The most cited articles published since 2010, extracted from Scopus
Volume 54, Issue 3, May 2011, Pages 136-154
In the paper, fundamental and technological issues associated with the development and exploitation of the most advanced infrared detector technologies are discussed. In this class of detectors both photon and thermal detectors are considered. Special attention is directed to HgCdTe ternary alloys on silicon, type-II superlattices, uncooled thermal bolometers, and novel uncooled micromechanical cantilever detectors. Despite serious competition from alternative technologies and slower progress than expected, HgCdTe is unlikely to be seriously challenged for high-performance applications, applications requiring multispectral capability and fast response. However, the nonuniformity is a serious problem in the case of LWIR and VLWIR HgCdTe detectors. In this context, it is predicted that type-II superlattice system seems to be an alternative to HgCdTe in long wavelength spectral region. In well established uncooled imaging, microbolometer arrays are clearly the most used technology. Present state-of-the-art microbolometers are based on polycrystalline or amorphous materials, typically vanadium oxide (VOx) or amorphous silicon (α-Si), with only modest temperature sensitivity and noise properties. Basic efforts today are mainly focused on pixel reduction and performance enhancement. Attractive alternatives consist of low-resistance α-SiGe monocrystalline SiGe quantum wells or quantum dots. In spite of successful commercialization of uncooled microbolometers, the infrared community is still searching for a platform for thermal imagers that combine affordability, convenience of operation, and excellent performance. Recent advances in MEMS systems have lead to the development of uncooled IR detectors operating as micromechanical thermal detectors. Between them the most important are biomaterial microcantilevers. © 2010 Elsevier Ltd. All rights reserved.
Volume 53, Issue 2, March 2010, Pages 120-126
Jenhong Tan | Yinkwee Ng | U. R. Acharya | Caroline Chee
Ocular surface temperature (OST) has been studied with numerous approach and Infrared (IR) thermography has proved to be the best way to capture temperature distribution over some surfaces. It is applied to a number of biomedical applications including studies in the field of ophthalmology. However, the analysis of an ocular thermogram is largely in nascent stage, and is usually achieved by first-order texture analysis. This current study conducted second-order texture analysis on ocular thermal images, mainly by cross co-occurrence matrix together with first-order texture analysis, moments and difference histogram. It was found that, for subjects aged above 35 years old their interocular difference in median, textural contrast, moment 2 and moment 3 (in absolute value) were significantly higher than younger peers. Several significant linear correlations among investigated features were observed. The features extracted from cross co-occurrence matrix may play an important role in the diagnosis of ocular diseases. © 2009 Elsevier B.V. All rights reserved.
Volume 55, Issue 4, July 2012, Pages 221-235
Basudev B. Lahiri | Shri S. Bagavathiappan | Tammana K. Jayakumar | John Philip
Abnormal body temperature is a natural indicator of illness. Infrared thermography (IRT) is a fast, passive, non-contact and non-invasive alternative to conventional clinical thermometers for monitoring body temperature. Besides, IRT can also map body surface temperature remotely. Last five decades witnessed a steady increase in the utility of thermal imaging cameras to obtain correlations between the thermal physiology and skin temperature. IRT has been successfully used in diagnosis of breast cancer, diabetes neuropathy and peripheral vascular disorders. It has also been used to detect problems associated with gynecology, kidney transplantation, dermatology, heart, neonatal physiology, fever screening and brain imaging. With the advent of modern infrared cameras, data acquisition and processing techniques, it is now possible to have real time high resolution thermographic images, which is likely to surge further research in this field. The present efforts are focused on automatic analysis of temperature distribution of regions of interest and their statistical analysis for detection of abnormalities. This critical review focuses on advances in the area of medical IRT. The basics of IRT, essential theoretical background, the procedures adopted for various measurements and applications of IRT in various medical fields are discussed in this review. Besides background information is provided for beginners for better understanding of the subject. © 2012 Elsevier B.V. All rights reserved.
