Analytical design of an Offner imaging spectrometer

Compact multi-spectral-resolution Wynne-Offner imaging spectrometer with a long slit

Current imaging spectrometers are developed towards a large field of view (FOV) as well as high resolution to obtain more spatial and spectral information. However, imaging spectrometers with a large FOV and high resolution produce a huge image data cube, which increases the difficulty of spectral data acquisition and processing. In practical applications, it is more reasonable and helpful to identify different targets within a large FOV with different spectral resolutions. In this paper, a compact multi-spectral-resolution Wynne-Offner imaging spectrometer with a long slit is proposed by introducing a special diffraction grating with multi-groove densities at different areas. With the increasing of the groove density and the slit length, the astigmatism of the Wynne-Offner imaging spectrometer increases sharply. Therefore, the relationships between the astigmatism and both the groove density and slit length are studied. Moreover, a holographic grating is introduced. The holographic aberrations produced are utilized to balance the residual astigmatism of the imaging spectrometer. The design results show that the system is only 60m m×115m m×103m m in volume but achieves both a long slit of 20 mm in length and a waveband from 400 nm to 760 nm with three kinds of spectral resolutions of 2 nm, 1 nm, and 0.5 nm. The designed compact multi-spectral-resolution Wynne-Offner imaging spectrometer can be widely applied in the fields of crop classification and pest detection, which require both a large FOV and multiple spectral resolutions.

Dual-channel snapshot imaging spectrometer with wide spectrum and high resolution

The comprehensive analysis of dynamic targets brings about the demand for capturing spatial and spectral dimensions of visual information instantaneously, which leads to the emergence of snapshot spectral imaging technologies. While current snapshot systems face major challenges in the development of wide working band range as well as high resolution, our novel dual-channel snapshot imaging spectrometer (DSIS), to the best of our knowlledge, demonstrates the capability to achieve both wide spectrum and high resolution in a compact structure. By dint of the interaction between the working band range and field of view (FOV), reasonable limits on FOV are set to avoid spectral overlap. Further, we develop a dual-channel imaging method specifically for DSIS to separate the whole spectral range into two parts, alleviating the spectral overlap on each image surface, improving the tolerance of the system for a wider working band range, and breaking through structural constraints. In addition, an optimal FOV perpendicular to the dispersion direction is determined by the trade-off between FOV and astigmatism. DSIS enables the acquisition of 53×11 spatial elements with up to 250 spectral channels in a wide spectrum from 400 to 795 nm. The theoretical study and optimal design of DSIS are further evaluated through the simulation experiments of spectral imaging.

Deep ultraviolet high-resolution microscopic hyperspectral imager and its biological tissue detection

Ultraviolet (UV) hyperspectral imaging technology is commonly used in the field of atmospheric remote sensing. In recent years, some in-laboratory research has been carried out for substance detection and identification. In this paper, UV hyperspectral imaging technology is introduced into microscopy to better utilize the obvious absorption characteristics of components, such as proteins and nucleic acids in biological tissues in the ultraviolet band. A deep UV microscopic hyperspectral imager based on the Offner structure with F # 2.5, low spectral keystone and smile is designed and developed. A 0.68 numerical aperture microscope objective is designed. The spectral range of the system is from 200 nm to 430 nm; the spectral resolution is better than 0.5 nm; and the spatial resolution is better than 1.3 µm. The K562 cells can be distinguished by transmission spectrum of nucleus. The UV microscopic hyperspectral image of the unstained mouse liver slices showed similar results to the microscopic image after hematoxylin and eosin staining, which could help to simplify the pathological examination process. Both results show a great performance in spatial and spectral detecting capabilities of our instrument, which has the potential for biomedical research and diagnosis.

