Yuheng Chen

Conceptual design of airborne daytime infrared star cameras

Star camera is a kind of sensitive attitude sensors used for navigation of space vehicles. In order to use it on aircrafts in daytime, the conceptual design and the principle of airborne daytime infrared star cameras are introduced in this paper, as there is enough number of stars in near infrared band to be used as reference of a star camera for calculating attitude. Through analyzing the atmospheric scattering background light intensity for different altitudes, observing angles, and solar angles with Modtran software, and considering IR FPA (infrared focal plane array) performance, shot noise and the required star magnitude for daytime star trackers and sensors, the optical system parameters, i.e. FOV (field of view), clear aperture diameter and effective focal length, are determined according to the required SNR (signal to noise ratio).

Convex grating imaging spectrograph with light weight and high fidelity

The optimized design, alignment and experimental results of a compact convex grating hyper-spectral imager with high fidelity are reported and evaluated. The imager works in visible wavelength range from 0.4 to 0.78 μm. The numerical aperture of system is 0.2, and the entrance slit images with -1 magnification and linearly dispersed into 7.5mm in width. In addition, the spectral sample resolution is 0.76nm/pixel with negligible distortion. In order to get good performance and facilitate alignment, the optical system is both imagery and objective telecentric. The efficiency of the convex grating can up to about 40 percents by ion-beam etching. The size is 190mm×180mm×90mm and the mass is less than 1kg. The light weighted compact system is portable, and it is feasible in remote sensing.

Wavelength calibration and spectral line bending determination of an imaging spectrometer

After alignment of an imaging spectrometer, the image of a special wavelength should in theory strictly meet with the design value and is focused on a certain column of the CCD focal plane. Since the imaging spectrometer is usually used in spatial or aerial environment, the optical components and the detector will departure from the regulated place and leads to focusing the image onto the deflected position of the focal plane in the spectral direction. Since the onboard readjustment of an inaccurate imaging spectrometer is usually unavailable, the equivalent task should be performed by certain post processing method. In this paper, we present a wavelength calibration method based on a fitting algorithm. Because of the linear diffraction feature of a grating, first order fit is adopted for the calibration. Using a standard mercury lamp as the light source during the calibration, the experimental imaging data collected from the whole CCD focal plane is used for the wavelength calibration to construct the actual wavelength distributing curve. Because of spectral line bending (smiling) of the imaging spectrometer, the wavelength calibration result of each row of the CCD plane differs so that a row-by-row calibration work should be carried out. The total row-by-row calibration result not only provides a full-scale and high-precision calibration effort, but also brings forward a smiling evaluation method for the whole imaging spectrometer. Using a standard Hg-Cd lamp as both the illuminating light source and the object, the spectroscopic image of the slit focusing onto the CCD focal plane of a calibrated imaging spectrometer is collected. In certain rows of the image, the center position of every spectral line is recorded. Through the comparison of recorded positions of different rows, the smiling of the calibrated imaging spectrometer is worked out, which meets with the design value.

CCD based digital optical transfer function testing instrument

As the development of image processing and area array detectors, CCD has been used widely in optical transfer function (OTF) measurement for it can realize fast measurement and avoid scanning operation. An OTF testing instrument using CCD camera is introduced in this paper. This instrument works in visible wavelength with 200mm entrance pupil diameter. The highest spatial frequency this instrument can measure is up to 200lp/mm. This instrument is composed of four parts, that is, object generator, image analyzer, control system and software. A 50mm plano-convex lens is used to calibrate this OTF testing instrument. According to the calibration results, measurement error of this testing instrument is less than 0.04, while its repeatability is less than 0.03.

A method for measuring modulation transfer function of CCD device in remote camera with grating pattern

The remote camera that is developed by us is the exclusive functional load of a micro-satellite. Modulation transfer function (MTF) is a direct and accurate parameter to evaluate the system performance of a remote camera, and the MTF of a camera is jointly decided by the MTF of camera lens and its CCD device. The MTF of the camera lens can be tested directly with commercial optical system testing instrument, but it is indispensable to measure the MTF of the CCD device accurately before setting up the whole camera to evaluate the performance of the whole camera in advance. Compared with other existed MTF measuring methods, this method using grating pattern requires less equipment and simpler arithmetic. Only one complete scan of the grating pattern and later data processing and interpolation are needed to get the continuous MTF curves of the whole camera and its CCD device. High-precision optical system testing instrument guarantees the precision of this indirect measuring method. This indirect method to measure MTF is of reference use for the method of testing MTF of electronic device and for gaining MTF indirectly from corresponding CTF.

Research on system modeling and data reconstruction for spatial coding compressive spectral imaging

Compressive spectral imaging is a kind of novel spectral imaging technique that combines traditional spectral imaging method with new concept of compressive sensing. Spatial coding compressive spectral imaging realizes snapshot imaging and the dimension reduction of the acquisition data cube by successive modulation, dispersion and stacking of the light signal. It reduces acquisition data amount, increases imaging signal-to-noise ratio, realizes snapshot imaging for large field of view and has already been applied in the occasions such as high-speed imaging, fluorescent imaging and so on. In this paper, the physical model for single dispersion spatial coding compressive spectral imaging is reviewed on which the data flow procession is analyzed and its reconstruction issue is concluded. The existing sparse reconstruction methods are investigated and specific module based on the two-step iterative shrinkage/thresholding algorithm is built so as to execute the imaging data reconstruction. A regularizer based on the total-variation form is included in the unconstrained minimization problem so that the smooth extent of the restored data cube can be controlled by altering its tuning parameter. To verify the system modeling and data reconstruction method, a simulation imaging experiment is carried out, for which a specific imaging scenery of both spatial and spectral features is firstly built. The root-mean-square error of the whole-band reconstructed spectral images under different regularization tuning parameters are calculated so that the relation between data fidelity and the tuning parameter is revealed. The imaging quality is also evaluated by visual observation and comparison on resulting image and spectral curve.

