Phase measuring deflectometry with active display registration

高精度的三维形状评估是精密制造和质量控制的基本要求。相位测量偏转计（PMD）通过跟踪从显示器通过被测表面反射到相机的光的路径，提供了快速，全场和无接触的镜面形状评估。通常，计算机显示器应用于这项任务，通过使用相移技术的空间编码来确定反射相机视觉光线的参考点。相移提供了显示表面上的二维定位。因此，三维空间坐标的评估需要知道显示器的形状。然而，形状不是恒定的，而是受物理应力（如重力）的影响，这取决于它的方向，特别是受热膨胀的影响。这种行为需要一个程序来同时确定每个PMD测量显示的形状。这种方法是由立体条纹投影技术提供的。在这里，两个摄像头从一个经过结构化照明编码的表面捕获的图像用于瞬态点的识别，通过确定相应视觉光线的相交提供相关坐标。提出的项目旨在通过在直接观察显示的设置中引入额外的立体摄像系统来提高PMD的精度和灵活性。由于PMD中的显示器已经提供了结构化照明，因此条纹投影原理在没有投影仪的情况下也适用。立体系统提供独立的显示信息，以改进PMD测试表面的形状确定。它还可以精确地确定显示器的姿势。因此，不再需要将显示器的姿态和形状存储为系统校准数据。相反，立体相机系统提供了与PMD测量的相位数据相关的绝对坐标。显示器可以自由定位，仅受所有摄像机视场在显示器表面充分重叠的条件的限制。大多数PMD系统缺乏访问弯曲样品的完整表面的能力，因为所有相机视觉射线与显示器相交的系统几何形状无法实现。所提出的PMD设置提供了显示器的可变定位，并通过对不同显示姿态的多相测量进行组合评估，改善了对被测表面的访问。使用多个显示姿态进一步允许通过为反射的视觉光线提供多个参考点来解决偏转测量的模糊性。最后，该方法通过提供独立的显示形状信息来避免系统误差。假设一个恒定的显示形状的校准系统不再需要。

Highly accurate three-dimensional shape assessment is a fundamental requirement for precise manufacturing and quality control. Phase measuring deflectometry (PMD) provides fast, full-field and contact-free shape assessment of specular surfaces by tracing the path of light from a display via the reflection at the surface under test to a camera. Typically, computer displays are applied for this task, enabling the determination of reference points of reflected camera vision rays by spatial coding using the phase shifting technique.Phase shifting provides two-dimensional localization on the display surface. Therefore, the assessment of three-dimensional spatial coordinates requires the shape of the display to be known. However, the shape is not constant but affected by physical stress such as gravity depending on its orientation and particularly by thermal expansion. This behavior requires a procedure for simultaneous shape determination of the display with each PMD measurement. Such an approach is provided by the stereoscopic fringe projection technique. Here, images captured by two cameras from a surface specially coded by structured illumination are used for identification of transient points, providing the correlated coordinates by determination of intersections of corresponding vision rays.The proposed project aims at the improvement of accuracy and flexibility of PMD by introducing an additional stereoscopic camera system into the setup directly observing the display. Since the display in PMD already provides structured illumination, the fringe projection principle is applicable without a projector. The stereo system provides independent information of the display for improved shape determination of the surface under test with PMD. It additionally enables precise determination of the pose of the display. Therefore, storing pose and shape of the display as system calibration data is no longer required. Instead, the stereo camera system provides absolute coordinates related to the phase data of the PMD measurement. The display can be positioned freely, only confined by the condition that fields of view of all cameras sufficiently overlap on the display surface. Most PMD systems lack the ability to access the full surface of curved samples, since system geometries with all camera vision rays intersecting the display cannot be realized. The proposed PMD setup provides variable positioning of the display and improves access to the surface under test by enabling combined evaluation of multiple phase measurements for varying display poses. Using multiple display poses furthermore allows to solve the ambiguity of deflectometric measurements by providing multiple reference points for reflected vision rays. Finally, the proposed approach avoids systematic errors by providing independent display shape information. Assuming a constant display shape for a calibrated system is not required any more.