A design method for direct vision coaxial linear dispersion spectrometers

A spectrometer design method based on the prism-prism-grating (PPG) dispersion module is proposed in this paper to correct the serious nonlinear dispersion that prism and grating spectrometers and other dispersive spectrometers suffer from. First, we determine the criteria for selecting the optical materials of the PPG module by analyzing the dispersion characteristics of prisms and gratings. Second, a loop traversal algorithm is used to optimize the system structure parameters after selecting optical materials. Next, the direct vision coaxial condition of the PPG module is derived according to basic optical principles and the geometrical relationship between optical elements. Then, the dispersion equation of the PPG module is used to establish the spectral linearity index of the system. Finally, combined with the design index, the structural parameters of the PPG module to meet the linear dispersion requirements are determined. A direct vision coaxial linear dispersion spectrometer is designed and realized under the condition that the working band is 400-990 nm, the deviation angle and offset of the emitted ray with a central wavelength of 695 nm with respect to the optical axis are 0, and the dispersion angle is not less than 15°. The results simulated by ZEMAX show that the actual simulation results are consistent with the theoretical calculation results, the spectral resolution of the spectrometer is less than 1.5 nm, and the spectral smile and keystone are less than 3.89% pixels. In the discussion section, the influences of the dispersion ability of optical materials and the incident angles of prisms and gratings on the spectral dispersion linearity of the PPG module are analyzed and studied. The universality of the spectrometer design method developed in this paper is discussed, and its universality is simulated and verified in the 1000-1600 nm and 1600-2200 nm bands. In addition, some advantages compared with other dispersion structures are analyzed.

High spectral resolution compact Offner spectrometer based on the aberration-reduced convex holographic gratings recorded by spherical waves under Rowland circle mounting

High spectral resolution, excellent imaging quality, and compact configuration have become a recent trend in push-broom imaging spectrometers. The concentric Offner imaging spectrometer has become popular due to its high optical performance and compactness. However, astigmatism is the dominant residual aberration in the Offner imaging spectrometer, which makes the meridional and sagittal images unable to be focused well and causes a deterioration in image quality and spectral resolution. In this paper, we present a compact Offner imaging spectrometer with a high resolution based on an aberration-reduced convex holographic grating (ACHG), which is recorded by spherical waves under Rowland circle mounting. The holographic aberration coefficients of ACHG and geometric aberration coefficients of the Offner imaging spectrometer are derived based on the analysis of the light-path function. Furthermore, we analyzed the relationship between holographic aberration coefficients and holographic recording parameters of ACHG under Rowland circle mounting. To balance the geometric aberration of the Offner imaging spectrometer, proper holographic aberration coefficients of the ACHG are achieved through adjusting the holographic recording parameters. The design result indicated that the Offner imaging spectrometer with ACHG provides better images than those with mechanically ruled convex grating (MRCG). Moreover, the spectral resolution is significantly improved. This lays down a theoretical basis for subsequent construction work in the Offner imaging spectrometer with holographic aberration-reduced gratings.

5D-fusion sensing via interference illumination and polarization imaging

This study proposes a polychromatic interferometric illumination and polarimetric sensor-based imaging method for spectrum, polarization, and 3D shape, which are significant physical parameters of feature analysis for target detection. 5D-fusion sensing refers to the joint detection and fusion of the above 5D information, which is currently a great challenge. The method generates a polychromatic interference pattern using a Sagnac lateral shearing interferometer and projects it to the target. Then, interferograms modulated by the target are acquired during scanning. Fast Fourier transform (FFT) is performed on the interferograms to obtain their frequency spectra. The spectral and polarization information is extracted from the moduli of the frequency spectra. The 3D shape is recovered from the phase of the frequency spectra using the calibration data. The theory of 5D-fusion sensing is investigated, and verification experiments are then performed. The experiments indicate that the proposed method can fulfill 5D-fusion sensing in one scanning and with FFT using only one device compared with other separate methods. Consequently, the proposed method can improve the sensing and recognition ability of optical imaging technology, which provides great application potential in biomedicine, food safety, material analysis, criminal investigation, archeology, and other fields.

Wide-field grating-prism imaging spectrometer: optical design and implementation

A wide-field imaging spectrometer based on a grating prism is proposed. The grating and prism parameters are discussed to balance spectral distortion over the entire band. The design method of the grating-prism (GP) dispersive module and the catadioptric optics of the spectrometer are discussed in detail. A high optical speed (F/2.4) and long slit (29.4 mm) visible and near-infrared design with high image quality and small distortion is presented. The results show that the optical performance of the GP imaging spectrometer is excellent. The tolerance analysis indicates that the GP spectrometer can be easily manufactured and implemented. The prototype has been tested in the laboratory and outdoors, and the results confirmed that the design will be useful in the fields of aeronautics and astronautics.

