Offner Imaging Spectrometer Research Articles

Development of an Imaging Spectrometer with a High Signal-to-Noise Ratio Based on High Energy Transmission Efficiency for Soil Organic Matter Detection.

Hyperspectral detection of the change rate of organic matter content in agricultural remote sensing requires a high signal-to-noise ratio (SNR). However, due to the large number and efficiency limitation of the components, it is difficult to improve the SNR. This study uses high-efficiency convex grating with a diffraction efficiency exceeding 50% across the 360-850 nm range, a back-illuminated Complementary Metal Oxide Semiconductor (CMOS) detector with a 95% efficiency in peak wavelength, and silver-coated mirrors to develop an imaging spectrometer for detecting soil organic matter (SOM). The designed system meets the spectral resolution of 10 nm in the 360-850 nm range and achieves a swath of 100 km and a spatial resolution of 100 m at an orbital height of 648.2 km. This study also uses the basic structure of Offner with fewer components in the design and sets the mirrors of the Offner structure to have the same sphere, which can achieve the rapid adjustment of the co-standard. This study performs a theoretical analysis of the developed Offner imaging spectrometer based on the classical Rowland circular structure, with a 21.8 mm slit length; simulates its capacity for suppressing the +2nd-order diffraction stray light with the filter; and analyzes the imaging quality after meeting the tolerance requirements, which is combined with the surface shape characteristics of the high-efficiency grating. After this test, the grating has a diffraction efficiency above 50%, and the silver-coated mirrors have a reflection value above 95% on average. Finally, the laboratory tests show that the SNR over the waveband exceeds 300 and reaches 800 at 550 nm, which is higher than some current instruments in orbit for soil observation. The proposed imaging spectrometer has a spectral resolution of 10 nm, and its modulation transfer function (MTF) is greater than 0.23 at the Nyquist frequency, making it suitable for remote sensing observation of SOM change rate. The manufacture of such a high-efficiency broadband grating and the development of the proposed instrument with high energy transmission efficiency can provide a feasible technical solution for observing faint targets with a high SNR.

Thermal Design of the High-Resolution Volatiles and Minerals Moon Mapper (HVM3) Instrument on the Lunar Trailblazer Mission (LTB)

The Lunar Trailblazer (LTB) Mission consists of a spacecraft that hosts two science instruments, the High-resolution Volatiles and Minerals Moon Mapper (HVM3) and the Lunar Thermal Mapper. Flight system delivery is scheduled for the Fall of 2022. The purpose of the mission is to understand the form, abundance and distribution of water on the Moon as well as the lunar water cycle. HVM3 is a pushbroom shortwave infrared (SWIR) Offner imaging spectrometer that is optimized for the detection of volatiles to map OH, bound H2O and water ice. It has a spatial resolution of 70 m/pixel over a 20 km swath width and a spectral resolution of 10 nm over a spectral range of 0.6 µm to 3.6 µm. The spacecraft will deploy from an ESPA Grande and enter a 100±30 km lunar polar orbit where it may operate over all beta angles. The HVM3 thermal control architecture consists of active and passive elements. It leverages the passive cryogenic cooler design developed for the Moon Minerology Mapper (M3) that underwent a similar orbit. The first stage of the passive cooler is used to reject the heat of a Lockheed Martin Micro1-2 cryocooler that is used to actively cool the focal plane array (FPA). The second stage of the passive cooler is used to passively cool the optics. An overview of the overall thermal control design approach is presented.

Compact freeform-surface-based Offner imaging spectrometer with both a long-slit and broadband.

Current imaging spectrometers with conventional optical elements face major challenges in achieving a large field of view (FOV), broadband and compact structure simultaneously. In this paper, a compact freeform-surface-based Offner imaging spectrometer with both a long-slit and a broadband (CISLS) is proposed. To keep a long slit and an anastigmatic imaging, the slit off-axis amount of the initial system is within a specific range theoretically. While to achieve a compact structure, the slit off-axis amount should be away from the specific range and as small as possible. Based on the vector aberration theory and the analytical study, Zernike polynomial terms Z5 and Z6 introduce the astigmatism independent of FOV. They are utilized to well balance the astigmatism when the slit off-axis amount is away from the specific range, helping a miniaturization of the system. Other Zernike polynomial terms below the eighth order introduce the astigmatism related to FOV. They contribute to balancing the astigmatism that produced with the increasing of the FOV, thus achieving a wide FOV. The design results show that the proposed CISLS with a high spectral resolution of 2.7 nm achieves a long slit of 30 mm in length but a small size of only 60 mm × 64 mm × 90 mm in volume under a broadband from 400 nm to 1000 nm.

