Spectral Imaging System Research Articles

Shift-variant color-coded diffractive spectral imaging system

State-of-the-art snapshot spectral imaging (SI) systems introduce color-coded apertures (CCAs) into their setups to obtain a flexible spatial-spectral modulation, allowing spectral information to be reconstructed from a set of coded measurements. Besides the CCA, other optical elements, such as lenses, prisms, or beam splitters, are usually employed, making systems large and impractical. Recently, diffractive optical elements (DOEs) have partially replaced refractive lenses to drastically reduce the size of the SI devices. The sensing model of these systems is represented as a projection modeled by a spatially shift-invariant convolution between the unknown scene and a point spread function (PSF) at each spectral band. However, the height maps of the DOE are the only free parameters that offer changes in the spectral modulation, which causes the ill-posedness of the reconstruction to increase significantly. To overcome this challenge, our work explores the advantages of the spectral modulation of an optical setup composed of a DOE and a CCA. Specifically, the light is diffracted by the DOE and then filtered by the CCA, located close to the sensor. A shift-variant property of the proposed system is clearly evidenced, resulting in a different PSF for each pixel, where a symmetric structure constraint is imposed on the CCA to reduce the high number of resulting PSFs. Additionally, we jointly design the DOE and the CCA parameters with a fully differentiable image formation model using an end-to-end approach to minimize the deviation between the true and reconstructed image over a large set of images. Simulation shows that the proposed system improves the spectral reconstruction quality in up to 4 dB compared with current state-of-the-art systems. Finally, experimental results with a fabricated prototype in indoor and outdoor scenes validate the proposed system, where it can recover up to 49 high-fidelity spectral bands in the 420–660 nm.

Underwater Spectral Imaging System Based on Liquid Crystal Tunable Filter

In the past decade, underwater spectral imaging (USI) has shown great potential in underwater exploration for its high spectral and spatial resolution. This proposal presents a stare-type USI system combined with the liquid crystal tunable filter (LCTF) spectral splitting device. Considering the working features of LCTF and the theoretical model of USI, the core structure containing “imaging lens-LCTF-imaging sensor” is designed and developed. The system is compact, and the optical geometry is constructed minimally. The spectral calibration test analysis proved that the spectral response range of the system covers a full band of 400 nm to 700 nm with the highest spectral resolution between 6.7 nm and 18.5 nm. The experiments show that the system can quickly collect high-quality spectral image data by switching between different spectral bands arbitrarily. The designed prototype provides a feasible and reliable spectral imaging solution for in situ underwater targets observation with high spectrum collecting efficiency.

Flexible bilayer terahertz metasurface for the manipulation of orbital angular momentum states.

Metasurfaces employed for generating orbital angular momentum (OAM) beams have drawn tremendous interest since they can offer extensive applications ranging from quantum optics to information processing over the subwavelength scale. In this study, a flexible bilayer metasurface is proposed and experimentally verified in the terahertz (THz) region. Based on Pancharatnam-Berry (P-B) phase, the proposed meta-atom satisfies perfect polarization-flipping at the design frequency and is implemented for the generation of vortex beams under circularly polarized (CP) illumination. Two metasurfaces are designed, fabricated and experimentally characterized with a THz spectral imaging system for linearly polarized (LP) illumination. The transmitted field intensity distribution of y component is petal-shaped of gradually varied pieces with the frequency due to the complementary symmetric structure, indicating OAM state transition between a single vortex beam and superposition of two vortex beams. The measured spectral imaging distributions of amplitude and phase show good agreement with the simulation results. Such designs open a pathway for modulation of THz OAM states and bring more possibilities for flexible metasurfaces in a THz application.

Multichannel imaging for monitoring chemical composition and germination capacity of cowpea (Vigna unguiculata) seeds during development and maturation

