Liquid Crystal Tunable Filter Research Articles

Multispectral integral imaging acquisition and processing using a monochrome camera and a liquid crystal tunable filter

This paper presents an acquisition system and a procedure to capture 3D scenes in different spectral bands. The acquisition system is formed by a monochrome camera, and a Liquid Crystal Tunable Filter (LCTF) that allows to acquire images at different spectral bands in the [480, 680]nm wavelength interval. The Synthetic Aperture Integral Imaging acquisition technique is used to obtain the elemental images for each wavelength. These elemental images are used to computationally obtain the reconstruction planes of the 3D scene at different depth planes. The 3D profile of the acquired scene is also obtained using a minimization of the variance of the contribution of the elemental images at each image pixel. Experimental results show the viability to recover the 3D multispectral information of the scene. Integration of 3D and multispectral information could have important benefits in different areas, including skin cancer detection, remote sensing and pattern recognition, among others.

Classification of visible point sources using hyperspectral chromotomosynthetic imagery

A hyperspectral chromotomosynthetic imaging (CTI) system is used to detect and classify a collection of 21 scattered, spectrally diverse point-like sources. The instrument operates in the visible to near IR (400 to 800 nm) and has the potential to collect spectral imagery at great than 10 Hz. A two-dimensional wideband spatial image of the target scene is used to detect and spatially characterize the targets leading to optimization of the three-dimensional (3-D) spatial/spectral reconstruction of the hyperspectral image cube. The instrument is assessed by directly comparing results to spatial data collected by a wideband image and hyperspectral data collected using a liquid crystal tunable filter (LCTF). Target classification using k-means clustering of observed spectra yielded 5 to 6 target classes for each methodology, indicating information obtained using CTI was similar to that collected by the LCTF. The wide-band spatial content of the scene reconstructed from the CTI data is of same or better quality in terms of background noise and target intensities as a single frame collected by the undispersed imaging system with projections taken at every 1 deg. Performance is dependent on the number of projections used, with projections at 5 deg producing adequate results in terms of target characterization. The CTI has 2 to 4 times the spectral resolution of the LCTF. The data collected by the CTI system can simultaneously provide spatial information of equal quality as a the bandpass imaging system, provide high-frame rate slitless one-dimensional spectra, and generate 3-D hyperspectral imagery which can be exploited to provide the same results as a traditional multiband spectral imaging system.

Hyperspectral image reconstruction of an array of extended targets using chromotomosynthesis

The ability of a chromotomosynthetic imaging (CTI) system to accurately represent spatial and spectral characteristics of a spectrally diverse set of extended sources was evaluated. The goal is to evaluate CTI hyperspectral imagery for the remote classification of widely separated combustion events against a dark background. Accuracy is determined by comparing results to those of a liquid crystal tunable filter (LCTF). The reconstructed CTI two-dimensional (2-D) image was equivalent to a wideband, undispersed image for images reconstructed using 1 deg projection separation. The Colsher discrepancy metric degrades from 0.44 for the 1 deg sampling to 0.72 for a 10 deg sampling, with 5 deg sampling still equivalent to the image obtained by integrating the spectral frames of the LCTF. Hyperspectral data cubes are reconstructed with image quality dependent primarily on source intensity, not wavelength. Through nearly the entire range, narrower spectral lineshapes are attained from CTI data with little degradation in results from the 1 deg, 5 deg, and 10 deg sampled projections. The measured spectral locations were in agreement between the CTI and LCTF for most elements in the mosaic. Spectral and spatial artifacts are observed in the CTI hyperspectral imagery particularly for dim objects adjacent to bright and spectrally similar sources. As a scene becomes more continuous with spatially and spectrally similar sources, more complex processing will be necessary to avoid degraded performance.

Pseudoisochromatic test plate colour representation dependence on printing technology

The aim of the study is to determine best printing technology for creation of colour vision deficiency tests. Valid tests for protanopia and deuteranopia were created from perceived colour matching experiments from printed colour samples by colour deficient individuals. Calibrated EpsonStylus Pro 7800 printer for ink prints and Noritsu HD 3701 digital printer for photographic prints were used. Multispectral imagery (by tunable liquid crystal filters system CRI Nuance Vis 07) data analysis show that in case of ink prints, the measured pixel colour coordinate dispersion (in the CIExy colour diagram) of similar colour arrays is smaller than in case of photographic printing. The print quality in terms of colour coordinate dispersion for printing methods used is much higher than in case of commercially available colour vision deficiency tests.

