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

There has been tremendous advancement in the optical imaging field that led to the development of structured illumination microscopy with applications in various areas such as surface profiling and super resolution imaging among others. Currently, the working principle of structured illumination microscopy is based on the modulating the input illumination light to provide the depth information. Alternatively, recent research has shown that scanning the focal plane of the illuminated pattern enables determination of a three-dimensional surface profile of a specimen under investigation. Even though the process is precise and accurate, the axial scanning is susceptible to vibration distortion due to the long image acquisition time.

To this note, several techniques have been developed to solve the axial scanning method. For instance, remote focusing method even though developed to eliminate the moving parts cannot work without motion especially for moving the mirror in the image plane. In addition, in most of the optical imaging systems, the magnification change problem associated with depth scanning cannot be ignored and more so for those based on the electrically tunable lens.

Dr. Ju Wan Kim and Professor Byeong Ha Lee of Gwangju Institute of Science and Technology recently developed a structured illumination technique for effective surface profiling process, together with Dr. Jae Sung Ahn and Dr. Joo Beom Eom from the Korea Photonics Technology Institute. Fundamentally, they developed the structured illumination system based electrically tunable lens that was technically free from the ill effects resulting from the magnification changes in focal length and surface reflections of optics. Additionally, the authors validated their work by scanning an object plane without motion using the electrically tunable lens. Their work is currently published in the journal, Optics Communications.

In brief, the research team first explored the possibility of using a telecentric system to achieve a better performance of the magnification-invariant structured illumination system. This, however, depended on the geometric analysis which helped in minimizing the alignments errors involved. Next, the factors affecting the electrically tunable lens based structured illumination profiling and their remedies were experimentally investigated. Eventually, the effects associated with the strong reflection emanating from the lens surfaces were examined. The polarizing beam splitter and the quarter-wave plate were used to reduce the unwanted reflection. With this, the efficiency of using an electrically tunable lens was verified.

The authors observed that the developed imaging technique can effectively scan a three-dimensional object to obtain a surface profile image. Consequently, the obtained image was depth-resolved due to the fine structure of the pattern mask. On the other hand, incorporating the electrically tunable lens significantly enhanced the depth scanning speed while a 4-f relay optical system was utilized in minimizing the change in the magnification ratio. This was attributed to the good reproducibility and hysteresis properties of the electrically tunable lens.

In summary, the researchers developed a simpler and faster method based on electrically tunable lens. Interestingly, the object under imaging underwent no motion during the scanning. For instance, by keeping the magnification ratio change below 0.03, scanning depth variation of 35mm was successfully achieved. In general, the study will advance the structured illumination imaging technologies especially in cases that require high speed and rapid scans. This will further enhance their applications in various fields.