Imaging spectrometers can simultaneously obtain spatial and spectral information of targets with high spectral resolution and high spatial resolution. They are widely used in fields such as resource surveys, environmental monitoring, and mineral exploration. Spectral distortion is directly related to data fidelity and is important indicator for evaluating the performance of imaging spectrometers. It mainly includes two main parameters: smile and keystone. In view of the shortcomings of existing spectral distortion measurement methods, such as low accuracy and low measurement efficiency, this paper proposes a multi-field and multi-band spectral distortion measurement method. This method uses a spectral line lamp to illuminate the periodic line target, and images the periodic line target to the incident slit through the relay system. After passing through the spectroscopic system, the small pixel detector receives its spectral signal, and finally completes the measurement of spectral distortion. Using this method, the spectral distortion of the Offner imaging spectrometer is measured. The smile and keystone are 0.30±0.02μm and 0.26±0.02μm, respectively, and its expanded uncertainty is better than the 0.12μm measured by the standard light source method. Experiments show that the method proposed in the paper can effectively improve the accuracy and efficiency of spectral distortion measurement, and provide a technical basis for further performance evaluation and instrument calibration of imaging spectrometers.
The preparation technology of high groove density gratings is generally complex and costly, while nanoimprint technology has the ability to prepare large quantities of high-precision micro-nano structures. Therefore, nanoimprint technology is used to study the replication process of grazing incidence high groove density gratings used in Littman-type external cavity semiconductor lasers (ECDL). A UV composite rolling technology is used to imprint the high groove density grating groove area on the ULE glass substrate, and the grating replication is completed after coating. Replicated grating performance was tested and characterized with high fidelity.
In this paper, a visible-near infrared imaging spectrometer optical system is designed, which is used to detect seawater elements and provides a favorable monitoring means for the protection of marine resources. According to the application background of ocean color detection instruments, the index parameters of the instrument are determined, and a high resolution imaging spectrum system is designed. The front telescope system adopts an off-axis three mirror anastigmat without intermediate real image, with the focal length of 590.75mm and the entrance diameter of 118.15mm;The system comprises an innovative spectrometer design with 0.6 magnification. The de-magnifying optical design allows the telescope to operate at F#/5, while the spectrometers are built in a more compact arrangement at F#/3.The main mirror and three mirrors use Zernike Fringe Sag surface to compensate for the aberration introduced by breaking the classical Offner concentric and isometric structure. The whole optical system comprises a single telescope feeding two functionally identical spectrometers. The spectral range is 0.4-0.9μm, field of view angle is 19 °, spectral resolution is 5nm and instantaneous field of view angle is 0.048mrad, the full field of view and full band MTF is more than 0.8, which is close to the diffraction limit, the RMS radius is less than 4 μm, and the smile and keystone of the spectrometer are less than 10% pixels.
The line laser triangulation instrument is a non-contact dimensional measuring instrument based on the triangulation principle. With the advantages of small size, high accuracy, good stability and wide application scenarios, it is widely used in the industrial production of product surface profile inspection. The measurement uncertainty reflects the stability of the instrument measurement results, it is one of the important indicators of the line laser triangulation instrument, whose theoretical limit is determined by the speckle contrast during the test. By suppressing the speckle on the sample surface, the measurement uncertainty can be effectively reduced. In this paper, we introduce the suppression of scattering during the instrument measurement by temporal averaging method and wavelength diversity method. The paper firstly introduces the speckle theory and the principles of speckle suppression by temporal averaging and wavelength diversity, simulates and analyzes the number of superimposed average images and the speckle suppression effect in the temporal averaging and the power ratio between different wavelength light sources and the speckle suppression effect in the wavelength diversity, respectively. Then, the above methods and effects are experimentally verified by combining with the developed line laser triangulation instrument. Finally, the improved line laser triangulation instrument is compared with the measurement results of the original instrument to calculate the degree of reduction in the measurement uncertainty of the instrument. The experimental results show that the optical method can effectively suppress the sample surface speckle, reduce the measurement uncertainty of the instrument, and lay the technical foundation for the development of a higher precision laser triangulation instrument. It has important application value.
