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数字摄像能见度观测系统中实用黑体技术的应用
APPLICATION OF PRACTICAL BLACKBODY TECHNIQUE TO DIGITAL PHOTOGRAPHY VISIOMETER SYSTEM
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摘要: 在数字摄像能见度观测系统 (DPVS) 中, 目标物的自身亮度难以实时测量, 给观测结果带来的不确定性不可忽略, 采用实用黑体技术可以大大减小乃至消除其造成的误差。作者对非黑体目标引起的测量误差进行了分析, 结果表明相对误差随着目标物反射率的增加和能见度的升高而增大, 为了保证测量精度, 需要采用反射率足够低的目标物。对于长方体结构的实用黑体腔, 采用多次反射法计算了其近轴向的半球-方向反射率与其特性参数之间的关系。在没有观测到侧壁的前提下, 其轴向反射率最大, 随着偏离轴线角度的增加, 反射率逐渐减小。轴向反射率随腔体长度与开口边长比值的增加而迅速减小, 而腔体宽度增加引起的变化不明显。另外, 作者还推导了在考虑弥散成像时对实用黑体开口尺寸最小需求的计算公式, 也给出了符合一定条件的实用黑体轴向反射率的近似计算公式, 从而提供了DPVS中实用黑体参数选择的基本原则。最后, 给出了DPVS采用非黑体和实用黑体目标计算能见度的对比试验结果, 充分说明了在DPVS中采用实用黑体技术的必要性和可行性。Abstract: In Digital Photography Visiometer System (DPVS), the uncertainty, caused by the difficulty in real time measurement of the inherent brightness of targets, can be reduced even eliminated by using practical blackbody technique.The visibility measurement error caused by non-blackbody target is analyzed.It is shown that the relative measurement error increases along with the increase of the visibility and the target's reflectivity.To ensure the measurement precision, the target with reflectivity low enough is required.The relations between the near-axis hemispherical-directional reflectivity and the feature parameters of practical blackbody with cuboid st ructure are calculated using multi-reflect method.Without observing the inner side of the practical blackbody, the axial reflectivity is maximum, and the reflectivity decreases as the angle that departs from axis increases.The axial reflectivity sharply deceases as the ratio of body length to aperture side length increases, while the change caused by increase of body width is not obvious.Additionally, in consideration of dispersion imaging, the equation used to compute the blackbody's minimum aperture size required is presented.These results provide the criteria for selecting parameters of practical blackbody in DPVS.Finally, the comparison results of visibilities calculated from observing a blackbody and from a non-blackbody target are show n, and they sufficiently indicate that it is necessary and feasible to adopt the practical blackbody technique in DPVS.
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Key words:
- Digital photography;
- Visibility;
- Practical blackbody;
- Reflectivity
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