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

形状测量是保证微小零件工业质量不可缺少的工具。为了在大规模生产中进行100%的质量评估，需要一种精确和健壮的测量技术，必须提供每件一秒以下的测量速率。目前，还没有技术可以在所有空间方向上为具有粗糙表面和尺寸为数百微米的微型零件提供这些能力。触觉和共焦方法太慢了。几何光学方法，如微条纹投影法，在没有对许多数据点进行平均的情况下，只能提供相对较大的5到10微米的测量不确定度。由于扫描过程，白光干涉仪也太慢。最后，对于尺寸为100微米的微小物体，干涉多波长方法是不可行的，因为相应的散斑场由于波长之间的巨大差异而去相关。该项目的目的是缩小这一差距，并提供这样一种测量技术。该概念的基本思想是在短相干光照射下进行剪切干涉测量。剪切干涉仪将来自x和x+S两个位置的光叠加在被剪切S分开的物面上，特别是在使用短相干光时，互相干函数伽玛(x，x+S)，即干涉图形的对比度和相位，取决于被剪切分开的两个位置之间的表面轮廓的有限差。因此，如果互相干函数伽玛(x，x+S)可以在任何位置x确定，那么应该可以通过数值积分的方法来计算物体的表面轮廓。该技术与白光干涉(或相干雷达)有很大的相似之处，并且应该得到关于测量不确定度的相似结果。然而，与这些方法不同的是，它不需要在被调查的标本的整个高度上顺序扫描。此外，由于采用公共路径配置，因此不需要参考波，从而消除了对机械稳定环境的需要。这些优点使得能够同时实现工业所需的显著更高的测量速度和增强的稳健性目标。

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.