Offner spectrometer is widely applied in hyperspectral imaging spectrometers, in which the design, fabrication, and testing of convex gratings are crucial to ensure the spectral and imaging performance of the whole system. A conical diffraction Offner spectrometer is proposed with the spectral range of 400 to 900 nm, spectral resolution of 5 nm, slit length of 1 mm, and spectral dispersive width of 10 mm. A finite-element analysis is adopted to optimize the groove parameters of the conical diffraction blazed convex grating that is used in the aforesaid spectrometer. Holographic scan ion beam etching method is employed to fabricate such convex grating. Experimental facilities for testing the diffraction efficiency are built in the lab, showing that the efficiency is higher than 50% in the whole waveband and the peak efficiency is over 75%, which is consistent with the design result. The result indicates that it is feasible to design and optimize the grating groove using the finite-element analysis method.
Target discrimination is of great significance in many applications such as remote sensing, security monitoring, production testing and so on. Nowadays accurate target discrimination 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 generalized angle analysis method is used to solve camouflage target discrimination problem. A self-developed visual-band hyper-spectral imaging device is adopted to collect data cubes of certain experimental scene before spectral generalized angle is worked out so as to discriminate abnormal target. Full-band spectral generalized angle is measured to evaluate target discrimination effect visually and quantitatively. This is proved to be an effective tool for target detection task and can be further developed for other imaging techniques beyond spectral imaging.
Anomaly detection is helpful in many applications such as food monitoring, production testing, security surveillance, military countermeasure and so on. Spectral imaging technique is often resorted to for accurate abnormal target discrimination due to its high-resolution spectral/spatial information acquisition ability and a great number of data processing methods. Anomaly detection methods for hyperspectral imagery are contrastively studied in this paper. A self-developed visual-band hyperspectral imaging spectrometer is adopted to collect data cubes of certain experimental scene before two kinds of spectral-domain descriptors are used to execute abnormal camouflage detection. Detection effect of information divergence and generalized angle that are utilized as detection descriptors is visually and quantitatively compared and time consumption is assessed. The study is proved to be of significance to meet the anomaly detection demand that is based on spectral signature comparison and can be developed for further detection descriptor study and other imaging techniques beyond spectral imaging.
A measurement system for diffraction efficiency of convex gratings is designed. The measurement system mainly
includes four components as a light source, a front system, a dispersing system that contains a convex grating, and a
detector. Based on the definition and measuring principle of diffraction efficiency, the optical scheme of the
measurement system is analyzed and the design result is given. Then, in order to validate the feasibility of the designed
system, the measurement system is set up and the diffraction efficiency of a convex grating with the aperture of 35 mm,
the curvature-radius of 72mm, the blazed angle of 6.4°, the grating period of 2.5μm and the working waveband of
400nm-900nm is tested. Based on GUM (Guide to the Expression of Uncertainty in Measurement), the uncertainties
in the measuring results are evaluated. The measured diffraction efficiency data are compared to the theoretical ones,
which are calculated based on the grating groove parameters got by an atomic force microscope and Rigorous Couple
Wave Analysis, and the reliability of the measurement system is illustrated. Finally, the measurement performance of the
system is analyzed and tested. The results show that, the testing accuracy, the testing stability and the testing
repeatability are 2.5%, 0.085% and 3.5% , respectively.
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.
Proc. SPIE. 9685, 8th International Symposium on Advanced Optical Manufacturing and Testing Technologies: Design, Manufacturing, and Testing of Micro- and Nano-Optical Devices and Systems; and Smart Structures and Materials
Optical design of a novel optical imaging system is presented. It can overcome the scaling of the aberrations by dividing the imaging task between a single objective lens that achieves a partially corrected intermediate image on a spherical surface, and an array of micro-lens, each of which relays a small portion of the intermediate image to its respective sensor, correcting the residual aberrations. The system is aimed for obtaining large field-of-view without deteriorating its resolution, of which traditionally designed optical imaging systems have met great difficult. This progress not only breaks through the traditional restrictions, but also allows a wider application for optical imaging systems. Firstly, proper configuration, which satisfies both the requirement of compactness and high performance, is determined according to the working principle of the novel system and through the research of the design idea in this paper. Then, a design example is presented with the field-of-view 50°and its resolution 0.2mrad, which remains as the field-of-view scales. But the optimized scalable system is of close packed structure and its dimension is less than 300mm along the ray incidence.