The quality of the optical element surface directly affects the performance of the element, and the inspection and evaluation of the optical element surface is necessary for the manufacturing and processing of optical elements. Optical Coherence Tomography (OCT) is a new type of optical inspection technology, which uses the low coherence characteristics of broadband light sources to obtain the position information in the depth direction of the sample, and has the advantages of non-contact, non-damaging and high resolution. In this paper, a convex lens with a diameter of 25.4 mm was inspected using an 850 nm band fiber optic type spectral domain OCT inspection system. The inspection system mainly consists of a spectral domain OCT inspection system and a scanning device. The inspection data were then fitted to the surface using the least squares method. The PV is 0.97 μm and the root mean square residual is 0.14 μm. This experiment verifies the feasibility of OCT in the field of optical element surface inspection and provides a technical accumulation for the further development of high precision inspection of optical element surface using OCT.
Climate change caused by global warming has serious implications for the sustainable development of human society. Increasing concentration of atmospheric greenhouse gas, mainly CO2 and CH4, is an important cause of global warming. The satellite-based greenhouse gas monitor can efficiently monitor the concentration and distribution of global greenhouse gases, which is of great significance to the study of global warming. One of the important development trends of satellite-borne greenhouse gas monitors is light weight and high performance. A satellite payload is designed in this paper which is applied to monitoring greenhouse gas emissions in certain area. After analyzing the relationship between spectral resolution, Signal-to-Noise Ratio (SNR) and detection accuracy, the requirements of spectral resolution and SNR to detect 1ppm change of CO2 column concentration are determined. Then, a fixed-cavity Fabry-Perot (F-P) etalon is selected to be the spectroscopic element and its parameters are determined. Other optical system parameters are obtained based on satellite remote sensing requirement. Finally, the optical system of compact Fabry-Perot imaging spectrometer is designed according to the parameters above. This imaging spectrometer consists of three parts: telescope system, a spectroscopic system based on F-P and imaging system. The whole size of optical system of spectrometer is 160mm*160mm*480mm, it is a miniature and simplify instrument with high performance.
The development of energy production and transportation industry is seriously threatened by the methane for its flammability and explosiveness. How to monitor the methane leaks in real time is an important and difficult problem. The gas filter correlation technology has strong selectivity and fast response, so it can accurately detect the presence and concentration of methane. According to whether an artificial lighting source is used, the gas filter correlation technology can be classified as active or passive types. Passive light source like the sun has the advantages of low cost, wide coverage and being harmless to operators. The detection precision can be improved by increasing the working band to gather more radiation energy on the basis of maximizing the ratio between the total absorption of the methane and the total optical transmission of the system. In this paper, the precision of using passive gas filter correlation technology to detect methane concentration is simulated and analyzed. The working band of the detection system is chosen to be 7μm - 8μm by considering factors such as atmospheric transmission and radiation interference, and it covers the characteristic absorption peak of methane at 7.658μm. The radiation intensity of saturated absorption reference channel and non-absorbing measurement channel detected by the system under normal temperature is analyzed and compared. Based on the Lambert Beer law, the numerical relationship between methane concentration and radiation intensity was determined by multi-point fitting of simulation data, and the linear correlation reaches 0.9998. The simulation results show that the theoretical minimum gas detection limit is 0.465PPM, and the relative error at 12PPM is 10%. With the increase of gas concentration, the error decreases rapidly, and it is 3% at 35PPM. Even if the water vapor with absorption interference in the working band is added, the relative error at 35PPM is only 4%, indicating that it is insensitive to interference. It shows great potential in gas quantitative detection in the future.
Chromatic confocal measurement is a type of non-contact measurement technology, which can be used to detect three-dimensional profile of surfaces without axial scanning. It has the characteristics of high precision and high efficiency. Line scanning chromatic confocal measurement system can obtain the depth information in the linear field of view, which can get the three-dimensional information by one-dimensional scanning, having higher efficiency compared with the point scanning type. There are two different types of design in the chromatic confocal measurement system: single- axis design and dual-axis design. The dual-axis design has several advantages over the single-axis design such as smaller effective axial spot size and higher axial resolution, and the separation of illumination system and detection system can decrease the stray light and the energy loss. In this paper, we analyze the parameters of optical system according to the practical requirements and design the optical system for a dual axis scanning confocal measurement system. Its system optical magnification is 1 and the F number is 2.5, waveband covers 450-700nm. The system uses linear variable filter to generate the spectrum, which is helpful to get the simpler system structure, the lower cost and the larger axial measurement range, compared with the traditional method using grating or prism to split the light. The visible light emitted from the light source permeates through the filter and the lens group, then the light with different wavelengths is focused on different positions on the focal plane of the measured surface. The results show that the measurement range of the system is up to 3 mm, the length of the scanning contour is up to 12 mm, and the average axial resolution is up to 1.6μm, meeting the designated measurement requirements.
