Normal planar laser induced fluorescence (N-PLIF) method has been widely applied in liquid film imaging and thickness measurements in circular tubes with large curvatures, but meets great challenges in liquid film thickness measurements in circular tubes with small curvatures or at plates. An alternative method is to place both the incident laser sheet and the camera below the bottom interface of a plate, forming the acute PLIF (A-PLIF) method with an acute laser optical path and camera alignment. However, the liquid film thickness measurements based on direct imaging with the A-PLIF method are still challenged by the decrease of image contrast and indistinct interfaces. This work further explores the A-PLIF method with a novel image processing approach through both static liquid film and moving liquid film measurements. In static liquid film measurements, brightness curves along the liquid film thickness direction are extracted to compare with numerical simulation results based on ray-tracing approach, in which the laser intensity attenuation, laser sheet thickness, fluorescence spectrum and top interface reflection are considered. The comparison indicates that out-of-focus imaging is the dominant factor for the expanding of measured brightness curves. Three distinct regions, i.e. the increasing region, slowly decreasing region and rapidly decreasing region, are identified on the brightness curves and the main fluorescence source of each region is analyzed theoretically. It is concluded that the total width of the two decreasing regions shows strong correlation to the liquid film thickness, which facilitates the evaluation of liquid film thickness through peak position determination of the brightness curve and linear fitting of the rapidly decreasing region. This method is further tested in the moving liquid film thickness measurement in a cross-flow tunnel at various cross-sections, which shows that it can be applied to the liquid film imaging and thickness measurements on small curvature or flat surfaces.

普通的平面激光诱导荧光（N-PLIF）方法已广泛用于大曲率圆形管的液膜成像和厚度测量，但在小曲率圆形板或板的液膜厚度测量中遇到了巨大挑战。另一种方法是将入射激光片和照相机都放置在板的底部界面下方，从而形成具有急性激光光路和照相机对准的急性PLIF（A-PLIF）方法。然而，基于A-PLIF方法的直接成像的液膜厚度测量仍然受到图像对比度降低和界面模糊的挑战。这项工作通过静态液膜和移动液膜的测量，进一步探索了具有新颖图像处理方法的A-PLIF方法。在静态液膜测量中，提取沿液膜厚度方向的亮度曲线，以与基于射线追踪法的数值模拟结果进行比较，其中考虑了激光强度衰减，激光片厚度，荧光光谱和顶部界面反射。比较表明，散焦成像是扩展测得的亮度曲线的主要因素。在亮度曲线上确定了三个不同的区域，即增加区域，缓慢减少区域和快速减少区域，并从理论上分析了每个区域的主要荧光源。结论是，两个减小区域的总宽度与液膜厚度密切相关，通过确定亮度曲线的峰值位置和快速减小区域的线性拟合，可以方便地评估液膜厚度。对该方法在横断面横断面的运动液膜厚度测量中的各种横截面进行了进一步测试，结果表明该方法可用于小曲率或平坦表面的液膜成像和厚度测量。