Magnification-invariant surface profiling technique for structured illumination imaging and microscopy

Abstract

The structured illumination (SI) surface profiling method that is fast and has a depth-independent image magnification ratio is presented. Utilizing an electrically tunable lens (ETL) makes it possible to scan the object plane rapidly and motionlessly. The change in magnification ratio that generally occurs with the variation in the focal length of an objective lens is minimized by using a 4f relay system and placing the ETL at the confocal plane. The high reflection at the uncoated membrane surfaces of the ETL is bypassed by using polarization optics. It is also proposed that the fast response of the ETL can be fully exploited by switching the scanning sequence from that of a conventional SI imaging system; performing the depth scan before the pattern shift. It is shown that the scanning speed was increased up to 25 times. The principle and the related problems of SI profiling based on an ETL are fully analyzed, and the concept is confirmed experimentally. In the experiment, scanning depth variation of 35 mm could be achieved while keeping the magnification ratio variation below 0.03. The ability of 3D profiling is demonstrated by SI imaging a cone-shape 3D object and a face-shape plaster figure. In the fields requiring fast scanning, such as an intraoral scanner or biomedical imaging, the proposing SI imaging method can be fully utilized.