Field of view is a principle parameter of the push-broom imaging spectrometer which mainly affects its working efficiency. Integration of multiple sub-modules with smaller field of view is an optional solution to gain wider field of view. An approach for field registration and test for multi-module imaging spectrometer is presented. A single beam is reflected by multiple angle-tunable mirrors into multiple beams with different directions, which then enter the multimodule imaging spectrometer. Thus, one sub-module responses to lights with multiple incident angles simultaneously. A field registration platform is designed, which includes a collimating beam simulator, a multi-field beam generator, and an instrument support frame. The collimating beam simulator is composed of a collimator, an illuminant source, and a twodimensional shift pinhole target placed in the collimator focal plane. The multi-field beam generator is composed of a rotary table and multiple angle-tunable mirrors. The postures and the numbers of the mirrors are determined by the field registration requirement of the multi-module imaging spectrometer. Field registration is operated by monitoring the pixel response of the sub-module detector, and is tested by the two-dimensional shift of the pinhole target. Field registration was implemented on the platform for an airborne multi-module imaging spectrometer. Test result showed that the field registration is less than 0.2 pixels. Flight data of the imaging spectrometer demonstrated its good field registration alignment, which verified the engineering feasibility and value of the field registration approach.
In the past decades, hyperspectral imaging technologies was well developed in the whole world. Visible and Near Infrared hyperspectral imagers play an important role in agriculture, land use, forestry, etc. Higher performance airborne hyperspectral imagery is strongly expected these years. Wider Field of View and higher resolution instrument can acquire data more efficiently. A VNIR PHI with 40 degree FOV, 0.125mrad IFOV, 256bands was integrated last year. The system can adapt to the Velocity to Height Ratio lower than 0.04. The system consists of 3 subsystems. Every subsystem consists of TMA fore optics, spectrometer with planar blazed grating and electronics. The 3 subsystems work for left, middle, right FOV, respectively. Thanks to CCD’s pixel binning function, the system can operate in high spectral resolution mode, high spatial resolution mode, and high sensitivity mode for different applications. The integration was finished, and airborne flight validation experiments were conducted.
In the past decades, hyper-spectral imaging technologies were well developed in SITP, CAS. Many innovations for system design and key parts of hyper-spectral imager were finished. First airborne hyper-spectral imager operating from VNIR to TIR in the world was emerged in SITP. It is well known as OMIS(Operational Modular Imaging Spectrometer). Some new technologies were introduced to improve the performance of hyper-spectral imaging system in these years. A high spatial space-borne hyper-spectral imager aboard Tiangong-1 spacecraft was launched on Sep.29, 2011. Thanks for ground motion compensation and high optical efficiency prismatic spectrometer, a large amount of hyper-spectral imagery with high sensitivity and good quality were acquired in the past years. Some important phenomena were observed. To diminish spectral distortion and expand field of view, new type of prismatic imaging spectrometer based curved prism were proposed by SITP. A prototype of hyper-spectral imager based spherical fused silica prism were manufactured, which can operate from 400nm~2500nm. We also made progress in the development of LWIR hyper-spectral imaging technology. Compact and low F number LWIR imaging spectrometer was designed, manufactured and integrated. The spectrometer operated in a cryogenically-cooled vacuum box for background radiation restraint. The system performed well during flight experiment in an airborne platform. Thanks high sensitivity FPA and high performance optics, spatial resolution and spectral resolution and SNR of system are improved enormously. However, more work should be done for high radiometric accuracy in the future.
Low dimensional semiconductors have attracted enormous attention in recent years. Owing to the special dimension confinement, photodetectors based on low dimensional materials and their hybrid systems exhibit considerable performance at room temperature. This differs from traditional thin-film infrared photodetectors which require liquid nitrogen cooling. In this paper, we introduce uncooled photodetectors based on one dimensional (1D) nanowires, two-dimensional (2D) materials, 2D hybrid structures and 1D/2D heterostructures. We illustrate their working mechanisms and reveal the potential for practical infrared detection.
