Rapid shape measurement based on the measurement of the mutual coherence function using a shear interferometer (Gamma-Profilometry)

形状测量是保证微细零件工业质量不可缺少的工具。为了在批量生产中实现100%的质量评估，需要一种精确而稳健的测量技术，必须提供每件低于一秒的测量速率。目前，还没有技术可以为具有粗糙表面和在所有空间方向上尺寸为100微米的微型部件提供这些功能。触觉和共聚焦方法太慢了。几何光学方法，如微条纹投影，只能提供5到10 μ m的测量不确定度，而不需要对许多数据点进行平均。由于扫描过程白光干涉仪也太慢。最后，干涉多波长方法不适用于尺寸为100微米的微小物体，因为所需的波长之间的较大差异导致相应的散斑场不相关。该项目的目的是缩小这一差距在艺术的状态，并提供这样的测量技术。提出的概念的基本思想是在短相干长度的照明下进行剪切干涉测量。剪切干涉仪在被剪切s隔开的物体平面上叠加来自x和x+s两个位置的光。特别是当使用短相干长度的光时，相互相干函数Gamma(x, x+s)，即干涉图的对比度和相位，取决于被剪切s隔开的位置之间的表面轮廓的有限差。因此，如果相互相干函数Gamma（x, x+s）可以在任意位置x确定，则应该可以通过数值积分的方法计算出物体的表面轮廓。所提出的技术与白光干涉测量（或相干雷达）有很大的相似之处，并且在测量不确定度方面应该产生相似的结果。然而，与这些方法相反，它不需要在被调查的标本的整个高度上顺序扫描。此外，由于共路配置，不需要参考波，从而消除了对机械稳定环境的需要。这些优点使得同时实现工业所需的更高的测量速度和增强的鲁棒性目标。

Shape measurement is an indispensable tool for industrial quality assurance of micro parts. For an aim of 100% quality assessment in mass production, a precise and robust measurement technique is needed that has to provide measurement rates below one second per piece. Currently, there is no technique available that offers these capabilities for micro parts with rough surfaces and with dimensions of several 100 micrometers in all spatial directions.Tactile and confocal approaches are much too slow. Geometrical-optical approaches such as micro fringe projection only provide, without averaging over many data points, comparably large measurement uncertainties of 5 to 10 µm. Due to the scanning process white light interferometers are too slow as well. Finally, interferometric multi-wavelengths methods are not practicable for micro objects that have sizes of several 100 micrometers, because the corresponding speckle fields decorrelate due to the required large difference between the wavelengths.The aim of the project is to close this gap in the state of the art and to provide such a measurement technique. The basic idea of the proposed concept is to perform shear interferometry under illumination with short coherence length. A shear interferometer superposes light originating from two positions x and x+s on the object plane separated by the shear s. Especially when using light with a short coherence length, the mutual coherence function Gamma(x, x+s), i.e. the contrast and the phase of the interference figure, depends on the finite difference of the surface profile between the positions separated by the shear. Hence, if the mutual coherence function Gamma(x, x+s) can be determined at any position x, it should be possible to calculate the surface profile of the object by means of numerical integration.The proposed technique shares larges similarities with white light interferometry (or coherence radar, respectively) and should yield similar results with respect to the measurement uncertainty. However, in contrast to these methods it is not required to sequentially scan over the entire height of the specimen under investigation. Additionally, due to the common-path-configuration there is no need for a reference wave, thereby eliminating the need for a mechanically stable environment. These benefits enable the simultaneous achievement of the industrially required goals of a substantially higher measurement speed and an increased robustness.