Optimization of the exposure time of aerospace camera through its signal-to-noise ratio

The aerospace camera developed is an exclusive functional load of a micro satellite. The signal-to-noise ratio of the aerospace camera reflects its radiance response and is the parameter that directly associates with the quality of its acquired images. The traditional way to calculate the signal-to-noise ratio of a camera is to substitute the related parameters of its subassemblies into the deduced formulas. This kind of method lacks the focalization on the diversities of its components and specific application occasions. The result tested by using standard uniform source can certainly be utilized to evaluate the work performance of the camera, but it ignores its actual orbital atmospheric condition and consequentially leads to unavoidable data deviation. The atmospheric transmission model is built and the radiation condition of the aerospace camera in orbit is simulated by means of MODTRAN. Instead of building the noise model based on electronic devices of the camera to get theoretical noise data, considering the difference of the noises of the camera between in-lab and on-orbit condition, we adopt the measured noise data of the CCD camera to calculate the signal-to-noise ratio so as to make it approach the real value as possible. The influences of the changes of solar altitude angle, earth surface albedo and weather condition on the signal-to-noise ratio of the camera are quantitatively determined. The result of the signal-to-noise ratio can be used as the basis to evaluate the remote sensing imaging quality and to decide the feasible exposure time.

Camouflage target detection via hyperspectral imaging plus information divergence measurement

Target detection is one of most important applications in remote sensing. Nowadays accurate camouflage target distinction is often resorted to spectral imaging technique due to its high-resolution spectral/spatial information acquisition ability as well as plenty of data processing methods. In this paper, hyper-spectral imaging technique together with spectral information divergence measure method is used to solve camouflage target detection problem. A self-developed visual-band hyper-spectral imaging device is adopted to collect data cubes of certain experimental scene before spectral information divergences are worked out so as to discriminate target camouflage and anomaly. Full-band information divergences are measured to evaluate target detection effect visually and quantitatively. Information divergence measurement is proved to be a low-cost and effective tool for target detection task and can be further developed to other target detection applications beyond spectral imaging technique.

Quality assessment for spectral imaging

Recent research in the area of image quality assessment has been focusing almost exclusively on greyscale and color images. The advent of technologies such as remote sensing, biomedical and industrial imaging however demands this research to be extended to multi/hyper spectral images. Spectral imaging has more judging essentials than greyscale or color imaging and its image quality assessment task intends to cover up all-around evaluating factors. This paper presents an integrating spectral imaging quality assessment project, in which spectral-based, spatial-based and radiometric-based quality evaluation behavior for one remote-sensing hyperspectral imager are jointly executed. Spectral response function is worked out and spectral performance is further judged according to its FWHM and spectral excursion value. Spatial quality assessment is worked out by MTF computing with an improved slanted edge analysis method. Radiometric response ability of different spectral channels is judged by SNR computing based upon local RMS extraction and statistics method. Improved noise elimination and parameter optimization method are adopted to improve the evaluation fidelity. This work on spectral imaging quality assessment not only has significance in the development of on-ground and in-orbit spectral imaging technique but also takes on reference value for index demonstration and design optimization for spectral instrument development.

The design and implementation of the full-Stokes imaging spectropolarimeter

The imaging spectro-polarimetry combines the spectral imaging technology and the imaging polarization technology. It assembles the functions of camera, spectrometer and polarimeter. So the optical information quantity is increased and the detection efficiency is improved. But the acquirement of the multi-dimensional information results in the detector complex construction and large volume. The moving part is used in the current method to realize the different polarization states or spectral filtering. The images are difficult for registration and the current method can’t be used to get the motion scene. This paper presents innovative imaging spectro-polarimetry method with no moving parts. The hyper-spectral information, full-Stokes polarization information and one-dimensional spatial information are obtained by the polarization modulating and spectrum dispersing. The designed imaging spectro-polarimeter is composed of two parts, a polarization module and the spectral dispersive module. They are all employed stationary configuration. The polarization module includes two birefringent crystal wave plates and a polarizer. The thickness of the birefringent wave-plates and the polarization axes of each component are optimized and the full-Stokes polarization information is loaded on the spectrum. The polarization information can be restored by the Fourier transform. The concentric Offner configuration is adopted for spectral dispersive module. It is composed of two concave spherical mirrors and a holographic aberration-corrected convex grating. The designed dispersive configuration is compact and aligned simply. And high quality linear dispersion, low distortion spectral image are implemented. The Full-stokes imaging spectro-polarimeter our designed is validated by the model simulation and the laboratory experiment. The mixed hyper-spectral information and accuracy polarization information can be obtained.