Volume 53, Issue 4, July 2010, Pages 267-273
Jianfu Li | Weiguo Gong | Weihong Li | Xiaoying Liu
A novel and robust pedestrian detection method in thermal infrared images based on the double-density dual-tree complex wavelet transform (DD-DT CWT) and wavelet entropy is presented in this paper. The regions of interest (ROIs) are located first making use of high brightness property of the pedestrian pixels caused by the self-emission of the pedestrians related to the Planck's law. The candidate ROIs are then decomposed by DD-DT CWT and the wavelet entropy features are extracted from the high frequency subbands. The true pedestrian regions are finally classified and recognized using the support vector machine (SVM) classifier. Comparisons between our approach and traditional approaches are presented and experimental results using several thermal infrared image databases show the proposed scheme to be very promising. © 2010 Elsevier B.V. All rights reserved.
Volume 53, Issue 2, March 2010, Pages 146-150
Songfeng Yin | Liangcai Cao | Yongshun Ling | Guofan Jin
Color constancy (Toet and Franken, 2003 ; Toet, 2003 ) and color contrast (Scribner et al., 2000 ; Lee et al., 2005 ) are two important topics for color image fusion. The paper focuses on the low color contrast problem of linear fusion algorithms with color transfer method. Color transfer technology is popular in infrared (IR) and visible image fusion to give the fused image a natural day-time color appearance (Toet, 2003 ; Wang et al., 2007 ; Zheng and Essock, 2008 ). However, in the color transfer step, all three channels of the color space are processed with the same linear mapping without color enhancement, resulting in low color contrast between the target and the background (Wang et al., 2007 ). Based on the characteristics of the IR image, we introduce a ratio of local to global divergence of the IR image to improve the color contrast. The enhancement ratios for both hot and cold targets are larger than one, while it tends to one for the background. As a result, the proposed method pops out both hot and cold targets in color, where hot targets will appear intense red, and cold targets will appear cyan. Subjective results show visible color contrast enhancement effects. Target detection experiments through hue and saturation components of the fused image show an improvement in the hit rate for target detection, owing to larger color distance between the target and the background. Crown Copyright © 2009.
Volume 60, Issue , May 2013, Pages 35-55
Shri S. Bagavathiappan | Basudev B. Lahiri | Thangavelu T. Saravanan | John Philip | Tammana K. Jayakumar
Temperature is one of the most common indicators of the structural health of equipment and components. Faulty machineries, corroded electrical connections, damaged material components, etc., can cause abnormal temperature distribution. By now, infrared thermography (IRT) has become a matured and widely accepted condition monitoring tool where the temperature is measured in real time in a non-contact manner. IRT enables early detection of equipment flaws and faulty industrial processes under operating condition thereby, reducing system down time, catastrophic breakdown and maintenance cost. Last three decades witnessed a steady growth in the use of IRT as a condition monitoring technique in civil structures, electrical installations, machineries and equipment, material deformation under various loading conditions, corrosion damages and welding processes. IRT has also found its application in nuclear, aerospace, food, paper, wood and plastic industries. With the advent of newer generations of infrared camera, IRT is becoming a more accurate, reliable and cost effective technique. This review focuses on the advances of IRT as a non-contact and non-invasive condition monitoring tool for machineries, equipment and processes. Various conditions monitoring applications are discussed in details, along with some basics of IRT, experimental procedures and data analysis techniques. Sufficient background information is also provided for the beginners and non-experts for easy understanding of the subject.© 2013 Elsevier B.V. All rights reserved.
Volume 54, Issue 5, September 2011, Pages 403-411
We introduce a spatial and temporal target detection method using spatial bilateral filter (BF) and temporal cross product (TCP) of temporal pixels in infrared (IR) image sequences. At first, the TCP is presented to extract the characteristics of temporal pixels by using temporal profile in respective spatial coordinates of pixels. The TCP represents the cross product values by the gray level distance vector of a current temporal pixel and the adjacent temporal pixel, as well as the horizontal distance vector of the current temporal pixel and a temporal pixel corresponding to potential target center. The summation of TCP values of temporal pixels in spatial coordinates makes the temporal target image (TTI), which represents the temporal target information of temporal pixels in spatial coordinates. And then the proposed BF filter is used to extract the spatial target information. In order to predict background without targets, the proposed BF filter uses standard deviations obtained by an exponential mapping of the TCP value corresponding to the coordinate of a pixel processed spatially. The spatial target image (STI) is made by subtracting the predicted image from the original image. Thus, the spatial and temporal target image (STTI) is achieved by multiplying the STI and the TTI, and then targets finally are detected in STTI. In experimental result, the receiver operating characteristics (ROC) curves were computed experimentally to compare the objective performance. From the results, the proposed algorithm shows better discrimination of target and clutters and lower false alarm rates than the existing target detection methods. © 2011 Elsevier B.V. All rights reserved.
