Acousto-optic tunable filters (AOTFs) can be used as spectral filters in multispectral imaging applications. Acousto-optic crystals diffract a single wavelength from a broadband light beam, depending on the applied radio frequency signal. However, experimental measurements show that the actual performance is far from the expected behavior. We present an experimental characterization of several commercial off-the-shelf AOTFs for the implementation of multispectral imaging instruments. The diffraction performance of three bare crystals is compared, while a fourth AOTF crystal is mounted on the optical path of a multispectral imager to evaluate its performance. The experiments show that the behavior of all the analyzed AOTFs differs from the theoretical expectations and presents uneven diffraction efficiency, a Gaussian dependency with the applied power, and a strong nonlinear relationship with the driving signal frequency. The different behavior of each AOTF in terms of all the analyzed parameters shows the necessity for an in-depth characterization of the AOTF performance once mounted on a multispectral imaging device if quantitative measurements are required. Finally, several recommendations for use are derived from these experimental results.
Acousto-optic tunable filters (AOTFs) can be used as spectral filters for the implementation of multispectral imaging systems. However, obtaining quality images is challenging. In this work, we propose several improvements that enable the use of these systems in quantitative spectroscopic imaging applications. The improvements are based on three pillars: 1. a finer spectral bandpass shaping by dynamically optimizing the radio frequency (rf) driving signal, 2. an extensive calibration process, and 3. careful image preprocessing that uses calibration data to correct some well known AOTF issues in imaging applications. A novel multispectral imaging instrument is built using commercial off-the-shelf components. The instrument includes an Isomet (Springfield, New Jersey) AOTF working in the visible and near-infrared range, and a new concept of rf generator based on a high-speed digital-to-analog converter that allows the generation of multiband signals. The ancillary control software performs the main part of the image optimization process: an initial calibration, a dynamic adjustment of the rf driving signal power and exposure time, and finally the radiometric preprocessing of the acquired multispectral images. Finally, some results of the instrument performance are presented that show the achieved spectral and spatial resolution on different imaging scenarios.
This paper presents a semi-supervised one-class support vector machine classifier for remote sensing applications.
In one-class image classification, one tries to detect pixels belonging to one class and reject the others. When
few labeled target pixels and no labeled outlier pixels are available, the selection of the support vector machine
free parameters is very challenging. This problem can be alleviated by introducing the information of the wealth
of unlabeled samples present in the scene. The proposed algorithm deforms the training kernel by modelling
the data marginal distribution with the graph Laplacian built with labeled and unlabeled samples. The good
performance of the proposed method is illustrated in challenging remote sensing image classification scenarios
where information of only one class of interest is available. In particular, we present results in multispectral
cloud screening, hyperspectral crop detection, and multisource urban monitoring. Experimental results show the
suitability of the proposal, specially in cases with few or poorly representative labeled samples.
Identification of land cover types is one of the most critical activities in remote sensing. Nowadays, managing
land resources by using remote sensing techniques is becoming a common procedure to speed up the process while
reducing costs. However, data analysis procedures should satisfy the accuracy figures demanded by institutions
and governments for further administrative actions.
This paper presents a methodological scheme to update the citrus Geographical Information Systems (GIS)
of the Comunidad Valenciana autonomous region (Spain). The proposed approach introduces a multi-stage
automatic scheme to reduce visual photointerpretation and ground validation tasks. First, an object-oriented
feature extraction process is carried out for each cadastral parcel from very high spatial resolution (VHR) images
(0.5m) acquired in the visible and near infrared. Next, several automatic classifiers (decision trees, multilayer
perceptron, and support vector machines) are trained and combined to improve the final accuracy of the results.
The proposed strategy fulfills the high accuracy demanded by policy makers by means of combining automatic
classification methods with visual photointerpretation available resources. A level of confidence based on the
agreement between classifiers allows us an effective management by fixing the quantity of parcels to be reviewed.
The proposed methodology can be applied to similar problems and applications.
