Demonstration of the Performance Characteristics of the Rain Imaging System (RIS)

最近开发的一种名为雨滴成像系统(RIS)的仪器有望提高测量雨滴大小、形状和空间浓度的能力。这一信息对降水物理和大气遥感中的某些应用至关重要。由NASA开发的RIS基本上由一个卤素光源和一个相距4米的CCD相机组成。当单个液滴通过散射体时，它会记录下它们的前向散射图像，使用非常短的曝光时间来冻结液滴的位置，并消除图像的污点。当水滴落在相机的焦点区域时，对水滴进行计数和测量。景深的大小至关重要，因为它决定了测量水滴的体积。焦点区域内的水滴可识别为黑色轮廓，光源在其中显示为一个“洞”。焦域外的液滴没有空洞。知道焦点区域到相机的距离使得能够根据图像大小确定液滴大小。在典型的设置中，系统将相机镜头对焦距离为2米，光圈为f/8。在这些设置下，相机拍摄的区域标称为32 mm x 24 mm的横截面，景深为175 mm。该项目的目的是使用实验室产生的已知大小的水滴通过焦点体积的不同部分来评估和量化RIS的性能。实验将确定(1)RIS可测量的最小液滴及其与焦点区域位置的关系；(2)焦点区域的三维形状及其可能与液滴大小和位置的关系；(3)RIS测量液滴大小和形状的精度。对这些问题的回答将记录RIS的性能，并使数据处理算法得以改进。这项工作将在克莱姆森大学与NASA的拉里·布利文博士合作进行，后者是该仪器的发明者。

A recently developed instrument called the Rain Imaging System (RIS) offers promise of an improved capability of measuring the size, shape, and spatial concentration of raindrops. This information is critical for precipitation physics and certain applications in atmospheric remote sensing. Developed by NASA, RIS consists basically of a halogen light source and a CCD camera separated by a distance of 4 m. It records the forward-scattering image of individual drops as they fall through the scattering volume using a very short exposure time to "freeze" the drops in position and eliminate smearing of the image. Drops are counted and measured as they fall in the focal region of the camera. The size of the depth of field is crucial, because it determines the volume in which drops are measured. Drops within the focal region are identifiable as dark silhouettes with the light source appearing as a "hole" within them. Drops outside the focal region do not have the hole. Knowing the distance of the focal region from the camera enables determination of drop size from the image size. In a typical setup, the system has the camera lens focused at a distance of 2 m with an aperture of f/8. With these settings the camera images a region nominally 32 mm x 24 mm in cross section with a 175 mm depth of field. The purpose of this project is to evaluate and quantify the performance of RIS using laboratory-generated water drops of known size dropped through different parts of the focal volume. The experiments will determine (1) the smallest drop measurable by RIS and its dependence on position in the focal region; (2) the 3-D shape of the focal region and its possible dependence on drop size and position; (3) the accuracy with which RIS can measure drop size and shape. Answers to these questions will document the performance of RIS and enable the improvement of data processing algorithms. The work will be carried out at Clemson University in collaboration with Dr. Larry Bliven of NASA, who originally developed the instrument.