Objective Hyperspectral imaging technology provides a novel and effective observational approach for precise quantitative monitoring scenarios, such as those in chemical engineering. The research aims to design a high-performance optical system of hyperspectral imager that combines a wide working band, large numerical aperture, high resolution, and compact volume. The primary goal is to overcome the inherent limitations of traditional Offner imaging spectrometers, such as astigmatism and large physical size, by introducing an advanced optical configuration. The proposed system is intended to support applications in chemical process monitoring, raw material quality inspection, surface defect detection, impurity screening, and environmental safety assessments, with high throughput and spectral fidelity.
Methods The research proposes an advanced Offner imaging spectrometer. Unlike conventional designs, this system incorporates a lens group between the entrance slit and the collimating mirror. This addition serves two key functions: first, it effectively shortens the distance between the slit and the collimator, significantly reducing the overall system volume; second, by designing the final surface of the lens group as a toroid surface, it mitigates astigmatism caused by the dimensional disparity between the spatial and spectral dimensions of the slit. The system maintains an object-space telecentric design and adheres to the Rowland circle condition virtually, ensuring superior optical performance (Fig.1). The analysis utilized Coddington conditions to derive equations of the system. By solving these equations, we obtain identical tangential and sagittal focal distances, thereby effectively eliminating astigmatism. The intermediate lens group is designed to form an achromatic triplet and integrated into the Offner optical path, and the entire system is co-optimized to obtain the key optimized parameters (Tab.1).
Results and Discussions The optical simulation results demonstrate excellent performances of the optical system. The Modulation Transfer Function (MTF) values across the entire field of view exceed 0.45 at the Nyquist frequency of 55 lp/mm for central and edge wavelengths. The root mean square (RMS) radii of the spot diagrams are consistently less than 4.2 μm for all fields and wavelengths (Fig.3). The designed system has a magnification of 1∶1. In the dispersion direction, the image width is approximately 8 mm, covering about 896 pixels. Given the 600 nm spectral range, the spectral sampling is about 0.67 nm per pixel. With the designed slit width of 35 μm (covering nearly 4 detector pixels), the calculated spectral resolution is 2.7 nm. Tolerance analysis is performed using the average MTF at 660 nm as the criterion. With relatively loose tolerances (tilt ≤ 2′, radius tolerance ≤0.05 mm, thickness tolerance ≤0.05 mm, decentering tolerance ≤0.05 mm), Monte Carlo simulations indicate an MTF degradation of less than 0.04 over 90% of systems, and an MTF degradation of less than 0.03 over 90% of systems. This suggests the design is robust and suitable for practical manufacturing and alignment. The spectral resolution of the prototype (Fig.4) is calibrated using a mercury lamp for initial verification and a tunable laser with a narrow linewidth (0.05 nm FWHM) for precise calibration (Fig.5), yielded a measured spectral resolution of 2.72 nm, closely matching the design value. This consistency maintains across the waveband, confirming uniform high spectral resolution. Imaging capability is tested by coupling the spectrometer with a fore telescope and performing scanning observations of a distant scene on a 2D precision stage. With a 30 ms integration time, clear monochromatic images at different wavelengths and a fused pseudo-color image are obtained (Fig.6), which successfully demonstrate practical imaging performances of the system.
Conclusions This research successfully designs, develops, and tests an advanced Offner imaging spectrometer with a wide band (400-1000 nm), large numerical aperture (≥0.17), and high spectral resolution (2.7 nm). The key innovation lies in introducing a toroidal lens group between the slit and collimator, which effectively reduces system volume and minimizes astigmatism while maintaining the inherent advantages of the Offner configuration. Theoretical analysis provided the foundation for the optimal optical layout. Simulation results confirmed excellent optical performances. Prototype testing verified that the spectral resolution meets design specifications and demonstrated imaging capabilities. The system's compact size, high performance, and relaxed tolerances make it highly suitable for practical industrial applications such as chemical process monitoring, environment monitoring, raw material evaluation, surface defect detection, and impurity screening.
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