Chlorophyll fluorescence (Chf) emission allows estimating the photosynthetic activity of vegetation - a key parameter for the carbon cycle models - in a quite direct way. However, measuring Chf is difficult because it represents a small fraction of the radiance to be measured by the sensor. This paper analyzes the relationship between the solar induced Chf emission and the photosynthetically active radiation (PAR) in plants under water stress condition. The solar induced fluorescence emission is measured at leaf level by means of three different methodologies. Firstly, an active modulated light fluorometer gives the relative fluorescence yield. Secondly, a quantitative measurement of the Chf signal is derived from the leaf radiance by using the Fraunhofer Line-Discriminator (FLD) principle, which allows the measurement of Chf in the atmospheric absorption bands. Finally, the actual radiance spectrum of the leaf fluorescence emission is measured by a field spectroradiometer using a device that filters out the incident light in the Chf emission spectral range. The diurnal cycle of fluorescence emission has been measured for both healthy and stressed plants in natural and simulated conditions. The main achievements of this work have been: (1) successful radiometric spectral measurement of the solar induced fluorescence; (2) identification of fluorescence behavior under stress conditions; and (3) establishing a relationship between full spectral measurements with the signal provided by the FLD method. These results suggest the best time of the day to maximize signal levels while identifying vegetation stress status.
In addition to typical random noise, remote sensing hyperspectral images are generally affected by non-periodic partially deterministic disturbance patterns due to the image formation process and characterized by a high degree of spatial and spectral coherence. This paper presents a new technique that faces the problem of removing the spatial coherent noise known as vertical stripping (VS) usually found in images acquired by push-broom sensors, in particular for the Compact High Resolution Imaging Spectrometer (CHRIS). The correction is based on the hypothesis that the vertical disturbance presents higher spatial frequencies than the surface radiance. The proposed method introduces a way to exclude the contribution of the spatial high frequencies of the surface from the destripping process that is based on the information contained in the spectral domain. Performance of the proposed algorithm is tested on sites of different nature, several acquisition modes (different spatial and spectral resolutions) and covering the full range of possible sensor temperatures. In addition, synthetic realistic scenes have been created, adding modeled noise for validation purposes. Results show an excellent rejection of the noise pattern with respect to the original CHRIS images. The analysis shows that high frequency VS is successfully removed, although some low frequency components remain. In addition, the dependency of the noise patterns with the sensor temperature has been found to agree with the theoretical one, which confirms the robustness of the presented approach. The approach has proven to be robust, stable in VS removal, and a tool for noise modeling. The general nature of the procedure allows it to be applied for destripping images from other spectral sensors.
We explicitly formulate a family of kernel-based methods for (supervised and partially supervised) multitemporal classification and change detection. The novel composite kernels developed account for the static and temporal cross-information between pixels of subsequent images simultaneously. The methodology also takes into account spectral, spatial, and temporal information, and contains the familiar difference and ratioing methods in the kernel space as a particular cases. The methodology also permits straightforward fusion of multisource information. Several scenarios are considered in which partial or complete labeled information at the prediction time is available. The developed methods are then tested under different classification frameworks: (1) inductive support vector machines (SVM), and (2) one-class support vector data description (SVDD) classifier, in which only samples of a class of interest are used for training. The proposed methods are tested in a challenging real problem for urban monitoring. The composite kernel approach is additionally used as a fusion methodology to combine synthetic aperture radar (SAR) and multispectral data, and to integrate the spatial and textural information at different scales and orientations through Gabor filters. Good results are observed in almost all scenarios; the SVDD classifier demonstrates robust multitemporal classification and adaptation capabilities when few labeled information is available, and SVMs show improved performance in the change detection approach.
An imaging spectrometer covering the 400– to 1000–nm band is conceived and developed. The system is based on an acousto-optic tunable filter (AOTF) attached to a high-performance digital camera. The AOTF enables the selection of spectral bands with an rf signal in the range of 70 to 218 MHz. It includes a telecentric optical system that enhances system efficiency. Additionally, a smart choice of integration time reduces the dependence of the efficiency on the frequency. Calibration includes filter characterization and compensation of crystal nonconstant diffraction efficiency and spatial nonhomogeneity. The system is controlled by a PC application specifically developed for this purpose, providing wide versatility, while enabling transparent and intuitive management to nonexpert users. The spectrometer is validated by estimating the light absorption of leaves and their chlorophyll content.
