Influence of Spectral Emissivity of Temperature-Varying Window on Medium Wave Imaging Detection
Objective Infrared windows find extensive use in industrial,military,and aerospace applications.Infrared windows are used in a variety of high-temperature operations,such as boilers,ovens,and kilns.They are also used in infrared optoelectronic systems for high-speed or ultra-high-speed aircraft and guide heads.Positioned at the vanguard of environmental exposure,these windows serve to shield internal mechanisms,facilitate signal transmission,and sustain the sensor's optical and structural integrity.The assessment of material performance under extreme conditions and the elucidation of thermal radiation's effect on detection capabilities are paramount for researchers.We evaluate the effect of thermal radiation from zinc sulphide(ZnS)infrared windows materials on mid-wave infrared imaging through simulation and experimental studies.We introduce an evaluation method that significantly enhances the efficacy of material development and selection processes.Methods We analyze the thermal radiation characteristics of a typical ZnS infrared window under external heating conditions,as well as the temperature distribution and thermal stress variation rules of the window with heating time,by establishing a thermal radiation calculation model and finite element simulation.The experiment procedures include the use of an infrared window thermal radiation measurement device to obtain the spectral radiation characteristics of the ZnS infrared window and face source blackbody at different temperatures.The spectral emissivity of the infrared window is calculated from the spectral data.At the same time,the images of the ZnS infrared window at the corresponding temperatures are obtained,and the influence of the thermal radiation interference of the window on the image quality is analyzed based on the mean grey value,contrast,and other parameters of the images.Results and Discussions The result shows that,by applying different temperatures to the outer surface of the ZnS infrared windows,the temperature escalation is directly proportional to the increase in external surface temperature,with a stabilization of the temperature rise curve after approximately 20 s(Fig.3).As the temperature increases,the temperature distribution within the ZnS infrared window changes.There is a temperature gradient,and the temperature of the window is conducted from the outer surface to the inner surface,resulting in a gradual decrease in temperature from the center to the edges.The material of the infrared windows experiences stress changes simultaneously with rapid changes in thermal radiation due to temperature fluctuations(Fig.5).Simultaneously,as the temperature increases,it can be observed that different positions are subjected to varying thermal stresses,although the difference between the center and edge of the window is not significant(Fig.4).The ZnS infrared window shows a significant increase in both the maximum and average intensity of the mid-wave spectral radiation as the temperature increases by approximately 200 K.Specifically,the maximum intensity increases by a factor of 11,and the average intensity increases by a factor of 23(Fig.7).The spectral emissivity of the window is calculated using the energy method.As the window temperature increases,the spectral emissivity increases with wavelength at low temperatures and decreases with wavelength at high temperatures(Fig.10).Upon analysis of the mid-wave infrared image,it appears blurred as the temperature of the window increases.The grey scale and contrast error of the image reveal that the average grey scale of the image target is approximately 3 times higher than the background in all directions,while the highest contrast value of the entire image is 3 times that of the lowest value.Conclusions The thermal radiation properties of ZnS infrared windows increase exponentially with increasing temperature.A pronounced temperature gradient is observed longitudinally,from the outer surface of the heated end to the inner surface,with a higher temperature gradient at the outer surface.Transversely,uniform temperature distribution is observed at the center of the window,with non-uniformity at the edges.The temperature increase induces internal stress variations in the infrared window material,leading to refractive index alteration.These refractive index changes are identified as the primary cause of the observed degradation in infrared spectrum(IR)imaging quality.Consequently,the contrast disparity between the target and the top,bottom,left,and right backgrounds has a negative effect.The spectral emissivity of the IR window is between 0.02 and 0.03,increasing with wavelength at lower temperatures and decreasing with window temperature.A reduction in spectral emissivity correlates with diminished image quality.With a temperature rise within 200 K,image contrast is reduced by a factor of three,culminating in a significant degradation of overall image quality.
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