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

最近开发的一种名为“雨成像系统”（RIS）的仪器有望提高测量雨滴大小、形状和空间浓度的能力。这些信息对降水物理和大气遥感中的某些应用至关重要。RIS由NASA开发，主要由一个卤素光源和一个CCD相机组成，两者相距4米。它记录单个液滴的前向散射图像，因为它们通过散射体积，使用非常短的曝光时间来“冻结”液滴的位置并消除图像的涂抹。当水滴落在相机的焦点区域时，会被计数和测量。景深的大小是至关重要的，因为它决定了测量液滴的体积。焦点区域内的水滴可识别为深色轮廓，光源在其中显示为“洞”。焦点区域以外的水滴没有孔洞。知道焦点区域到相机的距离可以从图像大小确定水滴大小。在一个典型的设置中，系统将相机镜头聚焦在2米的距离上，光圈为f/8。有了这些设置，相机图像一个区域名义上32毫米× 24毫米的横截面与175毫米的景深。本项目的目的是利用实验室生成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可测量的最小落差及其与焦点区域位置的依赖关系；(2)震源区域的三维形状及其可能与水滴大小和位置的关系；(3) RIS测量液滴大小和形状的精度。这些问题的答案将记录RIS的性能，并使数据处理算法得到改进。这项工作将在克莱姆森大学与最初开发该仪器的美国宇航局的拉里·布莱文博士合作进行。

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.