Analysis and design of a wide-field and large-numerical-aperture compact imaging spectrometer with a freeform surface.

Wide-field-of-view (FoV) Offner imaging spectrometers with freeform surfaces have been studied extensively in recent years. However, a design result with a large numerical aperture (NA) cannot be simultaneously obtained with this layout. We present the concept of a limited system in the tangential direction. Based on this insight, we present a new design method, to the best of our knowledge, based on the decenter anamorphic stop, which can achieve large NA in compact wide-FoV Offner imaging spectrometers with freeform surfaces. Compared to conventional imaging spectrometers with the same parameters, the light-gathering capacity of the decenter anamorphic stop-based imaging spectrometer is increased by more than 40%. In addition, based on the presented method, we design a compact imaging spectrometer with a wide FoV and large NA. The designed imaging spectrometer with a freeform surface has excellent performance. Finally, we fabricate and measure the freeform mirror. The surface irregularity of the freeform mirror is better than 1/30λ (λ=632.8n m). The result shows that the Offner imaging spectrometer with a freeform surface can be fabricated and will play a significant role in the fields of aeronautical and astronautical remote sensing.

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.

Optical Design of Improved Offner Imaging Spectrometer

An improved Offner imaging spectrometer was proposed based on the optical system characteristics of Offner imaging spectrometer, which can ensure perfect imaging quality in a wider annular region. The operating wavelength of the improved Offner imaging spectrometer ranges from 900nm to 1700nm, and the magnification is 1. Improved Offner imaging spectrometer can be obtained by changing the meniscus lens position and further optimizing the design. The results indicate that the improved Offner imaging spectrometer can effectively improve compactness and lightweight, and reduce the difficulty of optical adjustment, which is conducive to the stability of practical application.

First high-resolution tropospheric NO<sub>2</sub> observations from the Ultraviolet Visible Hyperspectral Imaging Spectrometer (UVHIS)

Abstract. We present a novel airborne imaging differential optical absorption spectroscopy (DOAS) instrument: the Ultraviolet Visible Hyperspectral Imaging Spectrometer (UVHIS), which is developed for trace gas monitoring and pollution mapping. Within a broad spectral range of 200 to 500 nm and operating in three channels, the spectral resolution of UVHIS is better than 0.5 nm. The optical design of each channel comprises a fore-optics with a field of view (FOV) of 40∘, an Offner imaging spectrometer and a charge-coupled device (CCD) array detector of 1032×1072 pixels. A first demonstration flight using UVHIS was conducted on 23 June 2018, above an area of approximately 600 km2 in Feicheng, China, with a spatial resolution of about 25 m×22 m. Measurements of nadir backscattered solar radiation of channel 3 are used to retrieve tropospheric vertical column densities (VCDs) of NO2 with a mean total error of 3.0×1015 molec cm−2. The UVHIS instrument clearly detected several emission plumes transporting from south to north, with a peak value of 3×1016 molec cm−2 in the dominant one. The UVHIS NO2 vertical columns are consistent with the ground-based mobile DOAS observations, with a correlation coefficient of 0.65 for all co-located measurements, a correlation coefficient of 0.86 for the co-located measurements that only circled the steel factory and a slight underestimation for the polluted observations. This study demonstrates the capability of UVHIS for NO2 local emission and transmission monitoring.

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.

Analysis method of the Offner hyperspectral imaging spectrometer based on vector aberration theory.

Compact hyperspectral imaging spectrometers with a wide field of view (FoV) have significant application value. However, the aberration field of such imaging spectrometers is sensitive, and varies for different wavelengths when reducing the spectrometer volume. It is difficult to explain the variation in the aberration field using traditional aberration theory. In this study, we extend the vector aberration theory (VAT) to the Offner imaging spectrometer. We deduce the expression of the aberration field decenter vector of the Offner spectrometer based on the real ray-trace method. Furthermore, we derive the expression of the third-order vector aberration of the system. Subsequently, we explain some common phenomena in the Offner imaging spectrometer. This new analysis method can provide useful guidance for designing a compact wide FoV Offner imaging spectrometer. With this new insight, we designed a compact wide FoV Offner imaging spectrometer with a freeform tertiary mirror. Compared to conventional spectrometers with the same specifications, the total length of the spectrometer decreased by 37%, and the volume by 75%. After the tolerance analysis, the freeform optics satisfied the existing machining technology. The analysis method presented in this paper furthers the designer's understanding of the aberration field of the Offner imaging spectrometer. The method is significant in the design of a compact wide FoV Offner imaging spectrometer with freeform optics.