Design and manufacture of miniaturized immersed imaging spectrometer for remote sensing

A miniaturized immersed imaging spectrometer possessing long slit and large relative aperture is proposed. It is integrated monolithically with three concentric optical components. Through the chief ray tracing, we analyzed its astigmatism characteristic and then deduced the anastigmatic condition for long slit. Meanwhile, it is athermal by matching the suitable lens materials to compensate thermal defocus. Based on the anastigmatic and athermal conditions, we have designed a miniaturized VNIR immersed imaging spectrometer and developed the prototype. Its slit is 48 mm long and it is optically fast with an F-number of 2.5. The new form shows about 12 times smaller in volume than the classic Offner-Chrisp imaging spectrometer and weighs only 2 kg, and has excellent thermal adaptability while temperature changes between -40 °C and 60 °C. Meanwhile, fabrication of core elements and gluing process of the immersed imaging spectrometer are presented. Test results of the prototype show superior performance with high imaging quality and small smile and keystone distortions. Such miniaturized immersed imaging spectrometer will greatly improve the performance and reduce the costs of wide swath hyperspectral remote sensing, and is desirable for usage on small plane or satellite platforms.

Freeform based hYperspectral imager for MOisture Sensing (FYMOS)

We present FYMOS, an all-aluminum, robust, light weight, freeform based, near infrared hYperspectral imager for MOisture Sensing. FYMOS was designed and built to remotely measure moisture content using spectral features from 0.7-1.7µm integrating an InGaAs sensor. The imaging system, operating at F/2.8, is based on the three-concentric-mirror (Offner) spectrograph configuration providing a spectral resolution of 8 nm optimized for broad spectral coverage with sufficient resolution to make assessments of water levels. To optimize the optical performance, whilst minimizing weight and size, the design incorporates a bespoke freeform blazed grating machined on a commercial 5 axis ultra precision diamond machine. We achieve a 30% improvement on the RMS wavefront error in the spatial and spectral fields compared to a conventional Offner-Chrisp design with similar aperture and the monolithic Primary/Tertiary mirror eases the manufacturing assembly whilst minimizing weight. We demonstrate the performance of FYMOS by measuring the evaporation rate of water on a soil sample and results are processed with a physical multilayer radiative transfer model (MARMIT) to estimate the mean water thickness.

Design and optimization method of a convex blazed grating in the Offner imaging spectrometer

The convex reflective diffraction grating is an essential optical component in Offner systems, which has been widely used in imaging spectrometers. We propose a new design and optimization method for the convex blazed grating in the Offner imaging spectrometer. The method integrates the macro- and microdesign of the optical system, and it can be used to design and optimize the convex blazed grating with high diffraction efficiency. Traditional geometric optics theory and image quality evaluation methods are used to design the macro-optical structure parameters of the Offner system. And then the incident ray information, such as the incident angle and the polarization states are calculated by using the three-dimensional polarization ray-tracing method. To improve the diffraction efficiency, we combine rigorous coupled wave analysis and a particle swarm optimization algorithm to optimize the microstructure parameters of the convex-blazed grating. Further, a convex-blazed grating in a mid-wave infrared Offner imaging spectrometer is designed as an example to illustrate our design method in detail. The design results indicate that the Offner imaging spectrometer has good imaging quality, and the average diffraction efficiency of the -1st diffraction order of the convex-blazed grating in the spectral coverage 3-5 µm is 82.24%. Compared to the traditional design method, the lowest spectral diffraction efficiency is improved from 59.88% to 69.24%, the highest spectral diffraction efficiency is improved from 90.45% to 91.84%, and the standard deviation is reduced from 7.82 to 6.62.