Accurate positioning and navigation during the vertebral pedicle puncture process in percutaneous vertebroplasty is the key to the operation. At present, there are many problems for the widely used multiple X-ray perspective, such as the nonreal-time observation and its radiation. A single-use video endoscope for vertebral pedicle puncture, which can monitor the puncture process in real time, is presented in this paper. Firstly, the specifications of the optical system are derived according to the surgical requirements. The maximum lens diameter of the designed endoscope is less than 1.4mm, and the field of view is 80°. Its focal length is 0.6mm, and the object resolution is 13lp/mm. The F/# of this system is 4.95, and the depth of field is between 3~25mm.Then, in order to fulfill the requirement of single-use endoscope, the optical system is designed with the low-cost CMOS image sensor and the injection plastic lens. After optimization, the imaging quality of the optical system is close to the diffraction limit. The MTF is above 0.5 at the spatial frequency of 130lp/mm, which is corresponding to 13lp/mm in the object plane at a working distance of 6mm. Finally, the stacked assembly method is used to design the opto-mechanical structure of a single-use video endoscope. After the above design, the video endoscope for vertebral pedicle puncture is expected to realize single-time use.
Geostationary (GEO) imaging spectrometer can provide spectral imaging data with high spectral resolution, high temporal frequency and wide coverage, and it is expected to be an important earth observation payload for the next-generation GEO satellite. For the GEO application, the imaging spectrometer is preferred to has ultra-long slit and compact size. Benefiting from the more degrees of freedom, freeform surfaces provide a feasible method to fulfil this requirement. The paper presents an optical design of the wide swath imaging spectrometer with freeform surfaces for the GEO remote sensing. The slit length is up to 240 mm. The designed imaging spectrometer is based on the Offner configuration, while both the mirrors and convex grating are freeform Zernike surfaces instead of traditional spherical surface. The introduction of the freeform surfaces can reduce the residual astigmatism induced by the long slit significantly. The root mean square radius of the spot diagram of the design is smaller than the Airy radius and the MTF is close to the diffraction limit at the Nyquist frequency. The size of the design is about 800mm×263mm×358mm. In order to make a comparison, the traditional Offner and Wynne-Offner spectrometers are also designed with the same optical parameters. Under the same imaging quality requirements, the size of the two spectrometers is about 5 times and 3 times larger than that of the proposed Offner imaging spectrometer with freeform surfaces respectively. Finally, the manufacturability of the freeform surfaces is analyzed and the tolerance analysis results are presented.
The cooled thermal infrared imaging spectrometer use blacken surface to absorb the stray radiation of outside the field of view and reduces the thermal radiation by decreasing the temperature. Though the system performance is high, it has problems such as high cost, large power consumption, large volume, and low reliability. The uncooled thermal infrared imaging spectrometer, which has the advantages of small size, low cost and long lifetime, has great potential to monitor high temperature targets such as forest fires, but the high emissivity of blacken surface will become a huge thermal radiation source, which will affect the imaging performance seriously .Its application and development are constrained by the disadvantages such as low signal to noise ratio, high infrared stray radiation and so on. In this paper, we present an uncooled long-wave infrared imaging spectrometer working between 3μm and 14μm. The system is composed of three components, including baffle, foreoptics and spectrometer. We adopt two different mechanical surface processing methods, i.e. the black nickel coating method and the surface polishing method. Then we analyzed the influence of the two methods on the internal thermal radiation at 300K, and compared two methods with the analysis results of the blacken method. We find that the radiation of the black nickel coated surface can reduce stray radiation by 20%, while the polished surface can reduce stray radiation by 50%. According to the analysis results, we can find that the polished surface method has better radiation suppression performance than the black nickel coating method for the designed uncooled thermal infrared imaging spectrometer.
