High-speed polarized low coherence scanning interferometry

Three-dimensional surface measurements based on the low coherence scanning interferometry is a common practice in various industrial fields. However, it involves several scanning procedures to recover the original correlogram which significantly lowers their measurement speed despite their high sensitivity. To date, several methods such as sub-sampling and lateral scanning techniques have been proposed to increase their measurement speed. Despite the advantages of these techniques, they have limitations that reduce their measurement speeds.

In a recently published literature, researchers utilized polarized camera interferometry based on spatial phase-shifting technique to instantly obtain surface measurements. Unlike the low coherence scanning interferometry, the spatial phase-shifting interferometry successfully measured the phase with a polarized pixelated camera and confirmed its accuracy. Based on these insights, various authors have predicted the potential of introducing polarization in enhancing the measurement speed of low coherence scanning interferometry.

To this note, Chosun University scientists: Jun Woo Jeon, Hee Won Jeong, Hyo Bin Jeong and led by Professor Ki-Nam Joo developed a polarized low coherence scanning interferometry and investigated its feasibility in enhancing the measurements speed. This approach was based on a spatial phase-shifting technique that used a polarized CMOS camera. The spatial phase-shifting algorithm was used to ensure the extraction of the correlogram visibility in every scanning step. This elimination the need for the scanning conditions that acted as barriers to the realization of the high measurement speeds. Their work is currently published in the research journal, Applied Optics.

Just like the confocal scanning microscopy, polarized low coherence scanning interferometry exhibited the ability to monitor visibilities that played a key role in improving the precision of the obtained surface profile through correct phase information. Additionally, the scanning conditions like a scanning step size smaller than that obtained by the Nyquist sampling limit as well as equidistance scanning steps were all eliminated to achieve high measurement speeds. The larger scanning step also reduced the measured data.

To prove the concept, the authors experimentally verified the proposed polarized low coherence scanning interferometry and assessed its ability to enhance the measurement speeds. Three different specimen types: plane mirror, concave mirror, and step height were measured at various scanning steps. Apart from the successful reconstruction of the surface profiles, the authors also explored the compensation technique of the surface profile determined by the phase information. This was successfully obtained by the spatial phase-shifting technique.

The high-speed measurement capability based on the real-time visibility extraction from the correlogram was the main important feature of the polarized low coherence scanning interferometry. By reducing the number of data due to the larger scanning step size, the data processing time was also minimized. Even though no scanning conditions were required, it was worth noting that the visibility of the correlogram could not be extracted when the measured intensities failed to change along with the scanning procedure for sampling at the zero-intensity position.

Based on the results, the lead author Professor Ki-Nam Joo , through a statement to the Advances in Engineering, expressed his confidence that the proposed polarized low coherence scanning interferometry will offer the much-awaited high measurement speed.