Multi-spectral video endoscopy system for the detection of cancerous tissue

Multi-spectral video endoscopy provides considerable potential for early stage cancer detection. Previous multi-spectral image acquisition systems were of limited use for endoscopy due to (i) the necessary spatial scanning of push-broom approaches or (ii) the impractical long switching times of liquid crystal tunable filters. Recent technological advances in the field of tuneable filters, in particular fast acousto-optical tunable filters (AOTF), make switching times below 1ms feasible. Thus, AOTFs represent a suitable technology for the acquisition of hyper-spectral image and multi-spectral video data with excellent spatial and temporal resolution. In this paper, we propose a hyper-spectral imaging endoscope using a fast AOTF synchronized with a highly sensitive EMCCD camera for the detection of cancerous tissue. The setup demonstrates that the acquisition of hyper-spectral image and multi-spectral video data is feasible and enables the augmentation of endoscopic videos with overlays indicating cancerous tissue regions.Using hyper-spectral measurements from biopsies acquired with the setup in a clinical environment it is shown that the spectral characteristic of cancerous regions is tissue dependent. Even a sophisticated classifier such as a Support Vector Machines (SVM) or a Mixture of Gaussian Classifier (MOGC) cannot generalize the discriminative information if the training set contains measurements from different tissue types (e.g. larynx vs. parotid). In contrast, a training data selection scheme that chooses similar training sets for a given test set achieves a better prediction accuracy using an approach based on a Quadratic Discriminant Classifier (QDC) with the important advantage of improved robustness and less liability to overtraining. Combined with an image registration removing motion-based acquisition artefacts, the spectral information allows the augmentation of the video stream with overlays indicating cancerous tissue regions.

Abstract 4292: In vivo and in vitro imaging of Cyclooxygenase-2 in tumor

Abstract Cyclooxygenase-2 (Cox-2) is a biomarker for tumor progression and inflammatory diseases. It is known that normal tissues express minimal amounts of Cox-2 proteins while inflammatory and malignant tumor cells express high levels. Thus, inhibition of Cox-2 is a strategy in the treatment of tumor progression and inflammatory diseases. Recently, many different kinds of molecular imaging reagents for Cox-2 have been developed. Among them, an indomethacin-based Cox-2 probe, fluorocoxib, has been shown to bind to Cox-2 in vitro and in vivo. The binding specificity to Cox-2 was shown using cell culture and tumor xenograph models. In our present study, in order to optimize fluorocoxib imaging conditions, we utilized HCT116 and HT29 cells as low and high Cox-2 expressers, respectively. First, cultured cells were grown overnight on cover slips. Cells were then fixed with paraformaldehyde and incubated with fluorocoxib probe. Multispectral images were taken using a camera equipped with liquid crystal tunable filters. Spectral unmixing was applied to these images to enhance fluorocoxib signals. Our results indicate that HCT116 cells express fair amounts of Cox-2 protein. We also transplanted HCT116 and HT29 cells into the flank regions of nu/nu mice subcutaneously. After tumors grew to sufficient size (∼100 mm3) we injected fluorocoxib intravenously and monitored probe binding to the tumors. Whole animal in vivo imaging indicated that fluorocoxib binds to HT29 tumors after 3 hours. All the probes were eliminated from the animals by 24 hours. On the other hand, HCT116 tumors did not show strong binding of fluorocoxib as expected. However, HCT116 tumors did show some weak binding at the 3 and 6 hr time points. Once there was no longer any probe detected in vivo, we dissected the primary tumors from the HCT116 and HT29 implantations. Tissue sections were made with paraffin embedded tumors. After deparaffinization, tissue sections were incubated with fluorocoxib and fluorescent images were taken using multispectral imaging technology. Spectral unmixing data confirmed that HCT116 tumor sections showed fair amounts of Cox-2 probe expression. The amount of binding was also quantitated using a corresponding software package. Our results demonstrated that fluorescent biomarker probes can be imaged in vitro and in vivo non-invasively. Furthermore, we showed that the expression level can be quantitated using spectral unmixing imaging technology. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4292. doi:1538-7445.AM2012-4292

Development of software for spectral imaging data acquisition using LabVIEW

Developing data acquisition software is a major challenge in integrating a spectral imaging system. This paper presents the design and implementation of a data acquisition program using LabVIEW for a liquid crystal tunable filter based spectral imaging system (900–1700nm). The module-based program was designed in a three-tier structure. The image acquisition process, modelled by a finite state machine, was implemented in LabVIEW to control the spectral imaging system to collect hyperspectral or multispectral images. The collected spectral images were encoded in general format and could be further processed by other common spectral image analysis tools. In addition, the program could be used to observe band ratio images of the test object in real-time, collect spectral images after ensemble averaging, and select region of interest for spectral image acquisitions. This program is a useful data acquisition tool for the filter-based spectral imaging system. The design and implementation techniques described in this article could also be used to develop similar spectral image acquisition programs.