A High Optical Throughput Spectral Imaging Technique Using Broadband Filters

To address the miniaturization of the spectral imaging system required by a mounted platform and to overcome the low luminous flux caused by current spectroscopic technology, we propose a method for the multichannel measurement of spectra using a broadband filter in this work. The broadband filter is placed in front of a lens, and the spectral absorption characteristics of the broadband filter are used to achieve the modulation of the incident spectrum of the detection target and to establish a mathematical model for the detection of the target. The spectral and spatial information of the target can be obtained by acquiring data using a push-broom method and reconstructing the spectrum using the GCV-based Tikhonov regularization algorithm. In this work, we compare the accuracy of the reconstructed spectra using the least-squares method and the Tikhonov algorithm based on the L-curve. The effect of errors in the spectral modulation function on the accuracy of the reconstructed spectra is analyzed. We also analyze the effect of the number of overdetermined equations on the accuracy of the reconstructed spectra and consider the effect of detector noise on the spectral recovery. A comparison between the known data cubes and our simulation results shows that the spectral image quality based on broadband filter reduction is better, which validates the feasibility of the method. The proposed method of combining broadband filter-based spectroscopy with a panchromatic imaging process for measurement modulation rather than spectroscopic modulation provides a new approach to spectral imaging.

Nonnull interferometric testing of spherical gratings under Littrow conditions with opposite diffraction orders

Diffracted wavefront measurements are qualitative and comprehensive verifications for the spherical grating that was manufactured to specifications. Direct interferometric testing of the diffracted wavefront is convenient and implemented by tilting the spherical grating at a Littrow angle to obtain autoreflection and then results in a nonnull interferometric testing configuration. The diffracted wavefront of the spherical grating contains not only wavefront errors induced by the manufacturing imperfections but also inherent wavefront contributions from the autoreflection testing setup. The magnitudes of the latter are affected by both the spherical substrate and the groove pattern. Through the analysis of geometric aberrations of spherical gratings, the groove pattern contributions are demonstrated to be contrary for the opposite diffraction orders. A nonnull interferometric testing of spherical gratings is proposed without foreknowledge of the groove pattern, in which the wavefront errors contributed only by the manufacturing imperfections are derived from dual measurements under Littrow conditions with opposite diffraction orders. Simulations are implemented for varied line spacing (VLS) spherical gratings with an F-number slower than 1.5 and groove density varying from 150 to 300 lp/mm, and the residual error less than 0.004λ RMS is obtained. The residual misalignment error after conventionally removing defocus and tilt is further analyzed and discussed. A VLS grating in which the NA is 0.13 and groove density is 200 lp/mm is chosen as an experimental sample, and the diffracted wavefront error with 0.018λ RMS is obtained.

An Airborne Offner Imaging Hyperspectrometer with Radially-Fastened Primary Elements

We propose a new layout for the Offner imaging hyperspectrometer that is utilized onboard small space vehicles. The layout is based on a method of adjusting the adaptive temperature-dependent optical scheme by moving just two coaxial optical components located inside the hyperspectrometer. We present the results of modeling for a temperature range of -40 to +45 °C and an optical experiment using a heat and cold chamber for the range of 12 to 40 °C, proving the basic functionality of the proposed layout. Based on simulation results, the new layout is supposed to allow the hyperspectrometer to operate in a temperature range of -40 to +45 °C without its optical characteristics deteriorating, thus making it suitable for work onboard space or stratospheric vehicles.

Analytical design of a high-performing +1st order diffraction convex grating imaging spectrometer

A novel concentric spectrometer having one convex grating and one concave mirror, working at ${+}{1}$+1st order diffraction, and with a small size, high resolution, and high diffraction efficiency, is proposed. It can simultaneously achieve high resolution and compactness by increasing the grating groove density. A compact spectrometer operating at a wavelength of 740-790 nm with an excellent imaging quality is designed. Its spectral resolution reaches 0.049 nm, and its diffraction efficiency improves by 27% compared to the conventional Offner spectrometer with convex grating working at ${-}{1}$-1st order diffraction. This is suitable for small, light, and low-cost atmospheric gas monitoring satellites.

Optical Design of a Miniaturized Airborne Push-Broom Spectrometer

Combining the requirements of spectrometers for unmanned aerial vehicle platforms, a miniaturized airborne wide-angle push-broom imaging spectrometer with an Offner configuration is designed. The system comprises an objective lens and an Offner-type spectrometer with a spectral range of 400~1000 nm and a spectral resolution of 15 nm. The objective lens and Offner spectrometer are designed in isolation before integration. The front objective lens is an inverted telephoto with a focal length of 13 mm, a relative aperture of 1/4.5, and a field of view of 54°. The frequency of the convex grating in the Offner configuration is 102 LP/mm, and the dispersion width is 2.6 mm. The modulation transfer function of the integrated system is greater than 0.4 at the Nyquist frequency in all spectral bands. To estimate the volume and weight of the system, a preliminary optical–mechanical design scheme is given in this paper. The entire spectrometer has a volume of 130 × 80 × 120 mm and is less than 3 kg, which realizes the miniaturization design of the imaging spectrometer with a wide field of view for unmanned aerial vehicle platforms.