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.
Wavefront coding can extend the depth of field of traditional optical system by inserting a phase mask into the pupil plane. In this paper, the point spread function (PSF) of wavefront coding system with annular aperture are analyzed. Stationary phase method and fast Fourier transform (FFT) method are used to compute the diffraction integral respectively. The OTF invariance is analyzed for the annular aperture with cubic phase mask under different obscuration ratio. With these analysis results, a wavefront coding system using Maksutov-Cassegrain configuration is designed finally. It is an F/8.21 catadioptric system with annular aperture, and its focal length is 821mm. The strength of the cubic phase mask is optimized with user-defined operand in Zemax. The Wiener filtering algorithm is used to restore the images and the numerical simulation proves the validity of the design.
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.
A novel snapshot imaging spectrometer with large field-of-view (FOV) up to 100° is achieved by taking the advantages of a multiscale fore-optics and a compact Offner imaging spectrograph. Based on the diffraction imaging theory, the multiscale fore-optics composed of a monocentric spherical lens and multi-channel microlens array is designed, over which panchromatic images with small FOV are of uniform image quality. And identical imaging spectrographs with a dimension less than 30 cubic millimeters and with a high spectral resolution of about 2nm are designed correspondingly. The presented imaging spectrometer works at the visible wavelength range which is from 400nm to 780nm. It is of a fast speed about F/2.4 and a compact configuration of only 200mm×300mm×300mm in dimension. But the smile and keystone distortions are negligible.
Image quality assessment is an essential value judgement approach for many applications. Multi & hyper spectral
imaging has more judging essentials than grey scale or RGB imaging and its image quality assessment job has to cover
up all-around evaluating factors. This paper presents an integrating spectral imaging quality assessment project, in which
spectral-based, radiometric-based and spatial-based statistical behavior for three hyperspectral imagers are jointly
executed. Spectral response function is worked out based on discrete illumination images and its spectral performance is
deduced according to its FWHM and spectral excursion value. Radiometric response ability of different spectral channel
under both on-ground and airborne imaging condition is judged by SNR computing based upon local RMS extraction
and statistics method. Spatial response evaluation of the spectral imaging instrument is worked out by MTF computing
with slanted edge analysis method. Reported pioneering systemic work in hyperspectral imaging quality assessment is
carried out with the help of several domestic dominating work units, which not only has significance in the development
of on-ground and in-orbit instrument performance evaluation technique but also takes on reference value for index
demonstration and design optimization for instrument development.
The radiometric calibration of imaging spectrometer plays an import role for scientific application of spectral data. The radiometric calibration accuracy is influenced by many factors, such as the stability and uniformity of light source, the transfer precision of radiation standard and so on. But the deviation from the linear response mode and the polarization effect of the imaging spectrometer are always neglected. In this paper, the linear radiometric calibration model is constructed and the radiometric linear response capacity is test by adjusting electric gain, exposure time and radiance level. The linear polarizer and the sine function fitting algorithm are utilized to measure polarization effect. The integrating sphere calibration system is constructed in our Lab and its spectral radiance is calibrated by a well-characterized and extremely stable NIST traceable transfer spectroradiometer. Our manufactured convex grating imaging spectrometer is relative and absolute calibrated based on the integrating sphere calibration system. The relative radiometric calibration data is used to remove or reduce the radiometric response non-uniformity every pixel of imaging spectrometer while the absolute radiometric calibration is used to construct the relationship between the physical radiant of the scene and the digital number of the image. The calibration coefficients are acquired at ten radiance levels. The diffraction noise in the images can be corrected by the calibration coefficients and the uniform radiance image can be got. The calibration result shows that our manufactured imaging spectrometer with convex grating has 3.0% degree of polarization and the uncertainties of the relative and absolute radiometric calibrations are 2.4% and 5.6% respectively.
Noise equivalent temperature difference (NETD) is the key parameter characterizing the detectivity of infrared systems.