As a key component of the hyperspectral imager, the use of high frame frequency CCD was more and more widely. CCD working principle was analyzed. Methods to realize high frame frequency and image programmable online were presented. Hardware driver design was designed. The image with high visibility, whose frame frequency was more than 320 Hz, noise was below 75e-, dynamic range was superior to 70dB, was acquired by the CCD. Based on the operating characteristic of the CCD, the temperature experiment was designed. Through the experiment, the relation curve of dark level and noise with the CCD detector temperature was given, which was helpful for the later image correction.
In order to solve the need for PC software of aerial imaging spectrometer USB3.0 high-speed large-capacity data acquisition system development. The project bases on MFC and DirectDraw technology by VS2010 development environment. The system can complete imaging spectrometer data acquisition , data storage and real-time imaging to monitor through the parsing of the spectral image data protocol. In addition, this paper also analyzes the factors that affect data acquisition bottleneck from computer architecture and software. The test results show that the PC acquisition platform can stabilize at 1.28 Gpbs data acquisition speed of some aerial imaging spectrometer requirements .It’s a new Light-weight and compact, low-cost and universal data acquisition approach for aerial imaging spectrometer.
With the emergence of UAV (unmanned aerial vehicle) platform for aerial imaging spectrometer, research of aerial
imaging spectrometer DAS(data acquisition system) faces new challenges. Due to the limitation of platform and other
factors, the aerial imaging spectrometer DAS requires small-light, low-cost and universal. Traditional aerial imaging
spectrometer DAS system is expensive, bulky, non-universal and unsupported plug-and-play based on PCIe. So that has
been unable to meet promotion and application of the aerial imaging spectrometer.
In order to solve these problems, the new data acquisition scheme bases on USB3.0 interface.USB3.0 can provide
guarantee of small-light, low-cost and universal relying on the forward-looking technology advantage. USB3.0
transmission theory is up to 5Gbps.And the GPIF programming interface achieves 3.2Gbps of the effective theoretical
data bandwidth.USB3.0 can fully meet the needs of the aerial imaging spectrometer data transmission rate. The scheme
uses the slave FIFO asynchronous data transmission mode between FPGA and USB3014 interface chip. Firstly system
collects spectral data from TLK2711 of high-speed serial interface chip. Then FPGA receives data in DDR2 cache after
ping-pong data processing. Finally USB3014 interface chip transmits data via automatic-dma approach and uploads to
PC by USB3.0 cable.
During the manufacture of aerial imaging spectrometer, the DAS can achieve image acquisition, transmission, storage
and display. All functions can provide the necessary test detection for aerial imaging spectrometer. The test shows that
system performs stable and no data lose. Average transmission speed and storage speed of writing SSD can stabilize at
1.28Gbps. Consequently ,this data acquisition system can meet application requirements for aerial imaging spectrometer.
With the development of remote sensing technology, shortwave infrared (SWIR) imaging technology has got more and more attention because of its ability through the fog and high spatial resolution. High spatial resolution SWIR imaging often requires high frame frequency. If the frame frequency is too high, it could cause the shortage of the image’s signal to noise ratio (SNR), seriously affecting image quality. In order to solve the contradiction between high spatial resolution and sensitivity, time delay and digital accumulation (TDDA) technology is proposed in this paper to improve system’s SNR and image quality. A prototype of SWIR imaging system based on a large format area InGaAs detector is designed, which demonstrates TDDA technology. The experiment results indicate that TDDA technology can increase system’s SNR of the square root of accumulative stage and improve image’s uniformity. The results in this paper are helpful for the improvement and application of high spatial resolution SWIR imaging technology.
Imaging spectrometer plays an important role in the remote sensing application. Imaging spectrometer can collects and provides a unique spectral signature of many materials. The spectral signature may be absorbing, reflecting, and emitting. Generally, optical spectral bands for earth observing consist of VNIR, SWIR, TIR/LWIR. VNIR band imaging spectrometer is well-known in vegetation remote sensing and ocean detection. SWIR band imaging spectrometer is widely applied in mineralogy investigation. For its uniquely capability of spectral radiance measurement, TIR/LWIR imaging spectrometer attracts much attention these years. This paper will present a new generation VNIR/SWIR/TIR imaging spectrometer. The preliminary result of its first flight will also be shared. The spectral sampling intervals of VNIR/SWIR/TIR are 2.4nm/3nm/30nm, respectively. The spatial pixel numbers are 2800/1400/700,respectively. It’s a push-broom imaging spectrometer.