Broadband astigmatism-free Offner imaging spectrometer with high resolution

An advanced optical design has been proposed for an astigmatism-free Offner imaging spectrometer with high resolution in the broadband spectrum. The anastigmatic theory is thoroughly analyzed. Astigmatism is corrected by two pairs of lens-mirror combinations. On the basis of the concentric structure, the lens is used to make the meridional image distance equal to sagittal image distance. An example of the design has been presented with f-number of 3 working in 400-1000 nm according to the optimized theory. By the ray-tracing results, the spectral sampling is 0.6nm/pixel, and the RMS spots radii in all fields of view are less than 5 µm. The spectral keystone distortion and smile distortion are less than 0.1%. The prototype is manufactured based on theory and tolerance analysis. The achieved prototype has advantages of high spectral resolution (3 nm) and compact configuration.

Analytical design of athermal ultra-compact concentric catadioptric imaging spectrometer

An ultra-compact concentric catadioptric imaging spectrometer with large relative aperture and long slit is proposed. It consists of three optical components integrated monolithically in a concentric layout. Its astigmatism theory is discussed through tracing its chief ray and its athermalization is realized by optimizing lens materials. A high-speed (F/2.25) long-slit (48mm) VNIR design with high imaging quality and small distortions is presented. Results show a 10× reduction in volume than classic designs based on Offner-Chrisp configuration and a 1.9× reduction in length than Dyson configuration. Moreover, the design shows superior thermal adaptability with negligible decline in imaging quality while operating temperature changes between -30 ℃ and 70 ℃.

Manufacture of the compact conical diffraction Offner hyperspectral imaging spectrometer

The conical diffraction Offner hyperspectral imaging spectrometer (CDO-HIS) is a hyperspectral calibrator for monitoring the radiation stability of an ocean color sensor. The spectrometer adopts the structure of the conical diffraction Offner configuration (CDO-CF), containing a convex blazed grating to produce a nearly nondistortion and high spectral fidelity image. The theory analysis of the conical diffraction Offner is discussed and introduced to the instrument design. The optimization procedure and design results of CDO-HIS and the conical diffraction grating are provided based on the design ideas, which show benefits of the employment of CDO-CF. The results of laboratory characterization are presented, including the grating diffraction efficiency and the instrument performance.

Geometric Aberration Theory of Offner Imaging Spectrometers

A third-order aberration theory has been developed for the Offner imaging spectrometer comprising an extended source; two concave mirrors; a convex diffraction grating; and an image plane. Analytic formulas of the spot diagram are derived for tracing rays through the system based on Fermat’s principle. The proposed theory can be used to discuss in detail individual aberrations of the system such as coma, spherical aberration and astigmatism, and distortion together with the focal conditions. It has been critically evaluated as well in a comparison with exact ray tracing constructed using the commercial software ZEMAX. In regard to the analytic formulas, the results show a high degree of practicality.

Optical design and evaluation of airborne prism-grating imaging spectrometer

We have focused on the optical form that is low cost while maintaining high performance for airborne application. We report the optical design as well as the alignment and test results for a push-broom imaging spectrometer. The smart architecture of the prism-grating based spectrometer ensures high uniformity and image quality. Moreover, an effective method for aligning the spectrometer is also proposed. The results of laboratory-based optical tests and a flight test confirm the easy manufacture and excellent performance. Thus, the proposed system should be suitable for use as a hyperspectral instrument that can be loaded onto airborne and unmanned aerial vehicles.

