Hyperspectral analysis of solar rays reflected from the Earth’s surface is expected to play an important role in future Earth observation. Two imaging liquid crystal tunable filter (LCTF) spectropolarimeters for the visible and near-infrared wavelength bands have been developed by NAL over the past several years for such analysis. In order to realize the practical application of these optical sensors, efforts are currently under way to develop them into sensor packages for airborne observation systems. This paper first presents the concept and architecture of an optical observation system using an LCTF spectropolarimeter which is sensitive to radiation in the 650-1100 nm near-infrared wavelength band, along with its construction. The results of a farm observation conducted using a visible wavelength LCTF imaging spectropolarimeter are then presented by the spectral images of the observed areas as an example of a preliminary application to agro-environmental sciences. The results of a second farm observation conducted using a near-infrared LCTF imaging spectropolarimeter are presented by spectral images of an observed crop specimen, and radiances of solar rays reflected from the specimen are also shown. Finally, the applicability of the LCTF spectropolarimeter to agriculture observation is summarized based on the results of these agricultural observations.
The National Aerospace Laboratory (NAL) has developed a new type of imaging spectropolarimeter that uses a liquid crystal tunable filter (LCTF) which makes it possible to measure the optical properties of solar rays reflected from land and sea surfaces. The aim of this development is to pave the way for the establishment of polarimetric analysis of solar rays reflected from the Earth’s surface as a method of Earth environment observation. Two imaging LCTF spectropolarimeters that cover different wavelength bands have been developed: a visible light sensor for the 400-720 nm wavelength band, and a near-infrared sensor that covers the 650-1100 nm band. Efforts are now under way to apply these optical sensors to practical applications, for airborne and ultimately spaceborn Earth environment remote sensing. This paper first outlines the imaging optical sensors, including their operational principles and construction. Next, various spectral images acquired using the visible light optical sensor in outdoor field and flight evaluation experiments to measure spectral characteristics of solar rays reflected from land and water surfaces are shown. Then, the results of outdoor experiments conducted using the near-infrared optical sensor are shown, including the analyzed relative radiance of solar rays reflected from observed spots, and spectral images acquired at various wavelengths and polarization angles. These experimental results demonstrate clearly that solar rays reflected from targets with differing characteristics have different spectropolarimetric properties. Finally it is concluded that the way has been paved for determining surface conditions from the properties of the spectral images acquired by LCTF spectropolarimeters at wavelengths of 400-1100 nm.
Polarimetric analysis of solar rays reflected from the Earth's surface is expected to play an important role in future Earth environment observation. Research on an imaging spectropolarimeter using a liquid crystal tunable filter (LCTF), which is able to measure the polarimetric properties at selected wavelengths of solar rays reflected from land or water surfaces, has been conducted over the past five years at NAL for such analysis. Efforts are now under way to put this sensor to practical use, for airborne and ultimately space-based Earth environment remote sensing.
This paper first presents the principle and construction of an LCTF spectropolarimeter which senses radiation in the 400-720 nm wavelength band. Next, an outline of an onboard observation system that incorporates an LCTF spectropolarimeter and its performance characteristics obtained by laboratory tests are presented. Third, the apparatus and procedures for the field experiment using such observation system are described, and the area for the field experiments is shown. Spectral characteristics of solar rays reflected from the observed spots are then shown by relative radiance as the analyzed results of experimental data and spectral images at various wavelengths and polarization angles are also shown as further analyzed results. It is made clear from the experimental results that solar rays reflected from targets with differing characteristics have different spectropolarimetric properties. Moreover, the result of the flight experiment conducted preliminarily to confirm the operational functions of the observation system in a flight environment is shown. Finally it is concluded that the way has been paved for determining the surface conditions from the properties of the images acquired by the LCTF spectropolarimeter.
There is an emerging demand for remote sensing technologies that can determine the surface characteristics of objects from the properties of reflected light. In particular, hyperspectral analysis of solar rays reflected from the Earth's surface is expected to play an increasingly important role in Earth environment observation. The National Aerospace Laboratory (NAL) has developed a new type of imaging spectropolarimeter for such analysis that uses a liquid crystal tunable filter (LCTF), and efforts are now under way to develop it into a practical aircraft or spacecraft on-board sensor system for Earth environment sensing. This paper first presents the concept and architecture of an Earth observation system using an LCTF optical sensor which can sense radiation in the 400-720 nm wavelength band. The results of laboratory experiments to evaluate the performance characteristics of the observation system, e.g. hyperspectral resolution, optional selection of the plane of polarization, etc. are then presented, and the results of preliminary image acquisition experiments that demonstrate the feasibility of acquiring of spectral images is also shown. Finally, the applicability of the LCTF spectropolarimeter to Earth observation is summarized based on the results of the laboratory and field evaluation experiments.