Reflective triplet (RT) optics is an optical form with decenters and tilts of all the three mirrors. It can be used in spectrometer as collimator and reimager to get fine optical and spectral performances. To alleviate thermal and assembly stress deformation, opto-mechanical integrated design suggests that as with all the machine elements and the mainframe, the mirrors substrates are aluminum. All the mirrors are manufactured by single-point diamond turning technology and measured by interferometer or profilometer. Because of retro-reflection by grating or prism and reimaging away from the object field, solo three mirrors optical path of RT has some aberrations. So its alignment and measurement needs an aberration corrected measuring optical system with auxiliary plane and sphere mirrors and in which the RT optics used in four pass. Manufacture, alignment and measurement for a RT optics used in long wave infrared grating spectrometer is discussed here.
We realized the manufacture, alignment and test for the RT optics of a longwave infrared spectromter by CMM and interferometer. Wavefront error test by interferometer and surface profiles measured by profilometer indicate that performances of the manufactured mirrors exceed the requirements. Interferogram of the assembled RT optics shows that wavefront error rms is less than 0.0493λ@10.6μm vs design result 0.0207λ.
Short Wave InfraRed(SWIR) spectral imager is good at detecting difference between materials and penetrating fog and mist. High spectral resolution SWIR hyperspectral imager plays a key role in developing earth observing technology. Hyperspectral data cube can help band selections that is very important for multispectral imager design. Up to now, the spectral resolution of many SWIR hyperspectral imagers is about 10nm. A high sensitivity airborne SWIR hyperspectral imager with narrower spectral band will be presented. The system consists of TMA telescope, slit, spectrometer with planar blazed grating and high sensitivity MCT FPA. The spectral sampling interval is about 3nm. The IFOV is 0.5mrad. To eliminate the influence of the thermal background, a cold shield is designed in the dewar. The pixel number of spatial dimension is 640. Performance measurement in laboratory and image analysis for flight test will also be presented.
In the infrared system, cooling down the optic components' temperature is a
better choice to decrease the background radiation and maximize the sensitivity. This
paper presented a 100K cryogenic optical system, for which an integrated designation
of mechanical cooler, flexible thermal link and optical bench was developed. The
whole infrared optic components which were assembled in a vacuum box were cooled
down to 100K by two mechanical coolers. Low thermal conductivity supports and
low emissivity multi-layers were used to reduce the cryogenic optical system's heat
loss. The experiment results showed that in about eight hours, the temperature of the
optical components reached 100K from room temperature, and the vibration from the
mechanical coolers nearly have no affection to the imaging process by using of
thermal links. Some experimental results of this cryogenic system will be discussed in
MWIR imaging spectrometer is promising in detecting spectral signature of high temperature object such as jet steam, guided missile and explosive gas. This paper introduces an optical design of a MWIR imaging spectrometer with a cold slit sharply reducing the stray radiation from exterior environment and interior structure. The spectrometer is composed of a slit, a spherical prism as disperser, two concentric spheres and a correction lens. It has a real entrance pupil to match the objective and for setting the infrared cold shield near the slit and a real exit pupil to match the cold shield of the focal plane array (FPA). There are two cooled parts, one includes the aperture stop and slit, and the other is the exit pupil and the FPA with two specially positioned cooled shields. A detailed stray radiation analysis is represented which demonstrates the outstanding effect of this system in background radiation restraint.
Space-borne hyperspectral imager is capable of providing large amount of image data with high spatial and spectral resolution. However, with the increase of the resolution, the volume and weight of conventional dispersive (prism/grating) imaging spectrometers also increase significantly. Meanwhile, the demand for the system sensitivity becomes even higher. In this paper, the application of Integrated Stepwise Filter (ISF) in infrared hyperspectral imaging technology was analyzed, including optical efficiency, instrumental background radiation suppression, spectral time delay integration (TDI) and other performances. Several methods which can enhance the system sensitivity were also provided. A compact short-wave infrared (SWIR) hyperspectral imager prototype covering the spectral range of 2.0μm-2.5μm with 6 TDI stages was designed and implemented. The results of the imaging experiment demonstrated that ISF has great application prospects in the field of high sensitivity space-borne hyperspectral imagery.