Optical Coherence Tomography (OCT) is a medical diagnostic method using low coherence optical interferometry imaging. It has the advantages of non-invasive, and high resolution, and can realize three-dimensional imaging of the internal structure of biological tissues. Traditional medical imaging techniques have some defects, such as low imaging depth of confocal microscope and low resolution of ultrasonic imaging. OCT makes up for these defects and has been widely used in various fields of medicine, which has greatly improved the accuracy of clinical diagnosis. Compared with the temporal OCT technology, the spectral OCT technology has a faster imaging speed and has basically replaced the temporal OCT technology. This paper briefly describes the imaging principle of OCT technology, and analyzes the optical properties of biological tissues, and determines the design index of OCT system. A high-resolution spectral domain optical coherence tomography system was designed according to the indicators. A high-resolution spectral domain optical coherence tomography system was designed according to the indicators. The central wavelength of the light source used was 840nm, the bandwidth was 180nm, the theoretical axial resolution was 1.73μm, the theoretical spectral resolution was 0.09nm, and the imaging depth was 1.96mm in the air.
Offner imaging spectrometer consists of a convex grating and two concave mirrors. The concentric characteristics of the optical structure make it have the advantages of large relative aperture, small distortion and compact structure. In order to reduce the alignment difficulty of the Offner imaging spectrometer and improve the efficiency, a fast alignment method of Offner imaging spectrometer is presented in this paper based on the concentric characteristic and spherical autostigmatic method. Firstly, the spherical autostigmatic device is built, which can generate point source, and when the point source is located at the spherical mirror’s center of curvature (CoC), its reflection image point and the point source coincide. By measuring the distance between the reflection image point and the point source, the position deviation of the spherical mirror’s CoC can be determined. The Offner imaging spectrometer is completed with this device by locating the CoC of its primary mirror, convex grating and tertiary mirrors. The results show that location error of the two off-axis concave mirrors’s CoC can be controlled within 10μm, and the imaging performance of the imaging spectral system is satisfied. Compared with the existing method, this method has the advantages such as easy to operate, low cost and fast alignment.
Asteroid exploration has become one of the important ways to understand the origin of the universe, exploit the space mineral resources and protect the earth from the asteroid impact. The imaging spectrometer, which can acquire the spatial and spectral information simultaneously, has become one of the most important payloads for the asteroid exploration. In this paper, the optical design of a broadband Offner imaging spectrometer for the asteroid exploration is presented. It covers from 0.4μm to 3μm, and the F/# is 6 and 3 for the VNIR band (0.4μm-1.0μm) and the SWIR band (1.0μm -3.0μm), respectively. The convex grating in the imaging spectrometer is divided into two regions of different groove spacing, and each region is optimized with different blazed angles to improve the spectral response of this imaging spectrometer. The diffraction efficiency is analyzed with the Comsol software and the result is greater than 0.36 in the wavelength range of 0.4μm to 3μm. After optimized with the optical design software, the diameter of the imaging spectrometer's spot diagram is less than one pixel, and the lowest MTF is 0.460 at the Nyquist frequency. The smile and keystone is less than 10% of the pixel.
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.
Many applications like laser manufacturing, homogeneous illumination or laser-induced fluorescence spectroscopy require a uniform intensity distribution and variable size of laser beam. Conventional laser beam shapers have a homogeneous but fixed-size laser spot. In this paper, a continuous zoom beam shaper based on microlens array is designed. It is essentially a multi-channel Kohler illumination system consisting of two identical microlens arrays and a zoom lens group, which transforms a Guassian or other complex spacial intensity distributions to a uniform square distribution of variable size on the target plane. The continuous zoom beam shaper adopts mechanical compensated optical configuration. Cam curve of the continuous zoom beam shaper is smooth enough and avoids inflection point. Compared with conventional laser beam shapers, the continuous zoom beam shaper has high intensity uniformity, variable size of uniform distribution and low cost. The design method and optimum result of continuous zoom beam shaper are presented. As an example, a continuous zoom beam shaper with a zoom ratio of 3× and variable size of uniform square distribution from 4.12×4.12 mm2 to 12.36×12.36 mm2 , is designed. The zoom lens group consists of the front fixed group, zoom group, compensation group and the rear fixed group. Intensity uniformity of output beam is greater than 90% in different zoom stages. It satisfies the needs of laser applications.
The Strehl ratio (SR) describes the ratio of the on-axis intensity between the aberrated system and the diffraction limited system. It can be used to build the merit function for the optimization of phase mask in the wavefront coding system. We propose a computationally efficient method for the SR and use it to compute the SR with different defocus errors for the wavefront coding system using the rotationally symmetric phase mask. With this method, the computation efficiency of the merit function can be improved greatly in the optimization of the rotationally symmetric phase mask.