The Study on Spectral Imaging Technique in Detection of Dermatosis

Dermatosis, a kind of common and frequent disease, is harmful to health and risk against the lives of human beings. The detection of dermatosis is of great significance, especially to the early diagnosis of skin disease. Nowadays, the studies of spectral imaging technology in the diagnosis of skin disease are attracting more and more attentions. In this paper, we firstly review some literary products related to dermatosis detection and then make a comparison among fluorescence spectrum method, filter wheel method, liquid crystal tunable filter (LCTF) method and mosaic multispectral imaging technology. Finally we propose that the smaller, lighter and more cost-effective equipment should be the trend.

Thermocapillary flow on a gas‐liquid surface under normal and reduced gravity

AbstractThe aim of the present work is the study of the thermocapillary flow phenomenon at 1‐g, μ‐g conditions and the transition between 1‐g and μ‐g. The experimental results were observed with a novel high speed imaging system using a combination of a monochrome high speed CCD camera and a liquid crystal tunable filter. The flow velocity and the temperature patterns could be measured simultaneously from the seeded thermochromic liquid crystals, which change colour depending on their temperature. We investigated the thermocapillary flow with an experimental setup, where a bubble is positioned in a liquid matrix under a heated wall. The µ‐g experiments were conducted in the drop tower of the ZARM‐Institute of the University of Bremen. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Raman studies of Japanese art objects by a portable Raman spectrometer using liquid crystal tunable filters

Raman spectroscopic studies of a few Japanese art objects have been performed by using a portable Raman spectrometer constructed with liquid crystal tunable filters as dispersive elements. Interesting information has been obtained from the Raman spectra observed from ukiyo‐e's (Japanese woodblock prints) and their woodblocks. The performance data and the imaging capabilities of the constructed spectrometer are presented. Copyright © 2011 John Wiley & Sons, Ltd.

A liquid crystal tunable filter based shortwave infrared spectral imaging system: Calibration and characterization

Calibration is a critical step for developing spectral imaging systems. This paper presents a systematic calibration and characterization approach for a liquid crystal tunable filter (LCTF) based shortwave infrared (SWIR) spectral imaging system. A series of tests were conducted to validate the linearity of the system output, measure the field of view of the spectral imager, increase the system spectral sensitivity, test the spatial and spectral resolution of the system, evaluate the system stability and image distortion, and reduce the spectral noise of the system output. Results showed that the system had an angle of view of 6.98° and a spatial resolution of 158 μm. The spectral sensitivity of the system was corrected by controlling the camera exposure time and gain, which increased the signal to noise ratio of the system by 16.5%. Test results also verified the system spectral accuracy and linearity ( r > 0.999). The system output was proven to be stable and image distortion was not perceivable. Results of calibration tests indicated that this system satisfied the design criteria in both spatial and spectral domains. The calibration methods presented here are applicable to the LCTF-based spectral imaging systems in other applications.

Stand-off detection of explosives particles by multispectral imaging Raman spectroscopy

A Raman multispectral imaging technique is presented, which can be used for stand-off detection of single explosives particles. A frequency-doubled Nd:YAG laser operating at 10 Hz illuminates the surface under investigation. The backscattered Raman signal is collected by a receiver subsystem consisting of a 150 mm Schmidt-Cassegrain telescope, a laser line edge filter, a liquid-crystal tunable filter, and a gated intensified charge-coupled device (ICCD) detector. A sequence of images is recorded by the ICCD, where, for each recording, a different wavelength is selected by the tunable filter. By this, a Raman spectrum is recorded for each pixel, which makes it possible to detect even single particles when compared to known spectra for possible explosives. The comparison is made using correlation and least-square fitting. The system is relatively insensitive to environment and light variations. Multispectral Raman images of sulfur, ammonium nitrate, 2,4-dinitrotoluene, and 2,4,6-trinitrotoluene were acquired at a stand-off distance of 10 m. Detection of sulfur particles was done at a distance of 10 m.