This study aimed to set a computer-integrated multichannel spectral imaging system as a high-throughput phenotyping tool for the analysis of individual cowpea seeds harvested at different developmental stages. The changes in germination capacity and variations in moisture, protein and different sugars during twelve stages of seed development from 10 to 32 days after anthesis were non-destructively monitored. Multispectral data at 20 discrete wavelengths in the ultraviolet, visible and near infrared regions were extracted from individual seeds and then modelled using partial least squares regression and linear discriminant analysis (LDA) models. The developed multivariate models were accurate enough for monitoring all possible changes occurred in moisture, protein and sugar contents with coefficients of determination in prediction Rp2 of 0.93, 0.80 and 0.78 and root mean square errors in prediction (RMSEP) of 6.045%, 2.236% and 0.890%, respectively. The accuracy of PLS models in predicting individual sugars such as verbascose and stachyose was reasonable with Rp2 of 0.87 and 0.87 and RMSEP of 0.071% and 0.485%, respectively; but for the prediction of sucrose and raffinose the accuracy was relatively limited with Rp2 of 0.24 and 0.66 and RMSEP of 0.567% and 0.045%, respectively. The developed LDA model was robust in classifying the seeds based on their germination capacity with overall correct classification of 96.33% and 95.67% in the training and validation datasets, respectively. With these levels of accuracy, the proposed multichannel spectral imaging system designed for single seeds could be an effective choice as a rapid screening and non-destructive technique for identifying the ideal harvesting time of cowpea seeds based on their chemical composition and germination capacity. Moreover, the development of chemical images of the major constituents along with classification images confirmed the usefulness of the proposed technique as a non-destructive tool for estimating the concentrations and spatial distributions of moisture, protein and sugars during different developmental stages of cowpea seeds.

Spectral shift correction and adaptive band selection for multispectral imaging

Spectral imaging systems using liquid crystal tunable filters (LCTFs) can be applied to color reproduction. However, the spatially-varying spectral shift caused by inherent LCTF characteristics affects the results’ accuracy. This paper proposes a spectral shift correction method employing high-chroma color patches as calibration targets, which reduces the color difference from 3.67 to 1.98 (CIEDE2000). Additionally, a method is developed for adaptively selecting wavebands for image acquisition according to the target’s pre-estimated spectral reflectance, whereby color differences are reduced by 21.2% over the conventional method with a limitation of 15 spectral images acquired. These methods avoid LCTF’s drawbacks and exploit its tunability, making it more suitable for color-relevant applications.

Compressive circular polarization snapshot spectral imaging

A compressive sensing based circular polarization snapshot spectral imaging system is proposed in this paper to acquire two-dimensional spatial, one-dimensional circular polarization (the right and left circular polarization), and one-dimensional spectral information by one snapshot measurement, simultaneously. The dispersed component combined with a double Amici prism and a Wollaston prism was used to implement the spectral shifting along two orthogonal directions. Theoretically, the spectral resolution could be improved compared with the traditional coded aperture snapshot spectral imager proposed by Brady et al. The right and left circular polarization components are extracted by the assembly with an achromatic quarter wave-plate and a Wollaston prism. The encoding and reconstruction are illustrated comprehensively. The feasibility and fidelity are verified from the aspects of image and spectral reconstruction evaluations. It provides us an alternative approach for circular polarization spectral imaging for the dynamic objects in the fields such as defogging, underwater imaging, insects detection, and so on.

Development of a real-time spectral imaging system using in-site micro-LED-based illumination and high-speed micro-camera for endoscopic applications.

We designed a compact, real-time LED-based endoscopic imaging system for the detection of various diseases including cancer. In gastrointestinal applications, conventional endoscopy cannot reliably differentiate tumor from normal tissue. Current hyperspectral imaging systems are too slow to be used for real-time endoscopic applications. We are investigating real-time spectral imaging for different tissue types. Our objective is to develop a catheter for real-time hyperspectral gastrointestinal endoscopy. The endoscope uses multiple wavelengths within UV, visible, and IR light spectra generated by a micro-LED array. We capture images with a monochrome micro camera, which is cost-effective and smaller than the current hyperspectral imagers. A wireless transceiver sends the captured images to a workstation for further processing, such as tumor detection. The spatial resolution of the system is defined by camera resolution and the distance to the object, while the number of LEDs in the multi-wavelength light source determines the spectral resolution. To investigate the properties and the limitations of our high-speed spectral imaging approach, we designed a prototype system. We conducted two experiments to measure the optimal forward voltages and lighting duration of the LEDs. These factors affect the maximum feasible imaging rate and resolution. The lighting duration of each LED can be shorter than 10 ms while producing an image with a high signal-to-noise ratio and no illumination interference. These results support the idea of using a high-speed camera and an LED-array for real-time hyperspectral endoscopic imaging.