Our developed pushbroom longwave infrared imaging spectrometer works in a waveband between 8μm to 10.5 μm. Its
temperature sensitivity property is not only affected by atmosphere attenuation, transmittance of the optical system and
the characteristics of electric circuit, but also restricted by the self-radiation. The NETD accurate calculation formula is
derived according to its definition. Radiation analysis model of a pushbroom image spectrometer is set up, and its
self-radiation is analyzed and calculated at different temperatures, such as 300K, 150K and 120K. Based on the obtained
accurate formula, the relationships between the NETD of imaging spectrometer and atmospheric attenuation, F-number,
effective pixel area of detector, equivalent noise bandwidth and CCD detectivity are analyzed in detail, and self-radiation
is particularly discussed. The work we have done is to provide the basis for parameters determination in spectrometer
Spectral calibration of imaging spectrometer plays an important role for acquiring target accurate spectrum. There are two spectral calibration types in essence, the wavelength scanning and characteristic line sampling. Only the calibrated pixel is used for the wavelength scanning methods and he spectral response function (SRF) is constructed by the calibrated pixel itself. The different wavelength can be generated by the monochromator. The SRF is constructed by adjacent pixels of the calibrated one for the characteristic line sampling methods. And the pixels are illuminated by the narrow spectrum line and the center wavelength of the spectral line is exactly known. The calibration result comes from scanning method is precise, but it takes much time and data to deal with. The wavelength scanning method cannot be used in field or space environment. The characteristic line sampling method is simple, but the calibration precision is not easy to confirm. The standard spectroscopic lamp is used to calibrate our manufactured convex grating imaging spectrometer which has Offner concentric structure and can supply high resolution and uniform spectral signal. Gaussian fitting algorithm is used to determine the center position and the Full-Width-Half-Maximum（FWHM）of the characteristic spectrum line. The central wavelengths and FWHMs of spectral pixels are calibrated by cubic polynomial fitting. By setting a fitting error thresh hold and abandoning the maximum deviation point, an optimization calculation is achieved. The integrated calibration experiment equipment for spectral calibration is developed to enhance calibration efficiency. The spectral calibration result comes from spectral lamp method are verified by monochromator wavelength scanning calibration technique. The result shows that spectral calibration uncertainty of FWHM and center wavelength are both less than 0.08nm, or 5.2% of spectral FWHM.
The optical compressive spectral imaging method is a novel spectral imaging technique that draws in the inspiration of compressed sensing, which takes on the advantages such as reducing acquisition data amount, realizing snapshot imaging, increasing signal to noise ratio and so on. Considering the influence of the sampling quality on the ultimate imaging quality, researchers match the sampling interval with the modulation interval in former reported imaging system, while the depressed sampling rate leads to the loss on the original spectral resolution. To overcome that technical defect, the demand for the matching between the sampling interval and the modulation interval is disposed of and the spectral channel number of the designed experimental device increases more than threefold comparing to that of the previous method. Imaging experiment is carried out by use of the experiment installation and the spectral data cube of the shooting target is reconstructed with the acquired compressed image by use of the two-step iterative shrinkage/thresholding algorithms. The experimental result indicates that the spectral channel number increases effectively and the reconstructed data stays high-fidelity. The images and spectral curves are able to accurately reflect the spatial and spectral character of the target.
Compressive spectral imaging combines traditional spectral imaging method with new concept of compressive sensing thus has the advantages such as reducing acquisition data amount, realizing snapshot imaging for large field of view and increasing image signal-to-noise and its preliminary application effectiveness has been explored by early usage on the occasions such as high-speed imaging and fluorescent imaging. In this paper, the application potentiality for spatial coding compressive spectral imaging technique on rural survey is revealed. The physical model for spatial coding compressive spectral imaging is built on which its data flow procession is analyzed and its data reconstruction issue is concluded. The existing sparse reconstruction methods are reviewed thus specific module based on the two-step iterative shrinkage/thresholding algorithm is built so as to execute the imaging data reconstruction. The simulating imaging experiment based on AVIRIS visible band data of a specific selected rural scene is carried out. The spatial identification and spectral featuring extraction capacity for different ground species are evaluated by visual judgment of both single band image and spectral curve. The data fidelity evaluation parameters (RMSE and PSNR) are put forward so as to verify the data fidelity maintaining ability of this compressive imaging method quantitatively. The application potentiality of spatial coding compressive spectral imaging on rural survey, crop monitoring, vegetation inspection and further agricultural development demand is verified in this paper.
Imaging spectrometer is a promising remote sensing instrument widely used in many filed, such as hazard forecasting,
environmental monitoring and so on. The reliability of the spectral data is the determination to the scientific communities.