A liquid crystal tunable filter based shortwave infrared spectral imaging system: Design and integration

This paper presents the methodology to design and integrate a liquid crystal tunable filter (LCTF) based shortwave infrared (SWIR) spectral imaging system. The system consisted of an LCTF-based SWIR spectral imager, an illumination unit, a frame grabber, and a computer with the data acquisition software. The spectral imager included an InGaAs camera (320 × 256 pixels), an SWIR lens (50 mm, F/1.4), and an LCTF (20 mm aperture). Four multifaceted reflector halogen lamps (35 W, 12 VDC) were used to build the illumination unit. The system was integrated by a LabVIEW program for data acquisition. It can capture hyperspectral or multispectral images of the test object in the spectral range of 900–1700 nm. The system was validated by differentiating sugar from wheat flour, and water from 95% ethanol. The results showed that the system can distinguish these materials in both spectral and spatial domains. This SWIR spectral imaging system could be a potential useful tool for nondestructive inspection of food quality and safety.

Assessing the wavelength dependence of intraocular scattering by a new optical approach

AbstractPurpose The wavelength dependence of the light scattering is related with the characteristics of the inhomogeneities in the ocular media, and therefore, an objective assessment could be used to the early detection of underlying ocular pathologies. We developed a new optical instrument for measuring light scattered in the human eye at different wavelengths.Methods Using a double‐pass configuration, series of disks of uniform radiance with an increasing angular dimension are sequentially projected on to the retina. As a light source, a white light halogen lamp is spectrally filtered by using a liquid crystal tunable filter, selecting three different wavelengths: 550, 600 and 650 nm (FWHM = ± 50 nm). The disk’s retinal images are recorded by a cooled electron‐multiplied CCD camera. The derivative of intensity at the center of each disk’s retinal image with respect to its radius provides the eye’s wide‐angle double‐pass point‐spread function (PSF). The method was applied in an artificial eye with different levels of induced scattering, and in a group of normal subjects.Results In the artificial eye, the increasing scattering induced is successfully discriminated at the more eccentric angular domain of the wide‐angle PSFs. In the group of subjects, the estimated wide‐angle PSFs differ from the lower to the larger wavelengths. The PSF at 550 nm is characterized by more intense light scatter in the angular range between 5 and 10 degrees, while the PSF at the 650nm showed a relatively increased scattered light between 0 and 2 degrees. This would be related to the deeper interaction of thConclusion An optical objective method is sensitive enough to detect consistently differences in light scatter for different wavelengths.

Anomaly detection with varied ground sample distance utilizing spectropolarimetric imagery collected using a liquid crystal tunable filter

Liquid crystal tunable filters (LCTFs) are a technology that can act as both a spectral and linear polarization filter for an imaging device. Paired with the appropriate hardware, a LCTF can be configured to collect hyperspectral Stokes imagery which contains both spectral as well as polarimetric information on a per-pixel level basis. This data is used to investigate the utility of spectro-polarimetric data with standard spectral analysis algorithms, in this case anomaly detection. A method to simulate different ground sample distances (GSDs) is used to illustrate the effect on algorithm performance. In this paper, a spectro-polarimetric imager is presented that can collect spectro-polarimetric image cubes in units of calibrated sensor reaching radiance. The system is used to collect imagery of two scenes, each containing die-cast scale vehicles and different background types. An anomaly detector is applied to the intensity and polarized image cubes to find those pixels that are different from the background spectrally and/or polarimetrically. The effect of changing the apparent GSD on the anomaly detection performance is explored. This shows that applying anomaly detection to spectro-polarimetric data can improve the false alarm rate over standard spectral data for finding certain types of man-made objects in complex backgrounds.

Demonstration of a single-wavelength spectral-imaging-based Thai jasmine rice identification

A single-wavelength spectral-imaging-based Thai jasmine rice breed identification is demonstrated. Our nondestructive identification approach relies on a combination of fluorescent imaging and simple image processing techniques. Especially, we apply simple image thresholding, blob filtering, and image subtracting processes to either a 545 or a 575 nm image in order to identify our desired Thai jasmine rice breed from others. Other key advantages include no waste product and fast identification time. In our demonstration, UVC light is used as our exciting light, a liquid crystal tunable optical filter is used as our wavelength seclector, and a digital camera with 640 active pixels × 480 active pixels is used to capture the desired spectral image. Eight Thai rice breeds having similar size and shape are tested. Our experimental proof of concept shows that by suitably applying image thresholding, blob filtering, and image subtracting processes to the selected fluorescent image, the Thai jasmine rice breed can be identified with measured false acceptance rates of <22.9% and <25.7% for spectral images at 545 and 575 nm wavelengths, respectively. A measured fast identification time is 25 ms, showing high potential for real-time applications.