Application of spectral imaging for safety inspection of fresh cut vegetables

Food safety remains a critical issue and receiving increasing attention in both developed and developing countries. Fresh-cut vegetables may contain foreign objects such as small plastic pieces or biological contaminants (bugs and worms). In order to comply with requirements for product safety and maintain consumer confidence, there is a need for high-throughput, non-invasive technique for foreign objects detection in and real-time sorting of fresh-cuts. In this work, a liquid crystal tunable filter (LCTF) based spectral imaging system in the spectral range from 420 nm to 730 nm has developed and integrated with a conveyor unit. A software-interface was developed to collect the selected band images of moving samples and real time image processing for potential foreign materials detection on fresh cut vegetables. Spectral images throughout the whole spectral region were first collected for both fresh-cuts and foreign materials. The optimal wavebands for discriminating between foreign materials contaminated and sound fresh-cut were investigated. The optimal wavebands for each fresh-cut were selected and thus the developed system was used in multispectral imaging mode. Image processing algorithms were developed for detection of all potential foreign materials in fresh cut processing. The obtained results demonstrate that several kinds of foreign materials can be detected effectively despite being similar in color or nearly transparent. The ultimate advantage of a developed system is that it can be used either as in fluorescence mode or for collection of multispectral images in visible regions.

Spectral imaging and spectral LIDAR systems: moving toward compact nanophotonics-based sensing

Abstract With the emerging trend of big data and internet-of-things, sensors with compact size, low cost and robust performance are highly desirable. Spectral imaging and spectral LIDAR systems enable measurement of spectral and 3D information of the ambient environment. These systems have been widely applied in different areas including environmental monitoring, autonomous driving, biomedical imaging, biometric identification, archaeology and art conservation. In this review, modern applications of state-of-the-art spectral imaging and spectral LIDAR systems in the past decade have been summarized and presented. Furthermore, the progress in the development of compact spectral imaging and LIDAR sensing systems has also been reviewed. These systems are based on the nanophotonics technology. The most updated research works on subwavelength scale nanostructure-based functional devices for spectral imaging and optical frequency comb-based LIDAR sensing works have been reviewed. These compact systems will drive the translation of spectral imaging and LIDAR sensing from table-top toward portable solutions for consumer electronics applications. In addition, the future perspectives on nanophotonics-based spectral imaging and LIDAR sensing are also presented.

High-throughput terahertz spectral line imaging using an echelon mirror.

This work demonstrates terahertz (THz) line imaging that acquires broadband spectral information by combining echelon-based single-shot THz spectroscopy with high-sensitivity phase-offset electrooptic detection. An approximately 40 dB signal-to-noise ratio is obtained for a THz spectrum from a single line of the camera, with a detection bandwidth up to 2 THz at the peak electric-field strength of 1.2 kV/cm. The spatial resolution of the image is confirmed to be diffraction limited for each spectral component of the THz wave. We use the system to image sugar tablets by quickly scanning the sample, which illustrates the capacity of the proposed spectral line imaging system for high-throughput applications.

Spectral Imaging to Differentiate Contaminant Levels

Mercury (Hg) and Arsenic (As) have been recognized as chemical threats to human health. Still, the detection of lower contamination levels using traditional image analysis remains challenging due to the small number of available data samples and the insufficient utilization of the spatial information contained in the sensor pad images. To overcome this challenge, we use the spectra data of the colorimetric response pads and propose two kinds of classification models for differentiating contaminant levels with high test accuracy. In the first model, we use the SMOTE method to solve the imbalanced data problem, then apply the sequential forward selection algorithm to select optimal wavelength features in combination with the k-NN classifier to discriminate five contaminant levels. The second technique comprises principal component analysis (PCA) used as a dimensionality reduction technique combined with the random forest (RF) classifier to classify five contaminant levels. Our proposed system is trained and evaluated on a limited dataset of 126 spectral responses of five contamination levels. Our algorithms can yield 77% and 87% average accuracy, respectively. We will present an overview of the base model, the pipelines and the comparison of our proposed two classification models, and the phone-based narrow-band spectral imaging system that can obtain the camera spectral response for accurate and precise heavy metals analyses with the aid of narrow bandpass filters in front of a cell phone’s camera lens.

Research Progress and Applications of Spectral Imaging System on Chip

Research Progress and Applications of Spectral Imaging System on Chip

Optical design of MWIR spectral imaging system encoding in spectral dimension

Optical design of MWIR spectral imaging system encoding in spectral dimension

Two-photon fluorescence imaging of subsurface tissue structures with volume holographic microscopy.