Volume 55, Issue 4, July 2012, Pages 236-245
Mohd Shawal Jadin | Soib Bin Taib
Infrared thermography (IRT) has gained more attention and become an interesting method in electrical preventive maintenance due to its high precision and sensitivity imaging characteristics. This paper provides a review of the application of IRT for diagnosing electrical equipment, including their thermal anomalies and methods of measurement. Improvement of the inspection techniques is highlighted in order to investigate the reliability of electrical equipments due to the effect of the environmental factors and equipment condition. Factors related to the target equipment and the inspection tool together with their characteristics is also presented. Due to the complex analysis, various automatic diagnostic systems are proposed for faster and more accurate analysis. Typical engineering solutions using recent technologies are reviewed which could be used to improve the quality of IRT inspection. © 2012 Elsevier B.V. All rights reserved.
Volume 53, Issue 4, July 2010, Pages 280-287
Xin Wang | Zhenmin Tang
Tracking infrared pedestrian targets is a crucial part in video surveillance. Many factors make this problem decidedly non-linear and non-Gaussian, and the appropriate solution at present is based on the particle filter technique which is powerful and simple to implement. But in many cases, the traditional particle filter tracking algorithm fails to track the targets robustly and accurately. To solve these problems, a modified particle filter algorithm that combines intensity and edge cues is proposed. The algorithm firstly extracts the intensity cue and edge cue of the target based on the visual models which are originally learnt from the first frame and will be updated during the tracking process according to an automatic model updating strategy. Secondly, these two cues are combined into the particle filter framework by an adaptive integration scheme. Furthermore, its performance is evaluated with real-world infrared pedestrian sequences and extensive experimental results show that the presented method can track the infrared pedestrian more effectively and reliably than the traditional particle filter algorithm. © 2010 Elsevier B.V. All rights reserved.
Volume 54, Issue 2, March 2011, Pages 61-69
Xiangzhi Bai | Fugen Zhou | Bindang Xue
Infrared imaging sensor is sensitive to the variation of imaging environment, which may affect the quality of the obtained images and blur the regions of interest in infrared image. So, it is very important to enhance infrared image. In infrared image, the gray values of the regions of interest are bright or dim image regions, which are different from the surrounding regions. The new top-hat transform could extract image regions which are different from its surrounding regions. In light of this, an infrared image enhancement algorithm through contrast enhancement is proposed in this paper based on multiscale new top-hat transform. Firstly, the multiscale white and black new top-hat transforms are used to extract the multiscale light and dark infrared image regions. Then, the final light and dark infrared image regions for image enhancement are constructed by using the extracted multiscale light and dark infrared image regions. Finally, the contrast of the infrared image is enhanced through a power strategy. Experimental results on different infrared images show that the proposed algorithm could well enhance infrared image and make the possible interested targets brighter, which is very helpful for target detection and recognition. © 2010 Elsevier B.V. All rights reserved.
Volume 53, Issue 4, July 2010, Pages 246-253
Ralf W. Arndt
A methodical approach for qualitative and quantitative non-destructive testing of near-surface structures in civil engineering (CE) with active thermography is presented. It adopts the non-destructive testing (NDT) method of pulsed phase thermography (PPT) for the special requirements of CE and cultural heritage. The concept might be understood as a square pulse thermography (SPT) in frequency domain or an amplitude-expanded PPT with square pulse heating. After a discussion of the material spanning concept and qualitative results in cultural heritage a new approach for quantitative non-destructive testing (NDT) of near-surface structures in CE with active thermography is introduced and tested by investigations on concrete specimen with artificial defects. It is based on the thermal diffusivity of the material and the characteristic frequency of the first extrema of phase and amplitude contrast and aims at complementing the established approaches for defect depth calculation for measurements with long heating and observation times. It should be easily extendable to other fields of application. © 2010 Elsevier B.V. All rights reserved.