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.
The grating imaging spectrometer has the characteristics of good linearity, wide dispersion range and is widely used in the field of remote sensing. Distortions (including smile and keystone) are one of the important parameters of the grating imaging spectrometer, which directly affects the quality of the image and spectral information obtained by the imaging spectrometer. In order to get the requirements of two kinds of distortions in the design process of the grating imaging spectrometer, the effect of the smile and keystone on the target detection is simulated and analyzed respectively. Based on the spectral response function with the Gaussian, the change of the spectral signal acquired by the grating imaging spectrometer with the amount of the different smile is calculated by combining with the spectral data of the atmospheric in the visible and near-infrared (0.4~1μm). The results show that the amount of smile should be no more than 1nm, 0.6nm and 0.2nm respectively when the spectral resolutions of the imaging spectrometer are 20nm, 10nm and 5nm. With the assumption that the spatial response function is the rectangle function, the effect of the different keystone on spectral signal acquisition of the imaging spectrometer is simulated by using the hyperspectral data. The results indicate that the offset of the keystone should be controlled within 0.04d (d is the pixel width).
In this paper, the generation mechanism of stray light is analyzed for a visible and near infrared imaging spectrometer with a spectral range of 400nm to 900nm. The optical mechanical model of the instrument was established and its stray light level was simulated. Based on the notch method, A stray light measuring device is built. The veiling glare index of the imaging spectrometer is measured to be 0.84%. The uncertainty of measurement is assessed by GUM method, and the influence of uncertainty components on the measurement results is analyzed. When the confidence probability of the measuring device is 95.45%, the measurement uncertainty of veiling glare index is 0.15%. Finally, a comparison and analysis are made between the simulated values of the veiling glare index and the actual measured values. This work provides technical support for the development of high resolution imaging spectrometer.
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.
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.
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.
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.
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.
Membrane mirror with flexible polymer film substrate is a new-concept ultra lightweight mirror for space applications.
Compared with traditional mirrors, membrane mirror has the advantages of lightweight, folding and deployable, low cost
and etc. Due to the surface shape of flexible membrane mirror is easy to deviate from the design surface shape, it will
bring wavefront aberration to the optical system. In order to solve this problem, a method of membrane mirror wavefront
aberration correction based on the liquid crystal spatial light modulator (LCSLM) will be studied in this paper.
The wavefront aberration correction principle of LCSLM is described and the phase modulation property of a LCSLM is
measured and analyzed firstly. Then the membrane mirror wavefront aberration correction system is designed and
established according to the optical properties of a membrane mirror. The LCSLM and a Hartmann-Shack sensor are
used as a wavefront corrector and a wavefront detector, respectively. The detected wavefront aberration is calculated and
converted into voltage value on LCSLM for the mirror wavefront aberration correction by programming in Matlab.
When in experiment, the wavefront aberration of a glass plane mirror with a diameter of 70 mm is measured and
corrected for verifying the feasibility of the experiment system and the correctness of the program. The PV value and
RMS value of distorted wavefront are reduced and near diffraction limited optical performance is achieved. On this basis,
the wavefront aberration of the aperture center Φ25 mm in a membrane mirror with a diameter of 200 mm is corrected
and the errors are analyzed. It provides a means of correcting the wavefront aberration of membrane mirror.
Phase retrieval method can determine pupil phase directly from image formed by the optical system with no extra hardware. This method has attracted more and more attentions for its unique advantage, and has been successfully applied in many domains such as large telescope alignment, X-ray imaging and so on. In this paper, we propose a hybrid phase retrieval method for annular pupil, which is composed of two steps. In the first step, the estimated spot images is computed with analytic expression, while fast Fourier transform is used in the second step. This method can achieve high retrieval speed without sacrificing the retrieval accuracy. Numerical simulation is performed to validate this phase retrieval method.
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.
Advanced optical imaging systems should have high imaging quality and robotic environmental suitability. Such a
near-infrared lens with the Pitzval style is designed and developed. Its operation wavelength is from 0.72μm to 1.0μm
and its relative aperture as high as 1:2. Its passive athermalization design to suit for the wide operation temperature range
from -45°C to 60°C is implemented through optimal selection of its optical glasses and opto-mechamical structure.