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.

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.

Espectrómetros de imagen Offner

A hyperspectral sensor or imaging spectrometer is a device that collects spatial and spectral information in hundreds of contiguous and closely spectral bands. Hyperspectral imagers have many applications in several areas such as agriculture, geology, oceanography, forestry, medicine, colorimetry, security, military and others. Most of the existing hyperspectral systems were developed considering satellite or high altitude flying aircrafts applications. However, the specific development of these technologies allows their use in many other less expensive applications. In this context, the work presented in this thesis allows building high quality and low cost Offner imaging spectrometers. Rapid and efficient methods were used for the design, assembly and characterization of this type of spectrometers. Two laboratory setups developed from analytical model-based designs have been assembled showing high spectral and spatial resolutions and low distortions without specific aberrations corrected gratings nor non-spherical elements.

Offner成像光谱仪的消像差技术

Offner成像光谱仪的消像差技术

中/长波红外双衍射级次共路Offner成像光谱仪

中/长波红外双衍射级次共路Offner成像光谱仪

High-power white LED-based system incorporating a CCD Offner imaging spectrometer for real-time fluorescence qPCR measurements

An optical system for qPCR fluorescence measurements which incorporates high-power white LEDs, PMMA plastic lenses and an Offner multichannel (imaging) CCD-based spectrometer has been developed and validated. The optical system can detect twenty reaction vessels in an asynchronous manner and up to seven different fluorescent dyes (7 plex) at 1 nM dye concentrations in each of the reaction vessels. Furthermore, PCR curves obtained using the optical measurement system for a genomic deoxyribonucleic acid (DNA) template containing HEX and Texas Red fluorescent probes (fluorophores) are discussed. The spectral resolution, dynamic range and repeatability of the measurement system are < 15 nm, > 3 decades and < 1% CV, respectively.

Compact Offner–Wynne imaging spectrometers

Some ideas that were developed by Wynne to improve the performance of the Offner unit magnification imaging system are used to design Offner imaging spectrometers to which a concentric meniscus lens has been added. The new degrees of freedom associated to the meniscus lens allow removing image aberrations far from the paraxial region in a wide annular area or even more, in two concentric annular areas. This provides a great flexibility of design as it is shown below in four examples. In these examples, the slit length, the f-number or the spectral resolution has been optimized while retaining almost diffraction-limited spot size, with rather separate slit and image, and getting quite compact designs in comparison to the original Offner system. In particular, the most compact system (~50×40×30mm3) has an f-number of 1.86, a slit length of 9.6mm and covers a spectral range from 400 to 1000nm, being diffraction-limited for wavelengths longer than 450nm.

高成像质量Offner成像光谱仪的理论分析与研制

The Offner imaging spectrometer was built on the basis of the double mirror system,and its initial structure was obtained through solving optical path function.Through simulating and optimizing the initial structure using professional optical design software(Code V),the high image quality of spectrometer was obtained.Its root mean square spot diameter decreased obviously,and modulation transfer function curves in meridianal and sagittial had good imaging results.The study can eliminate overly complex aberration derivation process,and obtain high image quality Offner spectrometer structure in a wide spectrum(400~1 300nm).It can provide a theoretical basis for the construction of the spectrometer.

Offner型成像光谱仪波长使用范围和光谱分辨率研究

The parameters and property of Offner imaging spectrometer were studied according to eliminating aberration structure.The calculating equations of measurable range of wavelength,linear dispersion and spectral resolution were deduced by geometry method based on eliminating aberration structure of Offner imaging spectrometer;the relationship of resolution and each factor were analyzed,these factors including width of entrance slit,convex grating resolution and pixel size of detector.When the system aberration was to be ignored,the spectral resolution could be enhanced by decreasing width of entrance slit,this conclusion was verified by imaging experiment.The techniques and methods of improving spectral resolution were summarized,the tradeoff balance between each parameter and spectral resolution were considered.The final conclusion was as follow:the resolution of Offner imaging spectrometer was determined by the lowest resolution of the entrance slit,convex grating and pixel size of CCD.

Offner成像光谱仪建模及像差分析

Offner成像光谱仪建模及像差分析

Offner成像光谱仪建模及像差分析

Offner成像光谱仪建模及像差分析