Yueming Wang

Application of advanced IR-FPA in high-sensitivity pushbroom SWIR hyperspectral imager

High sensitivity SWIR(ShortWave Infra-Red) hyperspectral imager is benefit of national resources sensing. Background radiation results in noise and fluctuation of systems dark level, which does much harm to instruments 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, its showed that ROICs 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.

A new SNR model for space-borne hyperspectral imagers including atmospheric scattering influence

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.

Optical design of wide swath hyperspectral imager

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 2500nm.

Optical design, laboratory test, and calibration of airborne long wave infrared imaging spectrometer

We discuss and evaluate a long wave infrared imaging spectrometer in terms of its opto-mechanical design and analysis, alignment, testing, and calibration. The instrument is a practical airborne sensor achieving high spectral resolution and sensitive noise equivalent delta temperature. The instrument operates in the 8 to 12.5 μm spectral region with 28.85 nm spectral sampling, 1 mrad instantaneous field of view, and >40° cross track field. The instrument comprises three uniform sub-modules with identical design parameters and performances. The sub-module design is based on a refractive foreoptics feeding an all-reflective spectrometer. The optical form of the spectrometer is a double-pass reflective triplet with a flat grating, which has a fast f/2 and high optical throughput. Cryogenic optics of 100 K is implemented only for the spectrometer. Assembly and thermal deformation and focusing adjustment design are particularly considered for this low temperature. All the mirrors of the spectrometer are opto-mechanical-integrated designed and manufactured by single-point diamond turning technology. We consider the center sub-module as an example, and we present its laboratory testing results and calibration; the results indicate the instruments potential value in airborne sensing.

Measurement of Ti–6Al–4V alloy ignition temperature by reflectivity detection

Fires resulting from titanium combustion are complex and violent processes which can instantly burn a titanium alloy once ignited. The occurrence of titanium combustion is a disaster for aircraft. Accurate measurement of the ignition temperature of titanium alloys is of significance in preventing such fires and in investigating combustion-resistance properties. In this study, monochromatic temperature and emissivity measurement methods based on reflectivity detection were used to determine the ignition temperature of a titanium alloy. Experiments were carried out using a titanium burning apparatus. The temperatures of titanium in the oxidation stage before ignition and in the combustion stage during the ignition process were measured using wavelengths of 1050 nm and 940 nm, respectively. Experimental results showed that the ignition temperature of the titanium alloy could be measured by reflectivity detection and that measurement precision during thermal oxidation (500-900 °C) was ±1 °C. The temperature of the ignition process ranged between 1653 and 1857 °C, and the ignition temperature was around 1680 °C.

Wide field of view visible and near infrared pushbroom airborne hyperspectral imager

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.

Research and application of high frame frequency CCD drive technology in hyperspectral spectrograph

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.

Uncooled infrared photodetectors based on one-dimensional nanowires and two-dimensional materials

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.

Recent progress of push-broom infrared hyper-spectral imager in SITP

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.

High spectral resolution airborne short wave infrared hyperspectral imager

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.