Sharp ghost image due to even reflection at optical surfaces is eliminated with our suggested means, and thus stray light
within its image plane is both low and uniform even under backlighting. The Modulation Transfer Function (MTF) of the
designed lens at the Niquest spatial frequency 90 lines/mm of focal plane array detector is higher than 0.6 within its
operation temperature range and its entire field of view. Eighty percent of its diffraction encircled energy is within one
pixel of the detector. Its point source transmittance (PST) when the illuminating off-angle of point source is from 5 to 60
degrees, which is just out of its field of view, is computed through modeling and simulation, and as low as between 10-3
to 10-11. The experimentally measured MTF values and veiling glare index of our developed lens reaches respectively to
0.61 and 0.372% and validates our suggested design in the paper.
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.
Wave aberration of optical system describes the deviation of the exiting wavefront from perfect spherical wavefront. For field imaging, wave aberration of each field is generally different due to the existence of field-dependent aberrations induced by imperfect optical design and alignment. Wave aberration for different field is required to fully characterize the optical system. Wavefront interferometry and Shack-Hartmann wavefront sensing method are two traditional methods to measure the wave aberration. Compared to these two methods, image-based wavefront retrieval method determines wave aberration directly from image formed by the optical system with no extra hardware. This method has attracted more and more attentions for its unique advantages, and has been successfully applied in many domains such as large telescope alignment, X-ray imaging and so on. In this paper, we propose a spot image based multi-field wavefront retrieval method. As we know, wave aberration function of optical system is a function of the pupil and the field coordinates. Double Zernike expansion, which performs Zernike polynomials expansion on the pupil plane and image plane respectively, is used to fulfill field-dependent wave aberration expansion. With the coefficients of this expansion, wave aberration of every field can be obtained. The objective function and its gradient function for the multi-field wavefront retrieval are given and conjugate gradient algorithm is used to retrieve the coefficients of field-dependent wavefront expansion with the spot intensity images at different fields. Numerical simulation of this wavefront retrieval method is completed with MATLAB. The retrieval results are analyzed for both ideal and noisy conditions with different number of field points.
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.
Compared with traditional mirrors, a membrane mirror made of flexible film material has the advantages of folding and deployable, lightweight, low cost and etc, and it is prospected to be used as large aperture space optical elements. In order to solve the problem of measuring the shape of the membrane mirror, a method based on the Shack-Hartmann wavefront sensing has been studied in this paper. The Shack-Hartmann wavefront sensing system and the principle of wavefront detection are introduced firstly. In this system, the measured wavefront is collected by the microlens of the lenslets and focus on the CCD, the light spot coordinate offsets relative to the ideal spots are got and then the wavefront can be reconstructed. Secondly, according to the optical properties of a membrane mirror, the membrane mirror wavefront detection system has been designed and established. The preprocessing of the light spot image of the testing wavefront focused by the microlens array, the determination of the centroid coordination and the algorithm of wavefront reconstruction modal based on Zernike polynomials are researched and a program for reconstructing the wavefront is written with in Matlab. When measuring, in order to eliminate the influence of other optical components in the detection system on the testing wavefront, a standard plane mirror is used to calibrate the system. A glass planar mirror made of glass with a diameter of 50mm and a known distribution of surface shape is used to verify the feasibility of the test system and the correction of the algorithm. Finally, the wavefront of a membrane mirror with a diameter of 85mm is measured and the errors are analyzed. It provides a means of measuring the shape of membrane mirror.
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.
In this paper, we propose an aberration retrieval method for annular pupils using proposed parametric model of point spread function (PSF). With this model, the diffraction integral for annular pupils is expressed with Bessel-series representation and annular Zernike coefficients. Conjugate gradient algorithm is used to retrieve aberration coefficients of annular pupil from the corresponding spot intensity images. The iterative calculations of objective function and its gradient function are required for this algorithm. These calculations are accelerated by using the parametric model’s analytical expression instead of Fourier transforms. Numerical simulation and experiment are performed to validate this aberration retrieval method.
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.