The wavelength position at the focal plane of the imaging spectrometer will change as the pressure and temperature vary,
or the mechanical vibration. It is difficult for the onboard calibration instrument itself to keep the spectrum reference
accuracy and it also occupies weight and the volume of the remote sensing platform. Because the spectral images suffer
from the atmospheric effects, the carbon oxide, water vapor, oxygen and solar Fraunhofer line, the onboard wavelength
calibration can be processed by the spectral images themselves. In this paper, wavelength calibration is based on the
modeled and measured atmospheric absorption spectra. The modeled spectra constructed by the atmospheric radiative
transfer code. The spectral angle is used to determine the best spectral similarity between the modeled spectra and
measured spectra and estimates the wavelength position. The smile shape can be obtained when the matching process
across all columns of the data. The present method is successful applied on the Hyperion data. The value of the
wavelength shift is obtained by shape matching of oxygen absorption feature and the characteristics are comparable to
that of the prelaunch measurements.
Hyperspectral imager is now widely used in many regions, such as resource development, environmental monitoring and so on. The reliability of spectral data is based on the instrument calibration. The smile, wavelengths at the center pixels of imaging spectrometer detector array are different from the marginal pixels, is a main factor in the spectral calibration because it can deteriorate the spectral data accuracy. When the spectral resolution is high, little smile can result in obvious signal deviation near weak atmospheric absorption peak. The traditional method of detecting smile is monochromator wavelength scanning which is time consuming and complex and can not be used in the field or at the flying platform. We present a new smile detection method based on the holmium oxide panel which has the rich of absorbed spectral features. The higher spectral resolution spectrometer and the under-test imaging spectrometer acquired the optical signal from the Spectralon panel and the holmium oxide panel respectively. The wavelength absorption peak positions of column pixels are determined by curve fitting method which includes spectral response function sequence model and spectral resampling. The iteration strategy and Pearson coefficient together are used to confirm the correlation between the measured and modeled spectral curve. The present smile detection method is posed on our designed imaging spectrometer and the result shows that it can satisfy precise smile detection requirement of high spectral resolution imaging spectrometer.
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.
The routine spectral calibration is time-consuming and is difficult to use in the field. In this paper, we present a new
rapid spectral calibration method based on the holmium oxide panel which has rich of absorbed feature. The ratio of the
Spectalon panel signals to the holmium oxide panel signals can be used to calibrate the imaging spectrometer. The
spectral resolution of the spectrometer and the wavelength position on the detector array are determined by curve fitting
method including Gaussian response model setting and spectral convoluting. The iteration strategy and Pearson
coefficient are mix used to confirm the correlation or similarity between the measured and modeled spectral curve.
Spectral calibration experiment shows that new method base on the absorbed feature panel can fulfill the quick, accurate
and repeatable spectral calibrating requirement.
This paper presents a radiometric calibration method based on visibility function and uniform source system.
The uniform system is mainly comprised of an integrating sphere and a monitoring silicon detector. The current of the
silicon detector with a visibility function filter corresponds to the luminance at the exit port of integrating sphere through
standard luminance meter transfer. The radiance at the camera entrance pupil is calculated for different solar zenith
angles and Earth surface albedos by the MODTRAN atmospheric code. To simplify the calibration process, the radiance
at its entrance pupil is integrated by visibility function. The shift smear of the frame transfer CCD is removed by the
radiometric calibration and the amending ratio factor is introduced in the retrieving methods. The imaging experiment
verifies the reliability of the calibration method and retrieves good quality image.
The huge data volume of hyperspectral image challenges its transportation and store. It is necessary to find an effective
method to compress the hyperspectral image. Through analysis and comparison of current various algorithms, a mixed
compression algorithm based on prediction, integer wavelet transform and embedded zero-tree wavelet (EZW) is
proposed in this paper. We adopt a high-powered Digital Signal Processor (DSP) of TMS320DM642 to realize the
proposed algorithm. Through modifying the mixed algorithm and optimizing its algorithmic language, the processing
efficiency of the program was significantly improved, compared the non-optimized one. Our experiment show that the
mixed algorithm based on DSP runs much faster than the algorithm on personal computer. The proposed method can
achieve the nearly real-time compression with excellent image quality and compression performance.