Volume 53, Issue 3, May 2010, Pages 208-217
Kang Huang | Xia Mao
Selecting the most appropriate algorithms for detecting small targets in varied infrared image scenes is frequently needed, since the relative characteristics between small targets and backgrounds in varied scenes are disparate. To solve that problem, a novel criterion is proposed in this article to measure the difficulty in distinguishing small targets from infrared images, which is named detectability of infrared small targets (DIST). Then a new method to detect small targets based on it was introduced. Detection probability and false alarm probability were employed to evaluate the performance. The results from the experiments further demonstrated the robustness of the proposed methodology under different backgrounds. © 2009 Elsevier B.V. All rights reserved.
Volume 54, Issue 3, May 2011, Pages 278-282
Gerasimos Konstantatos | E. H. Sargent
We review recent progress in light sensors based on solution-processed materials. Spin-coated semiconductors can readily be integrated with many substrates including as a post-process atop CMOS silicon and flexible electronics. We focus in particular on visible-, near-infrared, and short-wavelength infrared photodetectors based on size-effect-tuned semiconductor nanoparticles made using quantum-confined PbS, PbSe, Bi 2S3, and In2S3. These devices have in recent years achieved room-temperature D values above 1013 Jones, while fully-depleted photodiodes based on these same materials have achieved MHz response combined with 1012 Jones sensitivities. We discuss the nanoparticle synthesis, the materials processing, integrability, temperature stability, physical operation, and applied performance of this class of devices. © 2010 Elsevier Ltd. All rights reserved.
Volume 54, Issue 2, March 2011, Pages 92-99
Susana Lagüela | Higinio González-Jorge | Julia Armesto | P. Arias
Infrared thermography is a technique with an increasing degree of development and applications. Quality assessment in the measurements performed with the thermal cameras should be achieved through metrology calibration and verification. Infrared cameras acquire temperature and geometric information, although calibration and verification procedures are only usual for thermal data. Black bodies are used for these purposes. Moreover, the geometric information is important for many fields as architecture, civil engineering and industry. This work presents a calibration procedure that allows the photogrammetric restitution and a portable artefact to verify the geometric accuracy, repeatability and drift of thermographic cameras. These results allow the incorporation of this information into the quality control processes of the companies. A grid based on burning lamps is used for the geometric calibration of thermographic cameras. The artefact designed for the geometric verification consists of five delrin spheres and seven cubes of different sizes. Metrology traceability for the artefact is obtained from a coordinate measuring machine. Two sets of targets with different reflectivity are fixed to the spheres and cubes to make data processing and photogrammetric restitution possible. Reflectivity was the chosen material propriety due to the thermographic and visual cameras ability to detect it. Two thermographic cameras from Flir and Nec manufacturers, and one visible camera from Jai are calibrated, verified and compared using calibration grids and the standard artefact. The calibration system based on burning lamps shows its capability to perform the internal orientation of the thermal cameras. Verification results show repeatability better than 1 mm for all cases, being better than 0.5 mm for the visible one. As it must be expected, also accuracy appears higher in the visible camera, and the geometric comparison between thermographic cameras shows slightly better results for the Nec camera. © 2010 Elsevier B.V. All rights reserved.
Volume 53, Issue 5, September 2010, Pages 363-371
Lock-in and pulse phase infrared thermography measurement techniques have been exploited for quantitative assessment of subsurface defects in a reference specimen made of Plexiglas. Radiometric thermal images were post-processed using a contrast approach in the frequency domain, allowing defect depth to be resolved with a combined standard uncertainty of about 5% for thicknesses up to 3.6 mm. Conversely, significant radial heat diffusion next to the boundary of the discontinuities made accurate sizing of deeper subsurface defects more difficult, resulting in a combined standard uncertainty of about 17% for a 10 mm diameter flat-bottomed hole of 3.6 mm deep. The obtained results demonstrate the potentiality of active thermography as a fast, powerful contactless NDE measurement tool. © 2010 Elsevier B.V. All rights reserved.
Volume 53, Issue 1, January 2010, Pages 17-22
The effects of 2-D electromagnetic crystal substrate on the performance of a rectangular microstrip patch antennas at THz frequencies is simulated. Electromagnetic crystal substrate is used to obtain extremely broad-bandwidth with multi-frequency band operation of the proposed microstrip antennas. Multi-frequency band microstrip patch antennas are used in modern communication systems in order to enhance their capacity through frequency reuse. The simulated 10 dB impedance bandwidth of the rectangular patch microstrip antenna is 34.3% at THz frequency (0.6-0.95 THz). The radiation efficiency, gain and directivity of the proposed antenna are presented at different THz frequencies. The simulation has been performed using CST Microwave Studio, which is a commercially available electromagnetic simulator based on finite integral technique. © 2009 Elsevier B.V. All rights reserved.