Accurate and automatic detection of clouds in satellite scenes is a key issue for a wide range of remote sensing applications. With no accurate cloud masking, undetected clouds are one of the most significant source of error in both sea and land cover biophysical parameter retrieval. Sensors with spectral channels beyond 1 um have demonstrated good capabilities to perform cloud masking. This spectral range can not be exploited by recently developed hyperspectral sensors that work in the spectral range between 400- 1000 nm. However, one can take advantage of their high number of channels and spectral resolution to increase the cloud detection accuracy, and to describe properly the detected clouds (cloud type, height, subpixel coverage, could shadows, etc.) In this paper, we present a methodology for cloud detection that could be used by sensors working in the VNIR range. First, physically-inspired features are extracted (TOA reflectance and their spectral derivatives, atmospheric oxygen and water vapour absorptions, etc). Second, growing maps are built from cloud-like pixels to select regions which potentially could contain clouds. Then, an unsupervised clustering algorithm is applied in these regions using all extracted features. The obtained clusters are labeled into geo-physical classes taking into account the spectral signature of the cluster centers. Finally, an spectral unmixing algorithm is applied to the segmented image in order to obtain an abundance map of the cloud content in the cloud pixels. As a direct consequence of the detection scheme, the proposed system is capable to yield probabilistic outputs on cloud detected pixels in the image, rather than flags. Performance of the proposed algorithm is tested on six CHRIS/Proba Mode 1 images, which presents a spatial resolution of 32 m, 62 spectral bands with 6-20 nm bandwidth, and multiangularity.
In this communication, we evaluate the performance of the relevance vector machine (RVM) (Tipping,2000) for the estimation of biophysical parameters from remote sensing images. For illustration purposes, we focus on the estimation of chlorophyll concentrations from multispectral imagery, whose measurements are subject to high levels of uncertainty, both regarding the difficulties in ground-truth data acquisition, and when comparing in situ measurements against satellite-derived data. Moreover, acquired data are commonly affected by noise in the acquisition phase, and time mismatch between the acquired image and the recorded measurements, which is critical for instance for coastal water monitoring. In this context, robust and stable regressors that provide inverse models are desirable. Lately, the use of the support vector regressor (SVR) has produced good results to this end. However, the SVR has many deficiencies, which could be theoretically alleviated by the RVM. In this paper, performance of the RVM is benchmarked with SVR in terms of accuracy and bias of the estimations, sparseness of the solutions, distribution of the residuals, robustness to low number of training samples, and computational burden. In addition, some theoretical issues are discussed, such as the sensitivity to hyperparameters setting, kernel selection, and confidence intervals on the predictions. Results suggest that RVM offer an excellent compromise between accuracy and sparsity of the solution, and reveal itself as less sensitive to selection of the free parameters. Some disadvantages are also pointed, such as the unintuitive confidence intervals provided and the computational cost.
This paper presents a new portable instrument called Autonomous Tunable Filtering System (ATFS), developed for highly customisable imaging spectrometry in the VIS-NIR range. The ATFS instrument consists of an Acousto-Optic Tunable Filter (AOTF), an optical system, a Radio Frequency (RF) driver based on a Direct Digital Synthesiser (DDS) and control software. The ATFS can be attached to a variety of high-performance monochrome cameras. The system works as a spectral bandpass filter whose wavelength can be selected between 400nm and 1000nm and whose bandwidth can be adjusted between 4nm and 50nm. The filter can be tuned electronically at a very high speed and accuracy, thanks to the DDS versatility. The control software synchronises the camera with the RF generation and implements a smart auto-exposure algorithm that maximises the dynamic range of the instrument for each band. The software can take a set of spectral images sequentially and save them in ENVI® multispectral format or as multiple TIFF images. The system has been validated using a reference point spectrometer. An optional acquisition procedure has been developed, based on the acquisition of dark and white Spectralon® reference images, in order to use the system in applications involving quantitative (reflectance) measurements. Procedures have been established in order to fully calibrate the instrument. The system has been demonstrated in a real world application, which uses the ATFS to map the leaf chlorophyll content from multispectral reflectance images.