Modulation Transfer Function (MTF) is the spatial frequency response of imaging systems and now develops as an
objective merit performance for evaluating both quality of lens and camera. Slanted-edge method and its principle for
measuring MTF of digital camera are introduced in this paper. The setup and software for testing digital camera is
respectively established and developed. Measurement results with different tilt angle of the knife edge are compared to
discuss the influence of the tilt angle. Also carefully denoise of the knife edge image is performed to decrease the noise
sensitivity of knife edge measurement. Comparisons have been made between the testing results gained by slanted-edge
method and grating target technique, and their deviation is analyzed.
The designed hyperspectral imaging system is composed of three main parts, that is, optical subsystem, electronic
subsystem and capturing subsystem. And a three-dimensional "image cube" can be obtained through push-broom. The
fore-optics is commercial-off-the-shelf with high speed and three continuous zoom ratios. Since the dispersive imaging
part is based on Offner relay configuration with an aberration-corrected convex grating, high power of light collection
and variable view field are obtained. The holographic recording parameters of the convex grating are optimized, and the
aberration of the Offner configuration dispersive system is balanced. The electronic system adopts module design, which
can minimize size, mass, and power consumption. Frame transfer area-array CCD is chosen as the image sensor and the
spectral line can be binned to achieve better SNR and sensitivity without any deterioration in spatial resolution. The
capturing system based on the computer can set the capturing parameters, calibrate the spectrometer, process and display
spectral imaging data. Laboratory calibrations are prerequisite for using precise spectral data. The spatial and spectral
calibration minimize smile and keystone distortion caused by optical system, assembly and so on and fix positions of
spatial and spectral line on the frame area-array CCD. Gases excitation lamp is used in smile calibration and the keystone
calculation is carried out by different viewing field point source created by a series of narrow slit. The laboratory and
field imaging results show that this pushbroom hyperspectral imaging system can acquire high quality spectral images.
Remote sensing is one of the most defective means for environment monitor, resource management, national security
and so on, but existing conventional satellites are too expensive for common users to afford. Microsatellites can reduce
their cost and optimize their image products for specific applications. Space camera is one of their important payloads.
The trade-off faced in a cost driven camera design is how to reduce cost while still have the required reliability. This
paper introduces our path to develop reliable and low-cost space camera. The space camera has two main parts: optical
system and camera circuits. Commercial off-the-shelf (COTS) lenses are difficult to maintain their imaging performance
under space environment. Our designed optical system adopts catadioptric layout, so that its temperature sensitivity is
low. The material and structure of camera lens can bear the vibration and shock during its launch. Its mechanical
reliability is approved through mechanical test. A window made of synthetic fused silica is used to protect the lens and
CCD sensor from space radiation. Optical system is completed with compact structure, wide temperature range, large
relative aperture, high imaging quality and pass through the mechanical test, thermal cycling and vacuum thermal test.
Modular concept is developed within the space camera circuit, which is composed of seven modules which are power
supply unit, microcontroller unit, waveform generator unit, CCD unit, CCD signal processor unit, LVDS unit, and
current surge restrain unit. Module concept and the use of plastic-encapsulated microcircuits (PEMs) components can
simplify the design and the maintainability and can minimize size, mass, and power consumption. Through the
destructive physical analysis (DPA), screening, and board level burn-in select the PEMs than can replace the
hermetically sealed microcircuits(HSMs). Derating, redundancy, thermal dissipation, software error detection and so on
are adopted in the camera design phase. The degree of reliability of the circuits can achieve 0.98/0.5Year. Environmental
tests, including vacuum thermal test, thermal cycle test and radiation test, verify the component reliability in the space
Thermal environment adaptability is an important aspect which should be involved in the development and test of a
space camera. Generally, vacuum thermal test and thermal cycle test are two important thermal tests to ensure the
reliability of a space camera. In this paper, vacuum thermal test and thermal cycle test of a space camera are introduced.
During the test, we check if the camera can work normally and evaluate performance of the camera under different
temperature. The performance is evaluated by the modulation transfer function (MTF) of the camera. According to the
measured MTF curve, the influence of temperature on performance of this camera is evaluated.