This paper describes a design concept for wide swath hyperspectral imager. The challenge is to meet the requirement
of good image quality and high precision registration from 400nm to 2500nm. A new type spherical prism imaging
spectrometer is presented in the paper. The swath of system can reach 60 kilometer from a 600km sun-synchronous orbit
with 30 meter ground sample distance (GSD). The optical system consists of a TMA objective and 2 30mm-slit spherical
prism spectrometer operating both VNIR and SWIR. Key features of the design include (1) high signal to noise ratio for
high efficiency of F-silica prism; (2) high precision band registration for same spectrometer operating from 400nm to
Nonintrusive measurements of infrared characteristics from engine exhaust plume are required for emission control or target tracking, due to the advantage of online measurement without affecting the exhaust plume. Conventional nonintrusive measurement techniques, e.g. the passive Fourier-transform infrared (FTIR) absorption spectrometry, lack prior knowledge of backgrounds and consume time to measure the complete infrared characteristics. Hence, an improved but simple nonintrusive method is proposed. Accordingly, a prototype system with a Mid-wave infrared imager has been developed and tested for the measurement of vehicle engine exhaust plume. Subsequently, the time-variant effective transmittance and emissivity is determined. Compared to the passive FTIR absorption spectrometry, this method incorporates a known background into the measurement and is more adequate for recording the rapidly changing exhaust plume radiation. Therefore, the accurate value of the transmittance and emissivity can be obtained. Further analysis reveals that the imager could be replaced with a dispersive spectrometer, which makes it feasible to acquire the absolute transmittance and emissivity with respect to wavelength. Thus, the concentration of specific toxic gases could be calculated following the radiance inversion technique.
Space-borne hyperspectral imagery is widely used in the fields of earth science and mineral detection. High signal-to-noise ratio (SNR) of imaging spectrometers is required to guarantee the image data validity. To describe the system sensitivity, previous SNR models mainly focused on the optical parameters and the detector characteristics. However, the sensitivity of space-borne hyperspectral imagers is also limited by the atmospheric scattering effect to a large extent. A quantified and complete SNR model including atmospheric scattering influence is valuable for the development of imaging spectrometers.
In this paper, scattering influence on hyperspectral imaging quality was analyzed in the spectral range of 0.4μm-2.5μm. Atmospheric simulation was presented and system performance reduction caused by the scattering effect was also quantified. The results show that the scattered light will occupy a large proportion of the system dynamic range and bring additional shot noise, which causes evident SNR attenuation.
Based on the analysis a new SNR model including atmospheric parameters was provided. Hyperspectral imaging quality was calculated with both the new SNR model and the classical SNR model respectively, and comparative study of the two models was given in this paper.
In order to validate the new SNR model, a hyperspectral imaging system and a multiband camera were built, and the imaging experiments were conducted. The results show that the atmospheric scattering effect could lead to significant SNR reduction and contrast attenuation of spectral images, especially at visible bands. Using the new SNR model could allow designers to estimate the system performance more precisely. Corresponding instrument design measures were also proposed based on the analysis and experiments.
Balloon-borne or ground-based high resolution long range observation has extensive applications in border monitoring and area surveillance. Performance of long-distance oblique or horizontal imaging systems is closely related to the atmospheric transmittance of the observing spectral band. Compared with visible and near infrared, the shortwave infrared (SWIR) band benefits from less scattering effects, which enables it to provide better quality images under harsh atmospheric conditions. We present a signal-to-noise ratio (SNR) model including atmospheric influences. Based on the model, image SNR was calculated in the spectral range of 0.4 μm to 2.5 μm. In order to validate the imaging performance model of SWIR, a multi-band camera was designed and spectral imaging experiments were conducted. The results clearly demonstrated the advantage of SWIR imaging. The experiments show that the contrast and SNR of SWIR images reduced insignificantly for long distances and under low visibility conditions. This advantage makes SWIR multiband cameras suitable for long-distance remote sensing and for observing through haze.
