Research on multi-plane particle image velocimetry based on digital refocusing technology
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摘要:目的
光场相机在传统相机的CCD探测器前方增设微透镜阵列（Micro-lens Array，MLA），以同时捕获入射光线的空间信息和角度信息，可实现先拍摄、后对焦的数字重聚焦，进而获得物空间中任意深度平面的清晰对焦图像。为了获取三维流场中多个深度平面的速度场分布，
方法本文提出了一种基于逆向光线追迹数字重聚焦的多平面粒子图像测速技术（Particle Image Velocimetry, PIV）。为了实现光线追迹，本文重新推导了光场相机的
F 数匹配关系，计算了光场图像中子图像的渐晕区域和非渐晕区域，并通过逆向光线追迹技术，分别追迹渐晕区域和非渐晕区域发出的主光线，使其依次穿过微透镜阵列、主镜头并映射至三维物理空间，从而获得不同深度平面的粒子的重聚焦图像。随后，利用期望最大化 (Expectation-Maximization，EM)算法对重聚焦图像进行去模糊处理。最后，利用二维互相关技术计算三维流场中多平面的速度场分布。为了验证所提方法的有效性，搭建了淹没水射流实验装置并开展实验。结果实验结果表明，所提出的基于数字重聚焦技术的多平面PIV技术能实现三维流场中多平面速度场的测量。
Abstract:ObjectiveA light field camera incorporates a micro-lens array (MLA) in front of the CCD sensor to simultaneously capture both the spatial and angular information of the incident light rays, thereby enabling digital refocusing after image acquisition and generating sharply focused images at arbitrary depths in the object space. To achieve multi-plane velocity field measurements in three-dimensional (3D) flow fields,
Methoda multi-plane particle image velocimetry (PIV) method based on the backward ray tracing-based digital refocusing is proposed in this paper. First, the F-number matching relationship of the light field camera is re-derived to facilitate the implementation of the ray-tracing process. The vignetted and non-vignetted regions within the sub-images of the light field image are then calculated. Subsequently, backward ray tracing is employed to project the principal rays from different regions through the MLA and the main lens into the 3D physical space, thereby generating refocused particle images at different depth planes. Subsequently, the Expectation-Maximization (EM) algorithm is further applied to deblur the refocused particle images. Finally, the velocity field distributions at multiple depth planes are calculated using a two-dimensional (2D) cross-correlation algorithm. To validate the effectiveness of the proposed method, a submerged water jet experimental system is constructed and corresponding experiments are conducted.
ResultThe experimental results demonstrate that the proposed multi-plane PIV method based on the digital refocusing technique can achieve measurements of multi-plane velocity fields in 3D flow fields.
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Key words:
- particle image velocimetry /
- light field /
- digital refocusing technology /
- ray tracing /
- vignetting
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图 1 多平面2D PIV技术的原理示意图
Figure 1. Schematic Diagram of the Principle of Multi-Plane 2D PIV Technology
图 3 不同光场相机的F数匹配原理示意图
Figure 3. Schematic Diagram of F-Number Matching Principle for Different Light Field Cameras
图 5 子图像的渐晕和非渐晕示意
Figure 5. Schematic of the vignetting and non-vignetting of the sub-image
图 6 非渐晕区域的主光线的逆向光线追迹示意图
Figure 6. Schematic of the backward ray tracing of the chief ray in the non-vignetting region
图 7 渐晕区域的主光线的逆向光线追迹示意图
Figure 7. Schematic of the backward ray tracing of the chief ray in the vignetting region
图 9 淹没水射流的原始光场图像、重聚焦的粒子和去模糊的粒子图像
Figure 9. Examples of raw Light Field image, refocused particle image and deblurred image in the Submerged Water Jets
图 10 淹没水射流的多平面速度场分布
Figure 10. Multi-plane velocity field distribution of submerged water jets
表 1 Raytrix R29光场相机的光学参数
Table 1. Optical parameters of Raytrix R29 light field camera
d2
(mm)fm
(mm)fm1
(mm)fm2
(mm)fm3
(mm)f
(mm)l1
(mm)lv2
(mm)l′v2
(mm)lm
(mm)Pm
(mm)Px
(μm)Wx×Wy
微透镜数量Nx×Ny
像素数量1.317 - 1.936 1.626 2.335 100 - 200 200 193.07 0.1705 5.5 159×203 4320 ×6340 
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