Convex blazed grating of high diffraction efficiency fabricated by swing ion-beam etching method

A swing ion-beam etching method to fabricate convex blazed gratings used in shortwave infrared hyperspectral imaging spectrometers is presented. This method solves the consistency problem of blaze angles by swing etching through the meridian direction of the gratings. The mathematical relationship of the curvature, aperture, and diffraction efficiency of convex gratings is studied to demonstrate the limitation of conventional translational lithography and the necessity of swing etching. A geometric model is built to analyze the influence of swinging speed and beam slit width on groove evolution. Convex gratings with a 45.5 gr/mm groove density, 67 mm aperture, 156.88 mm radius of curvature, and 2.2° blaze angle have been fabricated and measured where the peak and average diffraction efficiency in the shortwave infrared band reach 90% and 70%, respectively. Experimental results validate that high-efficiency convex gratings of small blaze angle and high groove consistency can be produced by swing etching, which satisfy the requirements for high spectral resolution and miniaturization of imaging spectrometers.

Efficient method to measure the spectral distortions using periodically distributed slit in hyperspectral imager

The imaging spectrometer generally shows geometrical asymmetric distortions known as the keystone and smile that are different from the regular imaging optical system. The conventional method of measuring such distortions requires a precision movement control stage and specialized optical setup. Moreover, it is even harder to measure other characteristics such as the wave front error (WFE) simultaneously and to repeat the measurements since an accumulated vast number of statistical data is required to calculate the keystone and smile. To overcome these disadvantages, a new and simple method is proposed. The newly proposed method takes images separated in fields and wavelengths utilizing a simple tool called the field identifier (FI). Then, the keystone and the smile are calculated fast and repeatedly from a single measurement image while measuring the WFE with the Shack-Hartmann sensor with the minimum change of the measurement setup. With this method, hyperspectral imager is aligned and its geometrical distortions are measured.

Comparison of hyperspectral imaging spectrometer designs and the improvement of system performance with freeform surfaces

Hyperspectral-grating-based imaging spectrometer systems with F/3 and covering the visual-near-infrared (420-1000 nm) spectral range are investigated for monitoring Earth's environmental changes. The systems have an entrance slit of 24 μm and a 6.5 nm spectral resolution. Both smile and keystone distortions are smaller than 20% of the pixel pitch. We benefit from the development in freeform technology and design 15 different systems with the help of off-axis aspheric and freeform surfaces. The potential of each system is explored with the help of nonspherical surfaces. Cross comparisons between different system types are summarized to give their advantages and disadvantages. In the end, detailed tolerancing of one selected system is presented to show the feasibility for fabrication.

Freeform spectrometer enabling increased compactness

We present optical designs with freeform optics in the context of hyperspectral imaging. Results show designs that are 5 × more compact in volume than similar designs using conventional spherical or aspherical surfaces. We will show how combining the concepts of spatial and spectral-band broadening, which will be introduced in this paper, led to the improvement in compactness that is uniquely enabled by freeform optics.

Rotatable Offner imaging system for ellipsometric measurement

To realize high spatial resolution imaging ellipsometric measurement with large field of view, we developed a rotatable Offner system with unit magnification. When the conventional Offner imaging system is tilted relative to the sample plane for the ellipsometric measurement, only a small region of plane is in focus. The rotatable Offner system developed here renders the entire object in focus through all rotations. The performance of the prototype of the Offner system and imaging ellipsometer is tested by generating maps of the ellipsometric parameters Δ and Ψ for samples such as a silicon wafer and a resolution target made of chromium film evaporated on a glass substrate.

The spectral imaging facility: Setup characterization

The SPectral IMager (SPIM) facility is a laboratory visible infrared spectrometer developed to support space borne observations of rocky bodies of the solar system. Currently, this laboratory setup is used to support the DAWN mission, which is in its journey towards the asteroid 1-Ceres, and to support the 2018 Exo-Mars mission in the spectral investigation of the Martian subsurface. The main part of this setup is an imaging spectrometer that is a spare of the DAWN visible infrared spectrometer. The spectrometer has been assembled and calibrated at Selex ES and then installed in the facility developed at the INAF-IAPS laboratory in Rome. The goal of SPIM is to collect data to build spectral libraries for the interpretation of the space borne and in situ hyperspectral measurements of planetary materials. Given its very high spatial resolution combined with the imaging capability, this instrument can also help in the detailed study of minerals and rocks. In this paper, the instrument setup is first described, and then a series of test measurements, aimed to the characterization of the main subsystems, are reported. In particular, laboratory tests have been performed concerning (i) the radiation sources, (ii) the reference targets, and (iii) linearity of detector response; the instrumental imaging artifacts have also been investigated.