Proc. SPIE. 7156, 2008 International Conference on Optical Instruments and Technology: Optical Systems and Optoelectronic Instruments
KEYWORDS: Signal to noise ratio, Clocks, Image segmentation, Image processing, Field programmable gate arrays, CCD cameras, Signal processing, Charge-coupled devices, CCD image sensors, Camera shutters
Frame transfer charge-coupled device (CCD) sensors have several characteristics which are suitable for spectroscopic
analysis, scientific imaging, industrial measurement and so on. A simple platform for frame transfer CCD has been
developed in this paper. The platform can implement variable integral time and pixel binning which can dynamically
alter the "grain size" of frame transfer CCD and correspondingly alter the "photographic speed" of the device. With the
fixed optical system and pixel size, integral time changing and pixel binning can allow adjusting the dynamic range to
suit the source intensity. The integral time Elongation can detect the low intensity of the scene but depress the
recognition of the moving objects. Pixel binning can intensify the capability of charge collection and further reduce CCD
Readout noise. The platform we design can satisfy the complex and precise time sequences of CCD driving and process
sequences, including CCD driving clocks, electronic shutter signal, A/D and black pixels clamp clocks and double
correlation sampling clocks, pixel binning signal and so on. Images with various pixel binning and integral time have
been acquired by using this platform on the CCD circuit system board which has been designed by our team.
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.
Star trackers determine attitude by identifying stars imaged on the image sensor via an optical system whose
performance is required to meet the star identification algorithm. The method to determine parameters of the optical
system is proposed based on the identification algorithm. These parameters include focal length, aperture, and field of
view (FOV). Aberration correction requirements are also analyzed. Pyramid identification algorithm utilized in this
paper is investigated. Some improved approaches are presented for star map processing, onboard catalog organization
and star identification. A link table construction is designed to save brightness from the programmed APS sensor which
decreases data effectively and enhances the ability to calculate star positions in star maps. A method is developed to
organize the onboard catalog which avoids searching and comparing similar star pairs but makes for rapid and unique
identification. When performing star identification with Pyramid identification algorithm, only X brightest stars are
chosen from star maps to acquire high signal to noise ratio and decrease spikes. Star number statistics is fulfilled all over
the sky with any orientations by varying FOV and limited magnitude. Base on the requirement of the identification
algorithm, parameters of the optical system are determined with the given STAR1000 APS sensor by analyzing their
feasibility in optical design. According to these determined parameters, a star camera is designed. An onboard catalog on
which star identification relies is produced. Star identification simulation is implemented. Simulation result proves that
the designed system gets a satisfactory performance.
Frame transfer area-array CCD camera is the perfect solution for high-end real-time medical, scientific and industrial
applications because it has characteristics of high fill factor, low dark current, high resolving power, high sensitivity,
high linear dynamic range and electronic shutter capability. Time sequences of frame transfer area-array CCD camera
have two compact segments: CCD driving sequences and CCD signal processing sequences. Proper working of CCD
sensor lies on good driving sequences while accurate CCD signal processing sequences ensures high quality of CCD
image. The relationship among CCD camera time sequences is complex and precise. The conventional methods are
uneasy to implement time sequences of Frame transfer area-array CCD. Embedded designing method is introduced in
this paper and field programmable gate array device is chosen as the hardware design platform. Phase-locked loops are
used for precise phase shifting and embedded logic analyzer for waveform verification. CCD driving clocks, electronic
shutter signal, A/D and black pixels clamp clocks and double correlation sampling clocks have been attained on the
hardware platform and this timing generator can control exposure time flexibly. High quality images have been acquired
through using this timing generator on the CCD circuit system board which has been designed by our team.
Micro-satellite is characterized by miniaturized structure and low cost, so it is a good choice to use area array CCD space
camera as image system on micro-satellite. FT-18 is a monochrome frame transfer image sensor offering 1024×1024
pixels with excellent antiblooming and variable electronic shuttering. The main components of driving circuit for FT-18
include power supply unit, microcontroller unit, clock signal generator unit, and analog-to-digital (A/D) converter. The
microcontroller unit controls startup sequence of all voltage, the exposure time of CCD and the working status of A/D
converter; the clock signal generator unit generates sequence signals for CCD and A/D converter; the A/D converter
converts the output of FT-18 to a 12-bit digital output. Special attention should be paid to the reliability of this camera for
it will work in a condition different from ground. The camera may suffer from vacuum discharge, particle radiation,
strong shock, hypergravity and so on. All these should be considered in the design of space camera, and enough
environment tests should be done to ensure it can work normally in space.