3-D Surface Profilometry for Accurate Extraction of Depth Profile with LC Phase Modulator Kyung-Il Joo 1 , Hee Yeon Noh 1 ,Chang-sub Park 1 , Min-Kyu Park 1 , Seong-Woo Oh 1 , and Hak-Rin Kim 1,2 * 1 School of Electrical Engineering and Computer Science, Kyungpook National University, 1370 Sankyuk- dong, Buk-gu, Daegu, Korea 2 School of Electronics Engineering, Kyungpook National University, Buk-gu, Daegu, Korea Abstract We proposed a 3-D surface profilometry with a liquid crystal (LC) phase modulator for accurate extraction of the position and depth profile. The LC phase modulator was designed to develop the dynamic fringe patterns to be operated with multi-spatial frequencies, which were determined by changing the gap between micro-pinholes. The amount of phase shift was controlled by changing the applied voltage. In addition, the fictitious pattern scanning method and geometrical parameters were used to reconstruct position and depth profile of an object. As a result, the depth profile of object could be computed accurately with profilometry. Also, 3-D surface profilometry showed that integrated compact system could be constructed with the proposed scheme. 1. Introduction 3-D optical surface profilometry is a device for measuring a surface profile of objects and their position and depth informations without contact and it has been applied for robot imaging and medical equipment [1-3]. The projected fringe patterns are captured as deformed resulting from the surface profile of objects because the reference spot for measurement of projection and acquisition are separated spatially. So, the surface profile of the objects can be computed from the information of amount of deformed fringe patterns. In this work, a LC phase modulator with micro-pinholes was manufactured to generate the fringe patterns. The phase modulation and spatial frequency of fringe patterns were controlled by the LC phase modulator. With wavefront splitting interferometer, the amount of phase modulation was determined by the optical path difference (OPD) of two incident rays. Also, the effective refractive index of the LC phase modulator could be changed with an applied voltage. So, the phase modulation of fringe pattern was controlled with the applied voltage. The spatial frequency of the fringe pattern was controlled by changing the gap between two micro-pinholes. In this paper, we proposed the 3-D surface profilometry with the LC phase modulator that it could reconstruct accurately the surface profile of objects without phase unwrapping method. Also, our 3-D surface profilometry could be constructed compactly in an integrated optic system. 2. LC phase modulator The LC phase modulator was developed to implement a 3D optical profilometry which could be operated by the phase- shifting as well as the multi-spatial frequency. Figure 1 shows the structure of LC phase modulator. The LC phase modulator was implemented by single LC cell and micro-pinhole array which were formed with aluminum lift-off method. The gap between micro-pinhole pairs were g = 200, 400, 600, 800 m, respectively. The spatial frequency of fringe pattern was controlled by changing gap of the selected micro-pinhole pair. To control the phase-shifting of the fringe pattern for developing 4-step phase- shifting method, the transparent electrodes of indium thin oxide (ITO) were patterned through photo-lithography process. Figure 2 shows the operation principle of the LC phase modulator. Because of the alignment condition, the LC was aligned with the vertically aligned geometry at a field-off state, as shown in Fig. 2 (a). In field-off state, two rays passing through micro-pinhole pair have the same optical path length (OPL) due to same distribution of refractive index. According to increasing an applied voltage, the distribution of LC director was reoriented along the perpendicular direction to the applied field direction. The reorientation of LC director generated the voltage-variable OPL region (OPL I ) which was formed on the ITO electrode. In the fixed OPL region (OPL II ), the distribution of LC director was remained as the initial homeotropic distribution because of the etched ITO electrode. Therefore, the optical path difference (OPD) from the micro- pinhole pair was electrically controlled. Figure 1. Schematic of LC phase modulator for realizing four- step phase shifting and multi-spatial frequencies with a single LC cell. Figure 2. Operation principle of a LC phase modulator (a) without and (b) with an applied voltage. P-25 / K.-I. Joo SID 2012 DIGEST • 1141 ISSN 0097-966X/12/4303-1141-$1.00 © 2012 SID