Spectral broadband anastigmatic Wadsworth imaging spectrometer

A new advanced optical design based on the Wadsworth mounting for a broadband stigmatic, coma-free practical spectrometer with high imaging quality is presented. By the addition of an inclined cylindrical lens with a wedge angle, the stigmatic imaging conditions in a broad waveband have been obtained by our analysis. An example which presents excellent optical performances over a spectral broadband of 380nm centered at 570nm has been designed to certify the analysis.

A digital sensor simulator of the pushbroom Offner hyperspectral imaging spectrometer

Sensor simulators can be used in forecasting the imaging quality of a new hyperspectral imaging spectrometer, and generating simulated data for the development and validation of the data processing algorithms. This paper presents a novel digital sensor simulator for the pushbroom Offner hyperspectral imaging spectrometer, which is widely used in the hyperspectral remote sensing. Based on the imaging process, the sensor simulator consists of a spatial response module, a spectral response module, and a radiometric response module. In order to enhance the simulation accuracy, spatial interpolation-resampling, which is implemented before the spatial degradation, is developed to compromise the direction error and the extra aliasing effect. Instead of using the spectral response function (SRF), the dispersive imaging characteristics of the Offner convex grating optical system is accurately modeled by its configuration parameters. The non-uniformity characteristics, such as keystone and smile effects, are simulated in the corresponding modules. In this work, the spatial, spectral and radiometric calibration processes are simulated to provide the parameters of modulation transfer function (MTF), SRF and radiometric calibration parameters of the sensor simulator. Some uncertainty factors (the stability, band width of the monochromator for the spectral calibration, and the integrating sphere uncertainty for the radiometric calibration) are considered in the simulation of the calibration process. With the calibration parameters, several experiments were designed to validate the spatial, spectral and radiometric response of the sensor simulator, respectively. The experiment results indicate that the sensor simulator is valid.

Design and construction of an Offner spectrometer based on geometrical analysis of ring fields

A method to obtain an aberration-corrected Offner spectrometer without ray obstruction is proposed. A new, more efficient spectrometer optics design is suggested in order to increase its spectral resolution. The derivation of a new ring equation to eliminate ray obstruction is based on geometrical analysis of the ring fields for various numerical apertures. The analytical design applying this equation was demonstrated using the optical design software Code V in order to manufacture a spectrometer working in wavelengths of 900-1700 nm. The simulation results show that the new concept offers an analytical initial design taking the least time of calculation. The simulated spectrometer exhibited a modulation transfer function over 80% at Nyquist frequency, root-mean-square spot diameters under 8.6 μm, and a spectral resolution of 3.2 nm. The final design and its realization of a high resolution Offner spectrometer was demonstrated based on the simulation result. The equation and analytical design procedure shown here can be applied to most Offner systems regardless of the wavelength range.

Tandem gratings spectrometer for spectroscopy broadband anastigmatic imaging

A tandem gratings spectrometer with high imaging quality is designed. By applying the geometric analysis, the spectral broadband anastigmatic imaging conditions have been obtained. It offers an advanced design with low aberrations for the whole spectral range of the small-scale spectrometer both in the off-axis and coaxial telescope applications. A UV design exhibiting excellent optical performance is presented. The specifications of design have also been investigated.

Modified Schwarzschild imaging spectrometer with a low F-number and a long slit

A modified Schwarzschild imaging spectrometer utilizing three nonconcentric aspheric mirrors and a plane grating is designed that can handle low F-number, long slit, and broad spectral range. Based on the geometrical aberration theory and Rowland circle condition, the astigmatism-correcting method of the Schwarzschild imaging spectrometer is analyzed. The design procedure of initial parameters is programmed using Matlab software. As an example, a modified Schwarzschild imaging spectrometer operating in 400-1000 nm waveband with F-number of 2.5 and slit length of 13 mm is designed, and good imaging quality is obtained.

