Optical instruments: in particular interferometry has been found appropriate for measuring a huge variation of implant surfaces, from smooth to rough surface modifications. The technique uses reflecting light as an optical stylus (Fig. 1.2). The measuring range is within a few millimetres. The technique provides high resolution, down to the nanometre level in height direction but is limited in spatial direction to approximately 0.3 μm; thus, the technique is most appropriate for surface characterisation at the micrometre level. In relation to oral implants, one big advantage with the technique is the possibility to access all parts of the implant, even flank areas, which is important since these areas are the largest part of the implant that is in contact with the surrounding bone. One disadvantage with optical instruments is that studied objects need some reflecting capacity, at least 4 % of the incident light as a minimum.

Atomic force microscopy (AFM): AFM can work in principle in three different modes of operation, contact, noncontact and tapping mode. The instrument uses a tip and cantilever with similar principle as the stylus instrument but with a huge difference in size. The AFM tip is in the range of a few to approximately 20 nm in diameter. The tip can be in contact with the surface during measurements or above it with the help of van der Waals forces. The measuring area is rather small, typically in the submicron range. The resolution can approach molecular levels both in height and spatial directions which makes it possible to measure nanostructures. However, many implants have a surface that is too rough for the equipment; thus, for oral implants the application is limited to rather smooth surfaces. Furthermore, flank areas are difficult to measure, due to the same reasons as for the stylus instrument.