Ekberg Peter, Su Rong, Leach Richard
Opt Express. 2017 Aug 7;25(16):18703-18712. doi: 10.1364/OE.25.018703.
Lateral optical distortion is present in most optical imaging systems. In coherence scanning interferometry, distortion may cause field-dependent systematic errors in the measurement of surface topography. These errors become critical when high-precision surfaces, e.g. precision optics, are measured. Current calibration and correction methods for distortion require some form of calibration artefact that has a smooth local surface and a grid of high-precision manufactured features. Moreover, to ensure high accuracy and precision of the absolute and relative locations of the features of these artefacts, requires their positions to be determined using a traceable measuring instrument, e.g. a metrological atomic force microscope. Thus, the manufacturing and calibration processes for calibration artefacts are often expensive and complex. In this paper, we demonstrate for the first time the calibration and correction of optical distortion in a coherence scanning interferometer system by using an arbitrary surface that contains some deviations from flat and has some features (possibly just contamination), such that feature detection is possible. By using image processing and a self-calibration technique, a precision of a few nanometres is achieved for the distortion correction. An inexpensive metal surface, e.g. the surface of a coin, or a scratched and defected mirror, which can be easily found in a laboratory or workshop, may be used. The cost of the distortion correction with nanometre level precision is reduced to almost zero if the absolute scale is not required. Although an absolute scale is still needed to make the calibration traceable, the problem of obtaining the traceability is simplified as only a traceable measure of the distance between two arbitrary points is needed. Thus, the total cost of transferring the traceability may also be reduced significantly using the proposed method.
大多数光学成像系统中都存在横向光学畸变。在相干扫描干涉测量中，畸变可能会在表面形貌测量中引起与场相关的系统误差。当测量高精度表面（如精密光学元件）时，这些误差就变得至关重要。当前用于畸变的校准和校正方法需要某种形式的校准工件，该工件具有光滑的局部表面和高精度制造特征的网格。此外，为确保这些工件特征的绝对和相对位置的高精度和准确性，需要使用可溯源的测量仪器（如计量原子力显微镜）来确定其位置。因此，校准工件的制造和校准过程通常昂贵且复杂。在本文中，我们首次展示了通过使用一个包含一些与平面的偏差且具有一些特征（可能只是污染物）的任意表面，从而能够进行特征检测，来对相干扫描干涉仪系统中的光学畸变进行校准和校正。通过使用图像处理和自校准技术，畸变校正实现了几纳米的精度。可以使用在实验室或车间中容易找到的廉价金属表面，例如硬币表面或有划痕和缺陷的镜子。如果不需要绝对尺度，那么纳米级精度的畸变校正成本几乎可以降低到零。尽管仍然需要绝对尺度以使校准可溯源，但由于只需要对两个任意点之间的距离进行可溯源测量，因此获得可溯源性的问题得到了简化。因此，使用所提出的方法也可以显著降低传递可溯源性的总成本。