Spot-image based aberration retrieval method determines aberration directly from optical
system's spot image with no extra hardware. In this paper, we propose a new spot-image based
aberration retrieval method using the parametric model of point spread function (PSF). This model is
built with scalar diffraction integral and its Bessel series representation. In this model, complex amplitude of PSF is represented by linear combination of function series with Zernike coefficients of wave aberration at pupil plane. With this PSF model, intensity distribution of spot image can be calculated without fast Fourier transform or numerical integration. Conjugate gradient method is used in our aberration retrieval routine and the proposed method is simulated with MATLAB. The retrieval results are satisfying for both noise-free and noisy star image.
KEYWORDS: Modulation transfer functions, Signal processing, Data processing, Signal attenuation, Image analysis, Optical testing, Sensors, CCD cameras, Mirrors, Fourier transforms
The signal processing flow for the MTF test bench that is based on Fourier analysis method is
presented.
The signal processing flow mainly consists of three parts that are Fourier analyzing, background
correction and system attenuation elimination. The center of the pinhole area is recognized
automatically and the line spread functions (LSF) of both sagittal and tangential directions are
calculated. Second-order fast Fourier transform is executed so that a primary two-direction MTF result
is gained. Either auto Fourier-domain background correction or manual time-domain background
correction is executed. The attenuation of the tested MTF result due to the influence of the detector and
pinhole is eliminated finally.
A commercially available 50-mm plano-convex audit lens is tested as the sample to validate the
accuracy of the signal processing flow of the MTF test bench. The test error is below 0.01 under
200lp/mm.
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.
A real-time MTF test bench for visible optical systems is presented in this paper. This test bench can perform quick
on-axis and off-axis MTF measurement of optical systems whose aperture are less than 200mm in visible wavelength. A
high quality off-axis parabolic collimator is used as object generator of this test bench. The image analyzer is a
microscopy with CCD camera installed on a multi-axis motion stage. The software of this MTF test bench provides a
good interface for the operators to set measurement parameters and control this bench. Validation of this test bench,
performed with a 50mm plano-convex audit lens, shows that MTF measurement error of this bench is within 0.04.
Besides MTF measurement, this bench can also perform effective focal length (EFL) and back focal length (BFL)
without any hardware modification. Transmittance of optical system can also be performed on this bench with an
integrating sphere.
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.
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.
KEYWORDS: Charge-coupled devices, Clocks, Camera shutters, CCD cameras, Signal processing, Signal to noise ratio, CCD image sensors, Image processing, Field programmable gate arrays, Image segmentation
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.
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.
Based on Karman's equation for symmetrical circular membrane and Qian's theory of circular membrane under the
action of uniformly distributed loads in its central portion, the relationship between the curvature of the center cross
section of a membrane mirror on a substrate of polyethylene terephthalate (PET) film and the external pressure, and the
deviation between the surface shape of the membrane mirror and that of a paraboloid mirror with the same diameter and
F/number have been calculated in this paper. The changes of the surface shape of the mirror under various initial
boundary conditions and various external pressures applied on it have been analyzed theoretically. An experimental
setup for the figuration of a membrane mirror in a diameter of 100mm has been designed and established. The pre-shape
of the membrane, the negative air-pressured deformation and the electrostatic deformation were combined to shape a
membrane mirror with a relative small curvature radius. The focal lengths of the mirror under different initial boundary
condition and different external pressures have been measured and analyzed comparing to that of the theoretical one. The
experimental results show the possibility of the method for the figuration of membrane mirrors.
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.
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.
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.
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.
As space technology develops the optical systems with aperture more than 10 meters are needed. Constrained by the
system mass and transport ability of launch vehicle, it's impossible to manufacture these optical systems by conventional
technology. It is one of the most novel technologies to manufacture large aperture, ultra-lightweight space optical
systems by using membrane mirror as the primary mirror. The membrane mirror is made of thin polymer membrane
instead of glass, so it is characterized by ultra-lightweight and low cost. Figuring membrane mirror's surface by
electrostatic field is one of the most promising methods. This method is based on the elastic mechanics and electrostatic
theories. Membrane mirror is deformed by applying electrostatic voltage between membrane and electrodes behind the
membrane. A Φ200mm membrane mirror with electrostatic deformation system has been built and this membrane mirror
can be pulled from flat mirror to a 17m radius of curvature. It consists of membrane mounting device, electrode pad and
high direct current (DC) voltage control system. The deformation of membrane mirror was observed with ZYGO
interferometer and the radius of curvature of membrane mirror was measured by Moiré deflectometry.
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