Nowadays, spectrometers are being increasingly used in the remote sensing of the earth system processes and they can be
well conducted field spectral calibrated by using absorption features of atmospheric profile. The spectral calibration
accuracy of an ASD spectrometer in different atmospheric conditions is analyzed in this paper. First of all, set the atsensor
spectral radiance spectrum as the reference spectral curve, which is generated by the MODTRAN5 radiative
transfer model outputs convolved with the spectral response function of each channel. The results can be totally different
when input different water vapor contents. Meanwhile, under the same atmospheric conditions (the same observation
target, the sun elevation angle, azimuth, aerosol distribution, composition and concentration distribution of
atmosphere..etc. ), the diffuser whiteboard is measured by the ASD spectrometer to acquire observed spectral curve. The
best spectral matching algorithm is used to compare the observed spectrum with the reference one. Central wavelength of
the ASD is obtained by calculating the results of matching spectrum shifts within feature spectrum range. Finally,
quantitative analysis and calculation about the effect on the accuracy of instrument spectral calibration with different
water vapor contents is presented.
High sensitivity SWIR(ShortWave Infra-Red) hyperspectral imager is benefit of national resources sensing. Background
radiation results in noise and fluctuation of system's dark level, which does much harm to instrument's radiometric
performance. Basing on the analysis of background radiation and object reflectance signal, a SWIR hyperspectral system
solution is given which utilize advanced infrared focal plane array technology. By system integration and testing, it's
showed that ROIC's charge to voltage gain plays an important role in enhancing Signal to Noise Ratio(SNR). At lower
light condition, high gain of ROIC and long integration time will lead to smaller dynamic range because of background
radiation. A real-time background radiation monitoring method was validated which did a favor for eliminating
background radiation in the image data.
The hyperspectral imager is able to acquire space and spectral information of ground object
simultaneously. When using a prism splitting mode, different wavelengths of light will disperse nonlinearly in
spectral dimension after going through the slit and the prism. Due to the longer slit and different angles of
incidence, when going through the slit and the prism, the same wavelength of light will curve in space
dimension. For SWIR bands, the maximum shift is more than 1.5 bandwidth. The shift cannot be ignored, for is
alters the pixel spectral and reduces match accuracies between space and spectral information. In this paper, a correction method of non-uniform spectral radiance in hyperspectral image is put forward. First, the
laboratory spectral calibration is performed to acquire center wavelength and full width half maximum (FWHM)
of each band as well as each pixel. Secondly, for each band, the mean of center wavelength which is
calculated according to the results of the spectral calibration is regarded as each pixel's adjusted center
wavelength. For each band and each pixel, calculate the ratio coefficient based on adjacent bands, then
establish a ratio coefficient form of full pixels. At last, correct the image by looking up the form. By using MNF
transformation, a corrected image can be well evaluated, a brightness gradient of the images has been
removed and the phenomenon of image spectral radiance mixing has been reduced greatly, especially at the
edge of the image.
Space-borne hyper-spectral imagery is an important means for the studies and
applications of earth science. High cost efficiency could be acquired by optimized
system design. In this paper, an advanced scanning method is proposed, which
contributes to implement both high temporal and spatial resolution imaging system.
Revisit frequency and effective working time of space-borne hyper-spectral imagers
could be greatly improved by adopting two-axis scanning system if spatial resolution
and radiometric accuracy are not harshly demanded. In order to avoid the quality
degradation caused by image rotation, an idea of two-axis rotation has been presented
based on the analysis and simulation of two-dimensional scanning motion path and
features. Further improvement of the imagers' detection ability under the conditions
of small solar altitude angle and low surface reflectance can be realized by the Ground
Motion Compensation on pitch axis. The structure and control performance are also
described. An intelligent integration technology of two-dimensional scanning and
image motion compensation is elaborated in this paper. With this technology,
sun-synchronous hyper-spectral imagers are able to pay quick visit to hot spots,
acquiring both high spatial and temporal resolution hyper-spectral images, which
enables rapid response of emergencies. The result has reference value for developing
operational space-borne hyper-spectral imagers.