.Significance: Two-photon (2P) fluorescence imaging can provide background-free high-contrast images from the scattering tissues. However, obtaining a multiplane image is not straightforward. We present a two-photon volume holographic imaging (2P-VHI) system for multiplane imaging.Aim: Our goal was to design and implement a 2P-VHI system that can provide the high-contrast optically sectioned images at multiple planes.Approach: A 2P-VHI system is presented that incorporates angularly multiplexed volume holographic gratings and a femtosecond laser source for fluorescence excitation for multiplane imaging. A volume hologram with multiplexed gratings provides multifocal observation, whereas nonlinear excitation using a femtosecond laser helps in significantly enhancing both depth resolution and contrast of images.Results: Standard fluorescent beads are used to demonstrate the imaging performance of the 2P-VHI system. Two-depth resolved optical-sectioning images of fluorescently labeled thick mice intestine samples were obtained. In addition, the optical sectioning capability of our system is measured and compared with that of a conventional VHI system.Conclusions: Results demonstrated that 2P excitation in VHI systems provided the optical sectioning ability that helps in reducing background noise in the images. Integration of nonlinear fluorescence excitation in the VHI provides some unique advantages to the system and has potential to design multidepth optical sectioned spatial–spectral imaging systems.

MDM-PCCT: Multiple Dynamic Modulations for High-Performance Spectral PCCT Imaging

Photon counting computed tomography (PCCT) has the ability to identify individual photons, resulting in quantitative material identification. Meanwhile, several technical challenges still exist in current PCCT imaging systems, including increased noise and suboptimal bin selection. These nonideal effects can substantially degrade the reconstruction performance and material estimation accuracy. To address these issues, in this work, we present a novel system for high-performance spectral PCCT imaging, which is a combination of multiple dynamic modulations, interpolation-based measurements processing strategy and advanced reconstruction method. For simplicity, this new PCCT imaging system is referred to as "MDM-PCCT". Specifically, the multiple dynamic modulations consist of dynamic kVp modulation, dynamic spectrum modulation and dynamic energy threshold modulation. In the dynamic kVp modulation, three kVp values, i.e., 80, 110 and 140, are included, and the tube voltage waveform follows a sinusoidal curve which is more practical than the rectangular curve in the fast kV switching mode. In the dynamic spectrum modulation, the X-ray spectra are processed by selective spatial-spectral filters to balance the X-ray fluxes and increase the spectral separation. In the dynamic energy threshold modulation, the energy threshold is adaptively changed to determine the optimal bin selection. Furthermore, we propose an energy threshold determination method and interpolation-based measurements processing strategy to address the issue of non-uniform and sparse-view PCCT measurements, respectively. In addition, by considering the intrinsic characteristics of the MDM-PCCT images, we utilize an enhanced total variation regularized model for images reconstruction. Finally, numerical and preclinical studies demonstrate that the presented MDM-PCCT imaging system is capable of yielding uniform and high-fidelity PCCT measurements with noise consistency, and the presented reconstruction method further improves the image quality and material decomposition accuracy.

On the application of multi-parametric optical phenotyping of bacterial colonies for multipurpose microbiological diagnostics

The development of new diagnostics techniques and modalities is critical for early detection of microbial contamination. In this study, the novel integrated system for multi-parametric optical phenotyping and characterization of bacterial colonies, is presented. The system combines Mach-Zehnder interferometer with a spectral imaging system for capturing multispectral diffraction patterns and multispectral two-dimensional transmission maps of bacterial colonies, along with the simultaneous interferometric profilometry. The herein presented investigation was carried out on five representative bacteria species and nearly 3000 registered multispectral optical signatures. The interferograms were analyzed by four-step phase shift algorithm to reconstruct the colony profile to enable the obtaining of the comparable optical signatures. The dedicated image processing algorithms were used for extraction of quantitative features of these signatures. The random forest algorithm was applied for selection of the most predictive set of features, which were used in classification model based on Support-Vector Machine. Obtained results have shown that the use of multiple multispectral optical signatures provide a multi-parametric bacteria identification at an exceptionally high accuracy (99.4-100%), significantly better than in case of classification based on each of these signatures (multispectral diffraction patterns, two-dimensional transmission coefficient maps), separately. Obtained results revealed that analysis of multispectral signatures can also be applied for characterisation of physical, physicochemical and chemical properties of the bacterial colonies in the presence of the antimicrobial factors. Therefore, the proposed label-free, non-destructive optical technique has perspectives to be exploited in the multipurpose diagnostics and it can be used as a pre-screening tool in microbiological laboratories.

