI images were used as data source,and JHU and USGS spectral library data were assumed to be ground spectra of the test case.Since actual emissivity images in absorption bands are unavailable,accuracy analysis is conducted by comparing OLI reflectance images with its simulations generated by the proposed method.Total RSMEs of simulated OLI images are 0.045 and 0.049,respectively;which shows the image simulation method is feasible and can produce images with high accuracy.     

Aspects of operational atmospheric correction of hyperspectral imagery

The large number of spectral bands of hyperspectral instruments and the time required for the calculation of atmospheric look-up tables and the reflectance image cube pose very challenging requirements on an operational processing facility. This contribution presents some aspects and suggestions to reduce the processing time. Essential components are a precalculated database with a reduced number of spectral bands, an interactive phase to determine the appropriate atmospheric parameters, and a choice between medium and high accuracy levels for the atmospheric correction. The medium accuracy levels work with look-up tables for a reduced number of spectral bands employing interpolation for the channels omitted in the look-up tables. The high accuracy level uses tables for all channels and includes the scan angle dependence of the atmospheric radiance and transmittance functions. These ideas were successfully implemented and tested during several airborne hyperspectral campaigns resulting in an estimated time saving of a factor 3-7. The deviations of field measured reflectance spectra and spectra retrieved from airborne HyMap imagery are in the range of 2-3% or better.     

The reflectance factors of a rough ocean with foam—comment on ‘Remote sensing of the sea state using the 0·8-1·1 μm spectral band’ by L. Wald and J. M. Monget

Reflectance factors of a rough ocean surface with and without foam are presented. They are calculated from the Cox-Munk model for an ocean without foam. The isotropic reflectance of oceanic foam is added as function of the wind speed. The effective reflectance of foam patches is taken into account     

Soil surface illumination at micro-relief scale and soil BRDF data collected by a hyperspectral camera

The results of the paper draw attention to the fact that the hyperspectral image of soil surface at micro-relief scale may display variation in the soil spectral shape due to illumination conditions of the surface. The image of an extremely rough cultivated soil surface, very deeply ploughed, was obtained by a hyperspectral camera, in the range of 0.4–1.0 μm with 0.67–0.74 nm spectral resolution. It was found that the soil reflectance spectra of the studied surface, illuminated by the direct sunbeams, are clearly convex with distinct absorption features. Furthermore, the soil normalized reflectance spectra were used to distinguish the subtlety of the analysed shaded soil spectra shape. They show that depressions caused by the absorption features of O₂ and H₂O, contained in the atmosphere above directly illuminated soil fragments, transform into peaks, if the same soil is deeply shaded.     

Self-consistent theory of selective reflection

A relatively simple procedure is developed for calculation of transmission and selective reflection spectra in a system consisting of a layer of atomic vapor sandwiched between two transparent solids with parallel interfaces. The proposed method allows one to correctly take into account multiple reflections, absorption, and spatial dispersion of atomic medium and is applicable to arbitrary densities and optical lengths of atomic medium. The article is published in the original.     

Feasibility of hyperspectral remote sensing for mapping benthic macroalgal cover in turbid coastal waters—a Baltic Sea case study

Quantitative analysis of coastal marine benthic communities enables to adequately estimate the state of coastal marine environment, provide better evidence for environmental changes and describe processes that are conditioned by anthropogenic forces. Remote sensing could provide a tool for mapping bottom vegetation if the substrates are spectrally resolvable. We measured reflectance spectra of green (Cladophora glomerata), red (Furcellaria lumbricalis), and brown (Fucus vesiculosus) macroalgae and used a bio-optical model in estimating whether these algae distinguish optically from each other, from sandy bottom or deep water in turbid water conditions of the Baltic Sea. The simulation was carried out for three different water types: (1) CDOM-rich coastal water, (2) coastal waters not directly impacted by high CDOM discharge from rivers but with high concentration of cyanobacteria, (3) open Baltic waters. Our modelling results indicate that the reflectance spectra of C. glomerata, F. lumbricalis, F. vesiculosus differ from each other and also from sand and deep water reflectance spectra. The differences are detectable by remote sensing instruments at spectral resolution of 10 nm and SNR better than 1000:1. Thus, the lowest depth limits where the studied macroalgae grow do not exceed the depth where such remote sensing instruments could potentially detect the spectral differences between the studied species.