Volume 53, Issue 5, September 2010, Pages 348-357
Jun Liu | Wang Yang | Jingmin Dai
Lock-in thermography, an active IR thermography technique for NDT, is based on propagation and reflection of thermal waves which are launched from the surface into the inspected component by absorption of modulated radiation. In this paper, thermal wave image sequences were sampled by a Cedip JADE MWIR 550 FPA infrared camera. Thermal wave signal processing algorithms are investigated to obtain information on subsurface defects. The Fourier transform, four-point correlation, and digital lock-in correlation algorithms are applied to extract the amplitude and phase of thermal wave's harmonic component. A novel method called the time constant method (TCM) is proposed to analyze subsurface defects by using lock-in thermography. The experimental results confirm the thermal wave signal processing algorithms' efficiency on subsurface defect detection. © 2010 Elsevier B.V. All rights reserved.
Volume 53, Issue 2, March 2010, Pages 77-83
Hong Li. | Yantao Wei | Luoqing Li | Yuanyan Tang
This paper presents an algorithm for detecting and tracking dim moving target in IR image sequence with low SNR. The algorithm, which is designed based on the locality preserving projection, accepts tensors as inputs. The justification for the algorithm comes from the role of the generalized eigenvalue problem in providing an optimal embedding for the manifold. Not only does the proposed method inherit the attractive characteristics of the locality preserving projections in terms of exploiting the intrinsic manifold structure, it is also appealing in terms of significant reduction in both space complexity and time complexity. Experimental results on two IR image sequences demonstrate the effectiveness of the proposed algorithm. © 2009 Elsevier B.V. All rights reserved.
Volume 53, Issue 5, September 2010, Pages 308-314
Dan Constantin Dumitraş | Stefan Banita | Ana M. Bratu | Ramona Cernat | Doru C A Duţu | Consuela Elena Matei | Mihai Paţachia | Mioara Petruş | Cristina Popa
The present paper describes an extremely sensitive apparatus based upon laser photoacoustic spectroscopy (LPAS) methods which can be used for the detection and measurement of trace gases at very low concentrations (parts per trillion by volume - pptV). Two experimental set-ups were designed and characterized with the photoacoustic (PA) cell in an external configuration: the first one with a low power CO2 laser where the saturation effects are negligible, and a second one with a high power CO2 laser where the saturation effects are important and have to be taken into consideration. In the first case, the minimum detectable concentration was 0.9 ppbV (parts per billion by volume), while in the second case this parameter was improved to 0.29 ppbV. Comparing with the best results published previously in the literature, our minimum detectable concentration is better by a factor of 4.2 in the first case and by a factor of 13.1 in the second case. All measurements were done in nitrogen and ethylene with the 10P(14) line of a continuous wave CO2 laser. This technology can dramatically impact detection in numerous areas. © 2010 Elsevier B.V. All rights reserved.
Volume 55, Issue 4, July 2012, Pages 309-315
Kun Liang | Yong Ma | Yue Xie | Bo Zhou | Rui Wang
In infrared images, detail pixels are easily immerged in large quantity of low-contrast background pixels. According to these characteristics, an adaptive contrast enhancement algorithm based on double plateaus histogram equalization for infrared images was presented in this paper. Traditional double plateaus histogram equalization algorithm used constant threshold and could not change the threshold value in various scenes, so that its practical usage is limited. In the proposed algorithm, the upper and lower threshold value could be calculated by searching local maximum and predicting minimum gray interval and be updated in real time. With the proposed algorithm, the background of infrared image was constrained while the details could also be enhanced. Experimental results proved that the proposed algorithm can effectively enhance the contrast of infrared images, especially the details of infrared images. © 2012 Elsevier B.V. All rights reserved.
Volume 54, Issue 3, May 2011, Pages 302-305
Victor I. Ryzhiǐ | Maxim Ryzhii | Nadezhda L. Ryabova | Vladimir V. Mitin | Taiichi Otsuji
We consider newly proposed terahertz and infrared interband detectors based on multiple-graphene-layer structures with p-i-n junctions. Using the developed device model, we calculate the photodetector characteristics (responsivity and dark current limited detectivity) and compare them with the characteristics of other photodetectors. It is shown that due to relatively high quantum efficiency and weakened thermogeneration processes, the detectors under consideration can exhibit superior performance. © 2010 Elsevier Ltd. All rights reserved.