Fluorescence microspectroscopy as a tool to study mechanism of nanoparticles delivery into living cancer cells.

Lack of better understanding of nanoparticles targeted delivery into cancer cells calls for advanced optical microscopy methodologies. Here we present a development of fluorescence microspectroscopy (spectral imaging) based on a white light spinning disk confocal microscope with emission wavelength selection by a liquid crystal tunable filter. Spectral contrasting of images was used to localize polymer nanoparticles and cell membranes labeled with fluorophores that have substantially overlapping spectra. In addition, fluorescence microspectroscopy enabled spatially-resolved detection of small but significant effects of local molecular environment on the properties of environment-sensitive fluorescent probe. The observed spectral shift suggests that the delivery of suitably composed cancerostatic alkylphospholipid nanoparticles into living cancer cells might rely on the fusion with plasma cell membrane.

Simultaneous monitoring of intracellular pH changes and hemodynamic response during cortical spreading depression by fluorescence-corrected multimodal optical imaging

Cortical spreading depression (CSD) plays an important role in trauma, migraine and ischemia. CSD could induce pronounced hemodynamic changes and the disturbance of pH homeostasis which has been postulated to contribute to cell death following ischemia. In this study, we described a fluorescence-corrected multimodal optical imaging system to simultaneously monitor CSD associated intracellular pH (pH(i)) changes and hemodynamic response including hemoglobin concentrations and cerebral blood flow (CBF). CSD was elicited by application of KCl on rat cortex and direct current (DC) potential was recorded as a typical characteristic of CSD. The pH(i) shift was mapped by neutral red (NR) fluorescence which was excited at 516-556 nm and emitted at 625 nm. The changes in hemoglobin concentrations were determined by dual-wavelength optical intrinsic signal imaging (OISI) at 550 nm and 625 nm. Integration of fluorescence imaging and dual-wavelength OISI was achieved by a time-sharing camera equipped with a liquid crystal tunable filter (LCTF). CBF was visualized by laser speckle contrast imaging (LSCI) through a separate camera. Besides, based on the dual-wavelength optical intrinsic signals (OISs) obtained from our system, NR fluorescence was corrected according to our method of fluorescence correction. We found that a transient intracellular acidification followed by a small alkalization occurred during CSD. After CSD, there was a prolonged intracellular acidification and the recovery of pH(i) from CSD took much longer time than those of hemodynamic response. Our results suggested that the new multimodal optical imaging system had the potential to advance our knowledge of CSD and might work as a useful tool to exploit neurovascular coupling under physiological and pathological conditions.

Non-display bio-optic applications of liquid crystals

Liquid crystal (LC) devices have been largely developed for LC displays and in the last decade for optical telecommunications; however, their application in the field of optical imaging is just starting to emerge. They can be miniaturised and thus have a great potential to be used with miniature optical imaging systems for biomedical applications. LC devices specifically designed for integration into biomedical optical imaging systems are reviewed. Using a combination of a few LC retarders, spectropolarimetric imaging of tissue can be obtained. LC tunable filters with high dynamic range and large throughput are designed for hyperspectral imaging and for spectral domain optical coherence tomography. The designs are based on several concepts, both using the classical stack of retarders and using more modern designs based on single layer in a Fabry–Perot cavity, in a resonant waveguide structure or in a photonic crystal.

Obtaining absorption spectra from single textile fibers using a liquid crystal tunable filter microspectrophotometer

Visible absorption spectra were recorded for single textile fibers using a microspectrophotometer based on a liquid crystal tunable filter. Spectra compared well with results from a conventional instrument. Some advantages include very fast and simple sample preparation and easy comparison of multiple fibers at the same time. Advantages over extraction-dependent methods include the fact that it is applicable to extremely small sample size, not susceptible to artifacts induced by variable extraction efficiencies, non-destructive, and much easier. Because an immense amount of information is collected in one experiment, good signal averaging is possible, along with multiple comparisons for each data set. The addition of a camera, computer, and liquid crystal tunable filter can transform a standard microscope into a microspectrophotometer capable of performing similar work.