The National Aerospace Laboratory of Japan has developed a new type of optical sensor, an imaging spectropolarimeter which uses a liquid crystal tunable filter, for airborne and satellite-based remote sensing of the Earth's environment. Ground-based field experiments conducted as a preliminary to flight evaluations have demonstrated the feasibility of acquiring spectral images of objects irradiated by solar rays, and confirmed that solar rays reflected from different targets have characteristic spectropolarimetric properties. This paper first presents an outline of the developed spectropolarimeter. Next, the apparatus and procedures for the field experiments are described. The spectropolarimetric characteristics of solar rays reflected from a range of targets are then shown by relative radiance as the results of analyzed experimental data, and spectral images acquired at various wavelengths and polarization angles are shown. Plans to evaluate the sensor in a flight environment are described. Possible applications of the optical sensor are also introduced: observation of water quality deterioration in brackish lakes, applications to agro-environmental science, and applicability to a fish-finding system. Finally, it is concluded that results of the field experiments demonstrate that the way has been paved for determining surface characteristics from the optical sensor output.
A spectroscopic camera has been developed which has spectral resolution of less than 1.5nm in the ultraviolet (UV) and visible wavelength bands (320-580 nm). Its main components are a specially coated UV objective lens, a UV Acousto-Optic Tunable Filter (AOTF) with a thermo-electric cooling system, and a imaging system based on a high-gain avalanche rushing amorphous photoconductor (HARP) developed by NHK Science and Technical Research Laboratories. Research is currently under way to develop the hyperspectral camera into a sensor package for airborne and ultimately space-based remote sensing applications. This paper presents the basic principle and configuration of the hyperspectral camera, and gives details of tests to measure its performance. The results of spectral resolution tests analyzing very close two spectra from a helium-discharge lamp demonstrate the camera's high spectral resolution performance. Full color and spectral images obtained by a spectrometry experiment are also presented to demonstrate the camera's hyperspectral capabilities.
Various methods, techniques and sensors for Earth observation are being developed worldwide as the necessity of protecting the Earth's environment increases. In particular, polarimetric analysis of solar rays reflected from the Earth's surface is expected to play an important role in future Earth environment observation. A new type of spectro-polarimeter based on a liquid crystal tunable filter (LCTF) has been developed at NAL for such analysis. Efforts are now under way to put this sensor to practical use in airborne or satellite-based remote sensing of the Earth's environment by developing a sensor package and onboard observation system based around it. This paper first presents the operational principle and construction of the LCTF spectro-polarimeter, which captures images in the 400 - 720 nm wavelength band. Next, an outline of an onboard observation system incorporating the spectro-polarimeter is described and its applicability to airborne remote sensing discussed. The performance of the observation system is then shown based on experimental results. Other possible applications of the sensor are presented, and finally, the results of the evaluation of the observation system, e.g. hyper-spectral resolution of less than 10 nm, are summarized in the conclusion.
Spatial resolution of CCD imaging spectrometer is deteriorated by cross talk between the elements. Some examples of such a cross talk of CCD elements is shown, and the deteriorated spatial resolution is analyzed. Eye system of some biological things utilizes such a cross talk positively to improve the performance. How to improve the performance? The function is called lateral inhibition. The concept and function of the lateral inhibition is applied to improve the deteriorated performance of CCD imaging sensor. The results of analysis and simulations of the improvement by lateral inhibition are shown. Furthermore, it was clarified that this technology can be applied to synthesize high quality CCD imaging system.
An imaging radiometer is one of the most fundamental optics sensors for aircraft as well as satellite remote sensing. The important points on the future imaging radiometers are hyper-spectral resolution, wavelength running capability, and polarimetry would give us further information. The basic problem is the requirement for two simultaneous photometric measurements of every point in the sensor field of view for the two orthogonal polarization components. Then we developed a tunable spectral and polarimetric radiometer: an AOTF imaging spectro-polarization components. Then we developed a tunable spectral and polarimetric radiometer: an AOTF imaging spectro-polarimeter which uses an acousto-optic tunable filter (AOTF) as spectroscopic as well as polarimetric element. The specific point of the device is that we can simultaneously obtain two diffracted beams and images perpendicularly polarized to each other. The instrument employs an reviewing the concept, configuration and performance of the instrument, applications of the instrument will be presented: the laboratory measurements of light scattering by rough surfaces, the aerosol measurements from the ground, and more.
Acousto-Optic Tunable Filter (AOTF) is a solid state electro-optical device which acts as an electronically tunable spectral filer. The specific point on the device is that we can get two diffracted beams perpendicularly polarized each other. So we developed an imaging spectro- polarimeter employing a visible AOTF and two CCD cameras each of which shows horizontally or vertically polarized spectral image of the object. Typical specifications of the visible AOTF imaging spectro-polarimeter completed are Spectral Range: 450 - 700 nm, Spectral Resolution: 1.2 - 1.9 nm, Spatial Resolution: 20 line pairs/mm (MTF: 0.3), Diffraction Efficiency: over 90%.
Polarimetry will provide useful information for various application fields of remote sensing technology. In this paper, results of experimental study on polarimetric characteristics of metallic powders, rocky powders including lunar regolith simulant and volcanic ash are presented. It was clarified that the polarimetry is useful for characterization of various terrestrial materials and for remote sensing of the Earth and planets.
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