In some key operational domains, users are not specially interested in obtaining an exhaustive map with all the thematic classes present in an area of interest, but rather in identifying accurately a single class of interest. In this paper, we present a novel partially supervised classification technique that faces this interesting practical and methodological problem. We have adopted a two-stage classification scheme based on an unsupervised approach, which allows us to introduce supervised information about the class of interest without an additional sample labeling. The first stage of the process consists in an initial clustering of the image using the Self-Organizing Map algorithm. The second stage consists in a partially supervised hierarchical joint of clusters. We modify the employed criterion of similarity by introducing fuzzy membership functions that make use of the supervised information. The method is tested on urban monitoring, where the objective is to produce an automatic classification of 'Urban/Non-Urban' by using optical and radar data (Landsat TM and 35-days interferometric pairs of ERS2 SAR). We compare classification accuracy of the proposed method to its parametric version, which uses the Expectation-Maximization algorithm. The good performance confirms the validity of the proposed approach: 90% classification accuracy using supervised information only in the coherence map.
An imaging spectrometer covering the 400-1000 nm band has been conceived and developed. The system is based on an Acousto-Optic Tunable Filter (AOTF) attached to a high performance digital camera. The AOTF permits the selection of spectral bands with an RF signal in the range of 70-210 MHz. The range is covered using two transducers attached to a single crystal. Although the idea is not new it covers a broader spectrum than previous systems. It includes a telecentric optical system that enhances system efficiency, by ensuring that the chief ray of each light cone emerges out of this doublet parallel to the optical axes. Additionally, an smart choice of integration time reduces the dependence of the efficiency on the frequency. Calibration includes filter characterisation and compensation of crystal non-constant diffraction efficiency and spatial non-homogeneity. The system is controlled by a PC application, specifically developed for this purpose, providing wide versatility, while enabling transparent and intuitive management to typical users.
In this paper, we propose a kernel-based approach for hyperspectral knowledge discovery, which is defined as a process that involves three steps: pre-processing, modeling and analysis of the classifier. Firstly, we select the most representative bands analyzing the surrogate and main splits of a Classification And Regression Trees (CART) approach. This yields three datasets with different reduced input dimensionality (6, 3 and 2 bands, respectively) along with the original one (128 bands). Secondly, we develop several crop cover classifiers for each of them. We use Support Vector Machines (SVM) and analyze its performance in terms
of efficiency and robustness, as compared to multilayer perceptrons (MLP) and radial basis functions (RBF) neural networks. Suitability to real-time working conditions, whenever a preprocessing stage is not possible, is evaluated by considering models with and without the CART-based feature selection stage. Finally, we analyze the support vectors distribution in the input space and through Principal Component Analysis (PCA) in order to gain knowledge about the problem. Several conclusions are drawn: (1) SVM yield better
outcomes than neural networks; (2) training neural models is unfeasible when working with high dimensional spaces; (3) SVM perform similarly in the four classification scenarios, which indicates that noisy bands are successfully detected and (4) relevant bands for the classification are identified.
In this paper, we propose a new approach to the classification of hyperspectral images. The main problem with supervised methods is that the learning process heavily depends on the quality of the training data set. In remote sensing, the training set is useful only for simultaneous images or for images with the same classes taken under the same conditions; and, even worse, the training set is frequently not available. On the other hand, unsupervised methods are not sensitive to the number of labelled samples since they work on the whole image. Nevertheless, relationship between clusters and classes is not ensured. In this context, we propose a combined strategy of supervised and unsupervised learning methods that avoids these drawbacks and automates the classification process. The method is based on the general formulation of the expectation-maximization (EM) algorithm. This method is applied to crop cover recognition of six hyperspectral images from the same area acquired with HyMap spectrometer during the DAISEX99 campaign. For classification purposes, six different classes are considered in this area: corn, wheat, sugar beet, barley, alfalfa, and soil. Classification accuracy results are compared to common methods: ISODATA, Learning Vector Quantization, Gaussian Maximum Likelihood, Expectation-Maximization, and Neural Networks. The good performance confirms the validity of the proposed approach in terms of accuracy and robustness.