Hyper Spectral Image Characteristics of Aronia Melanocarpa Leaves under Saline Alkali Stress

<p>Objective: To obtain the characteristics analysis results of aronia melanocarpa leaves under saline alkali stress state and improve yield and quality of aronia melanocarpa. Methods: Using hyper spectral imaging system to obtain hyper spectral images of aronia melanocarpa leaves under saline alkali stress. The clear hyper spectral image is obtained by the conversion of reflectance, spectral envelope removal, and spectral denoising, and final hyper spectral image is obtained by the normalization of a clearer hyper spectral image. Results: Spectral information of aronia melanocarpa leaves under slight saline alkali stress: in the visible band, leaf reflectance is lower than leaf nutrient rich stress condition; in the near infrared band, the nutrient rich leaves was significantly higher than that of stress leaves. Spectral information of leaves under moderate saline alkali stress: spectral reflectance of different lesion spots for a same leaf in 550-680nm band is: severe > moderate > slight > normal, in the near infrared band is on the contrary; spectral reflectance for different lesion grades in 550-680 nm, severe lesion leaves have highest reflectance, and normal leaves have the lowest reflectance. Under severe saline alkali stress, the leaf spectral information is: there is no significant difference in leaf spectral reflectance between the attachment aphid and the damaged leaf, but the difference is obvious for normal leaves in the band of 450-500 nm, 560-680 nm and 750-900 nm. Comparative results analysis for the three of saline alkali stress degree is: the near infrared band of 560-680 nm and the visible band of 780-900 nm is the sensitive band for the diagnosis of three kinds of stress; slight saline alkali stress has the most significant differences at 550 nm, and 780-900 nm; severe saline alkali stress has at 680 nm and 780-900 nm. Conclusion: The proposed method can analyze hyper spectral image characteristics of aronia melanocarpa leaves under different saline alkali stress the condition of is a kind of plant leaves, which is an efficient method for the analysis of characteristics of plant leaves under saline alkali stress.</p>

Multichannel nanoplasmonic platform for imidacloprid and fipronil residues rapid screen detection

In recent years, imidacloprid and fipronil have been reported to harm beneficial insects, such as honey bees, and to potentially pose risks to mammals, including humans. Considering their widespread use and potential minimum toxic range from 10 ppb to 1 ppm (species dependent), a simple, rapid, sensitive, and reliable method for screening and detection is urgently needed. Here, we present a surface plasmon resonance (SPR)-based nanoplasmonic chip integrated with a multichannel spectral imaging system to detect ecosystem-harming pesticides. The pre-modification of the designed mercapto-haptens reduced detection time to 2.5 h. Moreover, owing to the multichannel configuration, it was possible to introduce an internal standard analytical method to effectively reduce matrix interference in real samples; thus, the concentration of the target pesticide could be determined more precisely. The strong linearity of the spiked sample test results indicated high accuracy in quantifying target pesticides. Considering the limit of detection (~10 ppb), the cutoffs for detection and quantification were set at 15 and 45 ppb, respectively, and were used as the detection criteria. The detection results of the blind tests of real samples were also compared with those of liquid chromatography electrospray ionization tandem mass spectrometry (standard method) and were highly consistent. The custom-made integrated SPR system allows much simpler, label-free, high-throughput, and reliable on-site identification and quantification of imidacloprid and fipronil. All test results validated the platform's capability in the on-site rapid screening and detection of pesticide residues at the parts per billion and parts per million levels.

Design of binary-phase diffusers for a compressed sensing snapshot spectral imaging system with two cameras.

We propose designs of pupil-domain optical diffusers for a snapshot spectral imaging system using binary-phase encoding. The suggested designs enable the creation of point-spread functions with defined optical response, having profiles that are dependent on incident wavefront wavelength. This efficient combination of dispersive and diffusive optical responses enables us to perform snapshot spectral imaging using compressed sensing algorithms while keeping a high optical throughput alongside a simple fabrication process. Experimental results are reported.

A High Optical Throughput Spectral Imaging Technique Using Broadband Filters.

To address the miniaturization of the spectral imaging system required by a mounted platform and to overcome the low luminous flux caused by current spectroscopic technology, we propose a method for the multichannel measurement of spectra using a broadband filter in this work. The broadband filter is placed in front of a lens, and the spectral absorption characteristics of the broadband filter are used to achieve the modulation of the incident spectrum of the detection target and to establish a mathematical model for the detection of the target. The spectral and spatial information of the target can be obtained by acquiring data using a push-broom method and reconstructing the spectrum using the GCV-based Tikhonov regularization algorithm. In this work, we compare the accuracy of the reconstructed spectra using the least-squares method and the Tikhonov algorithm based on the L-curve. The effect of errors in the spectral modulation function on the accuracy of the reconstructed spectra is analyzed. We also analyze the effect of the number of overdetermined equations on the accuracy of the reconstructed spectra and consider the effect of detector noise on the spectral recovery. A comparison between the known data cubes and our simulation results shows that the spectral image quality based on broadband filter reduction is better, which validates the feasibility of the method. The proposed method of combining broadband filter-based spectroscopy with a panchromatic imaging process for measurement modulation rather than spectroscopic modulation provides a new approach to spectral imaging.