Filled-Up-Microscopy (FUM): a non-destructive method for approximating the depth of sub-surface damage on ground surfaces

Subsurface Damages (SSDs) can cause a wide variety of defects to optical lenses and other components. In addition to the adhesion and quality of coatings, the mechanical stability, the transmission quality and the laser-induced damage threshold (LIDT) of the products, is also affected. It is, therefore, attempted to get components as SSD-free as possible at the end of the production chain. Already during the individual production steps, it is important to know the depth of the SSDs in order to remove them in the following manufacturing steps. To design the manufacturing processes efficiently and avoid damage, it is important to be able to measure the depth and characteristics of SSDs as precisely as possible. There are a several approaches and methods to determine SSDs known in literature. However, many of them inevitably lead to the destruction of the workpiece. Although others are non-destructive, but very complex in design and/or associated with large investments. Likewise, only a few are suitable for determining SSDs on ground rough surfaces. Filled-Up Miicroscopy (FUM) is an alternative approach to approximating the depth of SSDs, even on rough surfaces without destroying them. At a first glance at the method, the procedure is described in detail and all necessary steps of preparing the samples are shown. A first comparison with the known Ball Dimpling Method confirms the functionality of the concept.