Multi-spatial-frequency and phase-shifting profilometry using a liquid crystal phase modulator

Abstract

Optical profilometry is widely applied for measuring the morphology of objects by projecting predetermined patterns on them. In this technique, the compact size is one of the interesting issues for practical applications. The generation of pattern by the interference of coherent light sources has a potential to reduce the dimension of the illumination part. Moreover, this method can make fine patterns without projection optics, and the illumination part is free of restriction from the numerical aperture of the projection optics. In this paper, a phase-shifting profilometry is implemented by using a single liquid crystal (LC) cell. The LC phase modulator is designed to generate the interference patterns with several different spatial frequencies by changing selection of the spacing between the micro-pinholes. We manufactured the LC phase modulator and calibrated it by measuring the phase modulation amount depending on an applied voltage. Our optical profilometry using the single LC cell can generate multi-spatial frequency patterns as well as four steps of the phase-shifted patterns. This method can be implemented compactly, and the reconstructed depth profile is obtained without a phase-unwrapping algorithm.

A 3D Optical Surface Profilometer Using a Dual-Frequency Liquid Crystal-Based Dynamic Fringe Pattern Generator

“…In this manner, the whole scanning time required for the four-step multi-spatial frequencies and four-step phase shifting schemes could be completed in less than 800 ms. This means that the optical surface profilometry can capture 16 sets of fringe patterns distorted by an object in less than 800 ms. Our profilometry using the DFPG can exhibit quite fast scanning and capturing speeds suitable for hand-held dental applications, compared with our previous study of a 3D optical surface profilometry system constructed with a mechanically moving optical element [21]. …”

“…The fringe visibility of the dynamic fringe patterns was about 0.9 in our system. This fringe visibility was enough to obtain the 3D depth extraction with our four-step phase-shifting and four steps of multi-spatial frequency scheme [21]. The beam from the He-Ne laser was expanded by a beam expander and was passed through an iris to reduce the optical noise so that a suitable spot size with a uniform beam intensity can cover all of the five Al slits.…”

“…The fringe pattern by the DFPG was calculated using the equation M phase = angle [ X ( pN /Λ + 1)], as explained in our previous study, obtained using a mechanical moving slit system [21]. The values of the phase modulation were repeatedly calculated depending on the change of the applied voltage with an increase of 0.01 V per step.…”

“…These are generated by beam pattern projecting modules such as mechanically moving wedge plates [15], tunable gratings [16], switchable gradient index lenses [17], polymer dispersed liquid crystal (LC) techniques [18,19], and several types of phase modulators using SLMs [20,21]. Recently, to obtain color information of an object in addition to the 3D depth information, time-sequential projection of structured and RGB-colored beam patterns was also proposed using fast switching digital light processing (DLP) projectors [22].…”

Profilometry based on Structured Illumination with Hypercentric Optics

“…이 방법에서 얻은 위상 정보는 0에서 2π로 한정되어 있기 때문에 일반적으로 unwrapping 알고리즘을 사용해야 올 바른 깊이 정보를 추출할 수 있다. 하지만 다중 공간 주파수 를 이용하여 각각의 주파수에 대하여 위상 천이 기술을 적용 할 경우 unwrapping 알고리즘을 적용하지 않고 깊이 정보를 계산할 수 있어서 unwrapping 알고리즘의 오류로 인하여 오 차가 발생하는 것을 피할 수 있다 [13,14] …”