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首页> 外文期刊>Journal of Geophysical Research. Biogeosciences >Characterization and field use of a CCD camera system for retrieval of bidirectional reflectance distribution function
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Characterization and field use of a CCD camera system for retrieval of bidirectional reflectance distribution function

机译：CCD相机系统的特性描述和现场使用，以获取双向反射率分布函数

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Vicarious calibration and field validation is a critical aspect of NASA's Earth Observing System program. As part of calibration and validation research related to this project, the Remote Sensing Group (RSG) of the Optical Science Center at the University of Arizona has developed an imaging radiometer for ground-based measurements of directional reflectance. The system relies on a commercially available 1024 x 1024 pixel, silicon CCD array. Angular measurements are accomplished using a fish-eye lens that has a full 180 degrees field of view with each pixel on the CCD array having a nominal 0.2 degrees field of view. Spectral selection is through four interference filters centered at 470, 575, 660, and 835 nm. The system is designed such that the entire 180 degrees field is collected at one time with a complete multispectral data set collected in under 2 min. The results of laboratory experiments have been used to determine the gain and offset of each detector element as well as the effects of the lens on the system response. Measurements of a stable source using multiple integration times and at multiple distances for a set integration time indicate the sq stem is linear to better than 0.5% over the upper 88% of the dynamic range of the system. The point spread function (PSF) of the lens system was measured for several field angles, and the signal level was found to fall to less than 1% of the peak signal within 1.5 degrees for the on-axis case. The effect of this PSF on the retrieval of modeled BRDFs is shown to be less than 0.2% out to view angles of 70 degrees. The degree of polarization of the system is shown to be negligible for on-axis imaging but to have up to a 20% effect at a field angle of 70 degrees. The effect of the system polarization on the retrieval of modeled BRDFs is shown to be up to 3% for field angles of 70 degrees off nadir and with a solar zenith angle of 70 degrees. Field measurements are made by mounting the camera to a boom mounted to a large tripod that is aligned toward south. This tripod obstructs sampling of the surface reflectance past 25 degrees off nadir northward. The system is typically operated at a height of 1.5 m to view over a large sampling of surface features, such as cracks. To evaluate the surface BRDF, measurements are collected throughout the morning as a function of Sun angle. A single measurement consists of all four bands and a dark-current measurement. Data sets have been collected over several vicarious calibration sites and calibration tarpaulins. Comparisons with measurements made by a simple goniometer-based system indicate that the camera system is as accurate as the goniometer. Scattering phase function values derived from the camera system are fit to a modified Pinty-Verstraete equation. This function is shown to fit the data to better than 0.3% for data collected during an example RSG vicarious calibration experiment. Bidirectional reflectance data derived from the camera system also compare well to those predicted from the Walthall model. These BRDF models are critical for determining the applicability of measurements taken over small areas to represent the BRDF properties of an entire site, which in some cases is of the order of several kilometers in size. [References: 29]
机译：替代性校准和现场验证是NASA地球观测系统计划的关键方面。作为与此项目相关的校准和验证研究的一部分，亚利桑那大学光学科学中心的遥感小组（RSG）开发了一种成像辐射计，用于基于地面的方向反射率测量。该系统依赖于市售的1024 x 1024像素硅CCD阵列。使用鱼眼镜头可实现角测量，该鱼眼镜头具有完整的180度视场，而CCD阵列上的每个像素均具有0.2度的视场。光谱选择是通过四个以470、575、660和835 nm为中心的干涉滤光片进行的。该系统的设计使得可以一次收集整个180度视野，并在2分钟内收集完整的多光谱数据集。实验室实验的结果已用于确定每个检测器元件的增益和偏移，以及透镜对系统响应的影响。使用多个积分时间并在一段设定的积分时间内在多个距离处对稳定光源进行测量，表明平方根在系统动态范围的上限88％内线性优于0.5％。在几个视场角上测量了镜头系统的点扩展函数（PSF），对于轴上情况，发现信号水平在1.5度内降至峰值信号的1％以下。对于70度的视角，该PSF对建模BRDF的检索的影响显示小于0.2％。系统的偏振度对于轴上成像可以忽略不计，但在70度的视场角下具有高达20％的效果。对于偏离天底的70度视场角和70度的太阳天顶角，系统极化对建模BRDF的检索的影响显示高达3％。通过将摄像机安装到悬臂上进行现场测量，该悬臂安装在向南对齐的大型三脚架上。该三脚架阻碍了向北最低点25度以上的表面反射率采样。该系统通常在1.5 m的高度上运行，以查看大量表面特征（例如裂缝）的样本。为了评估表面BRDF，整个早晨根据太阳角度进行测量。单个测量包括所有四个频段和暗电流测量。已在多个替代校准站点和校准防水油布上收集了数据集。与简单的基于测角仪的系统进行的测量结果进行比较表明，摄像头系统与测角仪一样准确。从摄像机系统导出的散射相位函数值适合于修改后的Pinty-Verstraete方程。对于示例RSG替代校准实验中收集的数据，显示此功能可使数据拟合度优于0.3％。从摄像机系统导出的双向反射率数据也与从Walthall模型预测的数据进行了很好的比较。这些BRDF模型对于确定在小范围内进行测量以表示整个站点的BRDF属性（在某些情况下约为几千米）至关重要。 [参考：29]

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