Volume 54, Issue 2, March 2011, Pages 100-107
He Deng | Jianguo Liu
The achievement of high detection probability and low false alarm probability is a bottleneck problem in the detection of infrared small targets. Thus, a new small target detection approach which integrates the concept of self-information map (SINM) with the adaptive thresholding method followed by a region growing technique is proposed in this paper. The concepts of local signal-to-noise ratio, region nonuniformity, detection probability, and false alarm probability are used to evaluate the performance. The experiment results obtained from qualitative and quantitative comparisons testify to the robustness of the approach presented under different conditions. © 2010 Elsevier B.V. All rights reserved.
Volume 53, Issue 6, November 2010, Pages 439-449
Ronan O'Malley | Edward Jones | Martin E. Glavin
We present a night-time pedestrian detection system based on automotive infrared video processing. Far-infrared or thermal night vision is a technology well suited for automatic detection of pedestrians at night as they generally appear warmer than the background. However, the appearance of a pedestrian in IR video can vary dramatically depending on the physical properties of the clothing they wear, the time spent adjusting to the outside environment, and the ambient temperature. We highlight the difficulties of detection in low temperatures (below 8 °C) when pedestrians typically wear highly insulating clothing, which can lead to distortion of the IR signature of the pedestrian. A pre-processing step is presented, which compensates for this clothing-based distortion using vertically-biased morphological closing. Potential pedestrians (Regions of Interest) are then segmented using feature-based region-growing with high intensity seeds. Histogram of Oriented Gradients (HOG) features are extracted from candidates and utilised for Support Vector Machine classification. Positively classified targets are tracked between frames using a Kalman filter, adding robustness and increasing performance. The proposed system adapts not just to variations between images or video frames, but to variations in appearance between different pedestrians in the same image or frame. Results indicate improved performance compared to previous HOG-SVM automotive IR pedestrian detection systems, which utilised stereo IR cameras. © 2010 Elsevier B.V. All rights reserved.
Volume 55, Issue 6, November 2012, Pages 513-521
Xin Wang | Guofang Lv | Lizhong Xu
Accurate and fast detection of infrared (IR) dim target has very important meaning for infrared precise guidance, early warning, video surveillance, etc. Based on human visual attention mechanisms, an automatic detection algorithm for infrared dim target is presented. After analyzing the characteristics of infrared dim target images, the method firstly designs Difference of Gaussians (DoG) filters to compute the saliency map. Then the salient regions where the potential targets exist in are extracted by searching through the saliency map with a control mechanism of winner-take-all (WTA) competition and inhibition-of-return (IOR). At last, these regions are identified by the characteristics of the dim IR targets, so the true targets are detected, and the spurious objects are rejected. The experiments are performed for some real-life IR images, and the results prove that the proposed method has satisfying detection effectiveness and robustness. Meanwhile, it has high detection efficiency and can be used for real-time detection.
Volume 57, Issue , February 2013, Pages 101-109
Krithiga Ganesan | Leonid A. Dombrovsky | Wojciech Lipiński
Visible and near-infrared properties of optically thick ceria ceramics encountered in solar-driven thermochemical cycles are determined in the wavelength range 0.35-2 μm by employing a novel methodology. The methodology consists of an analytical solution based on the modified two-flux approximation and the Monte Carlo ray-tracing method. The analytical solution is used to identify transport scattering albedo from normal-hemispherical reflectance of optically thick samples. The Monte Carlo ray-tracing method is used to obtain transport scattering coefficient in the near-infrared wavelength range 0.9-1.4 μm from bi-normal narrow-cone transmittance. Mie theory is employed to obtain transport scattering coefficient in an extended spectral range. Further, the index of absorption of ceria (cerium dioxide) is estimated by the additive relation for optically soft materials in a short-wave part of the near-infrared range. The proposed methodology is suitable for problems where traditional identification procedures have limited applicability because of highly scattering, optically thick materials and unknown index of absorption in the spectral range of weak absorption. © 2013 Elsevier B.V. All rights reserved.