The field-integrated snapshot imaging spectrometer, widely used in astronomy, remote sensing, and biomedicine, captures spatial and spectral information simultaneously. Unlike traditional pushbroom systems, these spectrometers require specialized calibration methods due to their unique imaging technique. This paper explores and validates calibration methods tailored for snapshot systems, highlighting their differences from pushbroom methods. Then this paper introduces the working principles of these spectrometers, which utilize a pinhole array, and discusses challenges such as inconsistent dispersion at discrete sampling points, chromatic distortion, and temperature sensitivity. The proposed spectral calibration employs full-spectral monochromatic spots to overcome discrete spectral distribution issues, using pixel coordinate-wavelength fitting to enhance consistency and creating a three-dimensional data cube for real-time spectral recovery. Central wavelength calibration uncertainty is 0.34nm, 6.8% of the standard 5nm spectral resolution, with techniques implemented to counter temperature-induced spectral drift. Finally, a drone airborne experiment was conducted to verify the correctness of the calibration.. This work serves as a technical reference for further development of snapshot imaging spectrometers.
The demand for high-resolution imaging of large-sized samples is increasing in current biomedicine and life sciences. Nevertheless, traditional microscope objectives encounter a compromise between resolution and field of view. This paper introduces a mesoscope objective that has a high space-bandwidth product, allowing it to attain high resolution and large field of view (FOV) at the same time. Based on aberration correction principles, a mesoscope objective is designed with focal length of 60 mm and numerical aperture of 0.47. This design allows a maximum FOV of 6 mm by optimizing materials and structure. This objective contains 14 spherical lenses made of common optical glass materials, achieving flat-field apochromatism over the wavelength range of 400-700 nm. Consequently, the modulation transfer function (MTF) curve of the mesoscope objective is close to the diffraction limit, and the RMS wavefront error throughout the whole field of view is below 0.08λ. The distortion is intended to be less than 0.5% at the periphery of the field. The lateral resolution of the objective is 0.68μm, and the field curvature is less than 5 μm. Based on the tolerance analysis results, MTF at the Nyquist frequency exceeds 0.2 when using a suitable compensator. This meets the necessary criteria for processing and practical applications. The mesoscope objective proposed in this paper exhibits considerable promise in the realm of tumour cell imaging, cranial nerve imaging, and embryonic tissue visualization.
As a snapshot 3D imaging technology, light field imaging is able to obtain the 3D information of objects in a single exposure and widely used in numerous fields. The paper analyzed the 3D imaging principles of defocused light field camera and elaborated the relationships between its performance indexes and structural parameters, then a performance index system of defocused light field camera was established. An image-telecentric optical system of defocused light field camera with wavelength range of 0.4-0.9μm, field-of-view of 24.5°, F-number of 5 was designed detailly. Aimed at the lack of a unified image quality evaluation method for defocused light field camera, it was proposed to add pupil aberration as an additional evaluation index on the basis of classical image quality evaluation. The result indicates that the designed defocused light field camera achieved excellent image quality and all indexes met the requirements.
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.
Baffle is the primary instrument for suppressing stray radiation outside of the field of view of optical system. Infrared optical systems detect thermal radiation from objects, which can be severely affected by the stray thermal radiation. The reflectivity of the vane of the reflective baffle is so high that it can reduce the stray radiation entering the optical system, and its own radiation is small, which meets the requirements of stray radiation suppression of thermal infrared optical systems. The Stavroudis baffle is the reflective baffle with ellipse and hyperboloid vane, which has a stable structure and great capability of suppression of stray thermal radiation. But its volume will increase sharply with the increase of the field of view of the optical system. In addition, the special shape of the baffle increases the difficulty of design and processing. This paper proposes an optimize design about conical reflective baffle, which can quickly and accurately design the baffle. The volume of optimized baffle is reduced by 25% compared with the baffle designed by the original method. In order to illustrate the performance of optimized baffle, comparing with the baffle with straight vane and unoptimized baffle, we simulate the stray radiation of Three Mirror Anastigmat (TMA) with optimized baffle. The simulation results show that the Point Source Transmittance (PST) of the optimized baffle is 1×10-7 when the off-axis angle is more than 60 degrees, which is lower than the other two baffles. Finally, we propose a high-efficiency and low-cost processing method for this baffle, and the mass of baffle is within 300g. The design and processing method of the reflective baffle proposed in this paper can complete the manufacture with high efficiency and low cost, which provides technical support for infrared optical systems.
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.
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.
Due to the restriction of slit width, the SNR of onboard slit-based pushbroom imaging spectrometers with high spatial resolution is difficult to make a breakthrough. In order to achieve higher SNR, Hadamard transform imaging spectroscopy is used to design the imaging spectrometer with high spatial resolution by replacing the slit with coding mask while its concept will be introduced in this paper. The SNR performance of coding imaging spectrometer used this technology will be simulated with main specifications which include orbital height of 500km, spatial resolution of 1m and spectral resolution of 5nm. As a result of the simulation, its SNR performance is 5 times superior than slit-based design in the same specifications. Moreover, other advantages of using coding imaging spectroscopy will be introduced in this paper. In addition to solve the problem that lights from other orders can’t be excluded when we use grating elements, Fery prisms are introduced to the Offner relay system to design the imaging spectrometer. The result shows that the imaging quality is close to the diffraction limit, and the prisms’ dispersive nonlinearity come out to be well corrected. So coding imaging spectroscopy turns out to be a good solution for high spatial resolution design with high SNR.
The uncooled thermal infrared imaging spectrometer has advantages of small size, low cost and so on, can monitor high temperature events in the extreme thermal environment. However,the difficulties in detecting objects with low signal energy, serious thermal infrared background radiation and other problems limit its development and application. Restraining self-radiation stray light is the key to successfully overcome these difficulties. A long-wave infrared imaging spectrometer is designed in this paper, with 7 to 14μm wavelength range and F number of 2.7. By establishing its optomechamical model, two-step method of suppressing self-radiation are studied. The first step is using a mechanical structure with a gold film. Its reflectance is more than 98% in the thermal infrared band, thereby lowering the emissivity of the lens surface. Compared to the blackened mechanical parts, the polished one can reduce the self-radiation by one order of magnitude. However, it is still two orders of magnitude higher than the irradiance of the 500K target, and the spectral image signal is submerged. The next measure is to add a shutter device at the spectrometer entrance slit, so that signals reaching on the image plane when the shutter is opened and closed can be detected consequently. The effective spectral image signal is extracted by the background noise removal algorithm. The measures effectively solve the problem that the self-radiation stray light affects the imaging quality of the uncooled thermal infrared imaging spectrometer.
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.
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