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.

Spatial-spectral resolution tunable snapshot imaging spectrometer: analytical design and implementation.

A snapshot imaging spectrometer is a powerful tool for dynamic target tracking and real-time recognition compared with a scanning imaging spectrometer. However, all the current snapshot spectral imaging techniques suffer from a major trade-off between the spatial and spectral resolutions. In this paper, an integral field snapshot imaging spectrometer (TIF-SIS) with a continuously tunable spatial-spectral resolution and light throughput is proposed and demonstrated. The proposed TIF-SIS is formed by a fore optics, a lenslet array, and a collimated dispersive subsystem. Theoretical analyses indicate that the spatial-spectral resolution and light throughput of the system can be continuously tuned through adjusting the F number of the fore optics, the rotation angle of the lenslet array, or the focal length of the collimating lens. Analytical relationships between the spatial and spectral resolutions and the first-order parameters of the system with different geometric arrangements of the lenslet unit are obtained. An experimental TIF-SIS consisting of a self-fabricated lenslet array with a pixelated scale of 100×100 and a fill factor of 0.716 is built. The experimental results show that the spectral resolution of the system can be steadily improved from 4.17 to 0.82 nm with a data cube (N x×N y×N λ) continuously tuned from 35×35×36 to 40×40×183 in the visible wavelength range from 500 to 650 nm, which is consistent with the theoretical prediction. The proposed method for real-time tuning of the spatial-spectral resolution and light throughput opens new possibilities for broader applications, especially for recognition of things with weak spectral signature and biomedical investigations where a high light throughput and tunable resolution are needed.

Non-uniformity correction of wide field of view imaging system.

Requirements for wide field of view (FOV) imaging system reflect the need for both uniform illumination as well as excellent image quality across the entire FOV. As the monocentric lens combined with a parallel array of relay imagers achieves a wide-FOV while maintaining a high resolution, we studied the monocentric cascade imaging system (MCIS). However, the imaging experiment of the prototype shows two issues, including vignetting and non-uniform image quality over the full FOV. They affect the image stitching which is necessary for wide-FOV image acquisition. This paper studies how the position of the aperture stop affects the vignetting and the local aberrations in MCIS. Moving laws of the aperture stop and its relationship with the local aberrations are presented. Moreover, aspheric surfaces on proper surfaces are introduced and studied to balance the local aberrations. Accordingly, an MCIS with uniform illumination and good image quality is presented. The MCIS achieves a wide-FOV of 116.4° and an instantaneous FOV of 0.0021°. It keeps a relative illumination exceeding 97% during the full FOV. The modulation transfer function (MTF) is over 0.285 at the Nyquist frequency of 270 lp/mm. This paper provides a profound theorical reference for further applications and developments of MCIS.

Analytical design of a cemented-curved-prism based integral field spectrometer (CIFS) with high numerical aperture and high resolution.

Snapshot hyperspectral imaging is superior to scanning spectrometers due to its advantage in dimensionality, allowing longer pixel dwell time and higher data cube acquisition efficiency. Due to the trade-off between spatial and spectral resolution in snapshot spectral imaging technologies, further improvements in the performance of snapshot imaging spectrometers are limited. Therefore, we propose a cemented-curved-prism-based integral field spectrometer (CIFS), which achieves high spatial and high spectral resolution imaging with a high numerical aperture. It consists of a hemispherical lens, a cemented-curved-prism and a concave spherical mirror. The design idea of aplanatic imaging and sharing-optical-path lays the foundation for CIFS to exhibit high-resolution imaging in a compact structure. The numerical model between the parameters of optical elements and the spectral resolution of the system is established, and we analyze the system resolution influenced by the hemispherical lens and the cemented-curved-prism. Thus, the refractive index requirements of the hemispherical lens and the cemented-curved-prism for the optimal spatial and spectral resolution imaging of the system are obtained, providing guidance for the construction of CIFS. The designed CIFS achieves pupil matching with a 1.8 f-number lenslet array, sampling 268 × 76 spatial points with 403 spectral channels in the wavelength band of 400 to 760 nm. The spectral and spatial resolution are further evaluated through a simulation experiment of spectral imaging based on Zemax. It paves the way for developing integral field spectrometers exhibiting high spatial and high spectral resolution imaging with high numerical aperture.

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.