The off-axis four mirrors telescope has the advantages of high image quality, high resolution and high integration, which makes it a core opti-mechanical component to adapt to the future development of multiband co-aperture airborne observation and aiming systems. The simulation and suppression of external stray light in the telescope is imperative for the airborne application since this type of systems is easily affected by strong light outside the field of view. For the asymmetry of the telescope, point source transmittance (PST) as a two-dimensional function varying with the horizontal and vertical angle of stray light was set up to evaluate the influence of external stray light. Besides, the conversion relationship between the horizontal and vertical angle and the process quantity of rotations about local coordinate axis required for opto-mechanical modeling was established. Based on that, stray light simulation model of an off-axis four mirrors telescope was constructed, then PST distribution of stray lights with different spatial angles in the whole incident hemisphere space was calculated. Furthermore, the imaging effect of long-distance scene under the influence of several stray lights with large PST peak was simulated, showing that the stray lights had great interference to airborne observation. According to the found transmission paths, the internal baffles were designed to significantly reduce the peak value of PST to reach the application indices. From the re-simulation results of the imaging effect of scene, the interference of residual stray lights was finally not obvious, which could meet the use requirements of airborne photoelectric observation and aiming system.
During the transmission of the thermal radiation in the atmosphere, the radiant energy is attenuated due to physical processes such as refraction, absorption, and scattering, which causes a change in the detection capabilities of the thermal imaging system. As a key performance of thermal imaging system, the inspection of the operating range is of great significance to the theoretical research and practical application of thermal imaging systems. Actually, the environmental condition that does not match the specified test condition when testing the operating range which are not encountered. It's complex to judge whether the thermal imaging system meets the index requirements by the test result directly. By modeling the target, analyzing and comparing the spectral transmittance under different atmospheric conditions, the paper introduces a method for judging the operating range of the infrared thermal imaging system when inspecting environmental deviates from the specified environment proposed.
In order to achieve infrared imaging spectrometers for environmental gas monitoring in the medium-wave infrared and long-wave infrared spectra, meanwhile detecting the characteristic spectra of a variety of ambient gas molecules, it is necessary to do research on the design of the infrared wide spectrum (3.2μm~14μm) imaging spectrometers optical system with a large relative aperture. First of all, the lens power of reasonable distribution will be available by solving of the wide spectrum correcting chromatic aberrations equation and athermalization equation. Secondly, the list compares the common infrared optical material in the infrared wide spectrum dispersion characteristics and thermal characteristics differences, optimizes the three infrared optical materials of the Germanium single crystal, wide spectral Zinc sulfide, and Chalcogenide glass for transmission optical system design. Thirdly, using CODE V optical design software to optimize the optical parameters, the system uses four lenses, introduces four aspherical faces, the rest are spherical. Finally, the modeling gives the 3D layout of the system, and the image evaluation and tolerance analysis of the optical system are carried out. The results show that the modulation transfer function (MTF) value of the optical system at the spatial frequency 30lp/mm is greater than 0.5. The average square root (RMS) value of the diffused spot diameter is less than 17μm; The working band is 3.2μm ~14μm, F number 1, the optical system in the temperature range of -40°C ~+60 °C has a good imaging quality. The optical system has the characteristics of large relative aperture, wide working band, good process and compact structure, and can be used for 640×480 the infrared wide-spectral segment focal plane detector.
KEYWORDS: Scene based nonuniformity corrections, Spectroscopy, Long wavelength infrared, Optical imaging, Sensors, Calibration, Nonuniformity corrections, Diffraction, Point spread functions
For spatial-modulation imaging spectrometers, the conventional non-uniformity correction method cannot efficiently reduce the pattern noise caused by stationary interference fringes on the imaging plane. In this study, we apply optical the defocus method to realize scene-based non-uniformity correction. By applying severe defocusing, the diffraction effects can be neglected, and the point spread function of the system is equivalent to the geometric projection at the exit pupil. When the light of each spot is within the field of view, it illuminates the entire detector, and the detector obtains uniform irradiance. We can implement a single-point calibration or single-plus-two calibration using the uniform irradiance.
For most uncooled infrared imager, there locates a baffle between the window of detector and the last lens of the optical system to block the internal stray radiation produced by lens cone and other structural parts. On the other hand, the baffle itself also brings another infrared radiation, and it has long been identified as a serious issue. Optimizing the surface shape of the baffle by modeling and calculating the distribution of stray energy on image plane is necessary to minimize the effects of the scattered radiation on the focal plane array (FPA). The Monte Carlo (MC) method has been verified to be an effective ray tracing technology in the computation of stray light, but for the baffle with complex heterotypic surface, the calculation by this way is very complicated and costs much time. Based on previous studies, this paper will present a MC method to trace the amount of rays radiated from the outer surface of detector, scattered by baffle and directly transmitted back to the FPA. Compared with the conventional MC rays tracing method, the way of spatial mesh discretization and gradual mesh reduction is proposed to replace the way of solving equations to search the intersection point between rays and complex surface on the baffle. As a result, it has higher computational efficiency and applicability to different shape of surfaces. Accordingly, it can be applied to the optimization calculation of baffle’s surface structure.
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