Pupil aberrations in Offner spectrometers

The light path function (LPF) of an Offner spectrometer is presented. The evaluation of the LPF of this spectrometer enables its imaging properties to be studied for arbitrary object and image positions, while avoiding the more complicated analysis of intermediate images generated by the diffraction grating, which is often involved. A power series expansion of the LPF on the grating coordinates directly determines pupil aberrations of the generated spectrum and facilitates the search for configurations with small low-order aberrations. This analysis not only confirms the possibility of reducing low-order aberrations in Rowland-type mounts, namely astigmatism and coma, as predicted in previous studies, but also proves that all third-order terms in the series expansion of the aberration function can be canceled at the image of the design point and for the corresponding design wavelength, when the design point is located on a plane orthogonal to the optical axis. Furthermore, fourth-order terms are computed and shown to represent the most relevant contribution to image blurring. Third- and fourth-order aberrations are also evaluated for Rowland mounts with the design point located outside the aforementioned plane. The study described in this manuscript is not restricted to small angles of incidence, and, therefore, it goes beyond Seidel and Buchdahl aberrations.

Design of Dyson imaging spectrometers based on the Rowland circle concept

We aim to show that Dyson imaging spectrometers can be easily designed by applying the concept of the Rowland circle to refracting surfaces. This allows us to conceive an analytical procedure that is based on the removal of astigmatism at two wavelengths. Following this procedure, high-optical-quality spectrometers can be designed even for high speeds. Root-mean-square spot radii less than 2.5 μm are obtained for speeds as high as f/1.5, slit lengths of 15 mm, and wavelength ranges of 0.4-1.7 μm. Design examples are presented for classical Dyson spectrometers in which the detector is glued to the glass plane surface and for spectrometers with an air gap between this surface and the image plane.

Off-plane anastigmatic imaging in Offner spectrometers

In this paper, the imaging performance of an Offner concentric imaging spectrometer is analyzed when the spectrometer entrance slit is disposed arbitrarily on the plane that is parallel to the grating grooves and contains the common center of curvature. Astigmatism-corrected designs are obtained for off-plane incidence on the grating if one point on the slit is located on the Rowland circle of the primary mirror. In this case, the combined system of primary mirror plus diffraction grating provides two astigmatic line images oriented parallel and orthogonal to the plane of diffraction, with the former located on the same plane as the slit. Consequently, these images can be brought to a single focus on this plane by the tertiary mirror if its radius of curvature is chosen properly. In addition, coma aberration is simultaneously removed. These results can be applied to the design of two-mirror or three-mirror spectrometers, generalizing the concept of the best imaging circle and providing solutions to get anastigmatic imaging for two object points and two wavelengths.

Approaches to spectral imaging hardware

Instruments used for spectral, multispectral, and hyperspectral imaging in the biosciences have evolved significantly over the last 15 years. However, very few are calibrated and have had their performance validated. Now that spectral imaging systems are appearing in clinics and pathology laboratories, there is a growing need for calibration and validation according to universal standards. In addition, some systems produce spectral artifacts that, at the very least, challenge data integrity if left unrecognized. This unit includes a comparison of the band-pass and light-transmission characteristics of electronic tunable filters, interferometers, and wavelength-dispersive spectral imaging instruments, as well as a description of how they work. Methods are described to test wavelength accuracy and perform radiometric calibration. A real-life example of spectral artifacts is dissected in detail in order to show how to detect, diagnose, verify, and work around their presence when they cannot be eliminated.

Two-wavelength anastigmatic Dyson imaging spectrometers

High-quality Dyson imaging spectrometers are designed by applying a telecentric condition for off-axis image points. By imposing this condition for two different wavelengths, designs presenting low aberrations for the whole spectral range of the system are obtained. A UV-TO-NIR fast design (f/1.5) exhibiting excellent optical performance is presented.

The Offner imaging spectrometer in quadrature

This is a proposal and description of a new configuration for an Offner imaging spectrometer based on the theory of aberrations of off-plane classical-ruled spherical diffraction gratings. This new spectrometer comprises a concave mirror used in double reflection and a convex reflection grating operating in quadrature, in a concentric layout. A very simple procedure obtains designs that are anastigmatic for a given point on the entrance slit and a given wavelength. Specific examples show that the performance of this type of system improves the performance of analogous conventional in-plane systems, when compactness and/or high spectral resolution is of fundamental importance.