| Long-wave infrared(LWIR, from 8 to 12 ?m) imaging spectrometers have advantages in remote sensing, and are getting more attention in military and civilian areas with the rapid development of the infrared detectors recently, such as mine detection, chemical clouds drawing and forest fire monitoring.The optical system of the LWIR imaging spectrometer could be divided into two main subsystems, including the fore telescope objective and the spectrometer, and the latter is the core component. Remote sensing signals are weak at the wavelength range from 8 to 12?m. Therefore high luminous flux of the LWIR imaging spectrometer is required. However, high signal-to-noise ratio and miniaturization as well are hard to be realized by the traditional LWIR imaging spectrometer with plane grating. Fortunately, the concentric structure possesses the advantages of compact and simple structure, large numerical aperture, and low smile and keystone, which is suitable in LWIR imaging spectrometer. Dyson form is a typical concentric structure, and the spectrometers based on Dyson form still maintain the advantages of the concentric structure.Zn Se is chosen as the lens material according to the characters of Dyson form, and the method to get the initial structure of the Dyson spectrometer in the band of LWIR is proposed, which meets the requirements of smaller volume and higher spectral resolution. In the comparisons to spectrometers based on another typical concentric structure, Offner form, with the same slit, spectral resolution and detector, the advantages of the Dyson spectrometers in the luminous flux and the volume are proved under different F-numbers and different grating constants.A cooled LWIR imaging spectrometer with an F-number of 2 is designed based on the Dyson form. We choose a cooled Hg Cd Te detector with the F-number of 2, pixel array of 256×256, and pixel size of 40 ?m. In order to get reasonable signal-to-noise ratio and spectral resolution, we make the width of the slit equal to 40?m. A three-mirror off-axis aspherical optical system which can provide excellent slit-shaped images is selected as the fore telescope objective. In the process of the imaging spectrometer integrative design, the re-imaging method is applied to obtain a cold stop efficiency of 100%. The corrector lens in traditional Dyson spectrometers is removed and the correction of the spherical aberrations is realized by the re-imaging lenses. The designed LWIR imaging spectrometer provides a spectral resolution of 25 nm, a spatial resolution of 0.2mard and a field of view of 2.93°, and it is with a relatively small volume of 300×250×116mm3. The smile is 27?m, which is little bigger than the half of the width of the slit. This smile could be easily corrected in the subsequent data processing step. The keystone is 18?m which is in the accessible range.An uncooled LWIR imaging spectrometer with an F-number of 1.25 is designed based on the Dyson form. We choose a microbolometer with the pixel array of 640×480 and the pixel size of 17?m as the detector. In order to collect much more luminous flux, we make the width of the slit double of the pixel size of 34?m. The refractive optical system which can realize relative large numerical aperture and field of view is selected as the fore telescope objective. The designed LWIR imaging spectrometer provides a spectral resolution of 25 nm, a spatial resolution of 0.3mard, a field of view of 9.35°, and it is with a relatively small volume of 142×180×63mm3. The smile and keystone is 0.03?m and 17?m, respectively. It is obvious that the geometric distortions are all smaller than the half of the width of the slit. |
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