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首先考虑图1所示的三维尾焰模型。
使用改进的SHDOM对该模型的辐射亮度进行估算,取紫外辐射波长280 nm,角度分辨率为16个天顶角,32个方位角,观测方位角与x轴夹角0°,则不同观测角下尾焰的辐射亮度分布结果如图4所示。可以看到,尾焰的辐射亮度分布会随尾焰轴向不断变化,在尾焰前部辐射亮度随半径成对称的双峰分布,而在尾焰尾部,辐射逐渐减弱并慢慢汇聚成单峰分布;当观测方向与尾焰方向垂直时,尾焰辐射亮度分布与其介质分布形状相似,而当观测方向趋近于尾焰轴向时,尾焰在xoy平面上的投影被拉伸,且整体更亮,尾焰辐射逐渐受到整个尾焰介质的影响。
根据以上得到的三维尾焰辐射亮度分布图,使用灰度级共生矩阵算法进行计算,可以分别得到火箭尾焰辐射图像特征的定量化准确描述及分级后尾焰紫外辐射的灰度图,结果如表1和图5所示。可以看出,当观测方向与尾焰方向接近时,如图5(b)所示,尾焰辐射亮度分布比图5(a)整体更强、更亮,在GLCM能量、对比度和熵的数值上更大,辐射强点主要出现在尾焰近外围区域。
Table 1. Results of image feature calculation
针对图5中不同观测角下的尾焰辐射分布,根据灰度分级算法计算出尾焰图像辐射强点的具体位置之后,采用α-shape和橡皮筋方法实现尾焰辐亮度轮廓特征的准确提取,图6(a)~(c)给出了垂直观测时的尾焰强点轮廓提取步骤,图6(d)~(f)则分别给出了观测角70°、45°和20°时的尾焰强点的轮廓提取结果。
如图6所示,辐亮度峰值部分约占全部尾焰的30%左右,且在不同观测角度下,尾焰的辐射中心分布图有着明显的差异。垂直观测时,尾焰的主要能量集中在尾焰中央区域,而近轴向观测时,尾焰强点主要逐渐出现在近喷嘴处的尾焰外围。文中提出的三维尾焰紫外辐亮度模型可以显著体现这种与实际观测相符的特征。同时,通过结合辐射中心提取的α-shape算法和橡皮筋算法可以获得更为精确的辐射强度图像轮廓模型,有效提高尾焰紫外辐射特征提取的准确率,为飞行器目标和尾焰的高分辨辐射图像特征识别提供依据和途径。
Image simulation and feature extraction of UV high resolution radiation of plume
doi: 10.3788/IRLA20200466
- Received Date: 2020-12-02
- Rev Recd Date: 2021-03-01
- Publish Date: 2021-10-20
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Key words:
- plume /
- UV radiation /
- rubber band algorithm /
- improved SHDOM
Abstract: A simulation model of the ultraviolet (UV) radiance of the aircraft plume was established based on the improved spherical harmonic discrete coordinate method (SHDOM). Based on the thermal radiation of the plume, the modified model took into account the chemiluminescence of gas molecules and multiple scattering source caused by the average optical properties of cluster particles in the plume. On account of the basic radiation transfer equation, the UV radiation of the plume at different observation angles and medium distribution was numerically calculated in the spherical harmonic discrete coordinate system, and the space distribution images simulation of high resolution three-dimensional (3-D) plume radiance was presented. At the same time, the gray-scale co-occurrence matrix (GLCM) algorithm was used to obtain the gray-scale map of the plume radiation distribution. Combined with two-dimensional super resolution rotational invariant subspace algorithm, α-shape method and the rubber band algorithm, the central position and peak contour curve of the plume radiation was calculated, and the accurate extraction of the plume radiance feature was obtained, which provide an important reference and approach for the high resolution radiation image feature recognition of aircraft target and plume.