Biomedical subjects

Lujie Chen

Publications and source records attributed to Lujie Chen.

5 recordsLinked to original sources

PubMed2006

Carrier phase component removal: a generalized least-squares approach.

In fringe projection profilometry, an object shape is evaluated through phase distribution extracted from a projected fringe pattern. For parallel illumination geometry, the carrier phase component introduced by the fringes is spatially linear, whereas nonparallel illumination would lead to a nonlinear carrier. In this study, a general approach for the removal of a nonlinear-carrier phase component is proposed. A series expansion technique is used to approximate the carrier phase function, and a least-squares method is developed to estimate the unknown coefficients of the series. The theoretical analysis is given on the basis of a divergent illumination geometry with carrier fringes in the x direction. The method is also extended to include a curved surface-fitting approach, which is applicable to various measurement system geometries.

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PubMed2005

Fringe projection profilometry with nonparallel illumination: a least-squares approach.

Under a nonparallel illumination condition, fringe patterns projected on an object have unequal fringe spacing that would introduce a nonlinear carrier phase component. This Letter describes a nonlinear carrier removal technique based on a least-squares approach. In contrast with conventional methods, the proposed algorithm would not magnify phase measurement uncertainty, nor does it require direct estimation of system geometrical parameters. The theoretical expression of the carrier phase function on the reference is derived and expanded in a power series. The unknown coefficients in the series are determined by a least-squares method. By subtracting the calculated carrier phase function from the unwrapped phase map, the phase distribution of the object profile is obtained.

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PubMed2005

Fringe-density estimation by continuous wavelet transform.

For many phase extraction algorithms, a priori knowledge of a fringe-pattern density distribution is beneficial for later processing. A fringe-density estimation method based on a continuous wavelet transform (CWT) is proposed. For a one-dimensional signal the instantaneous frequency detected at the CWT ridge is directly adopted as a measure of the local fringe density. For a two-dimensional signal the instantaneous frequency components in both the x and the y directions are detected. Their reliability is evaluated by the CWT coefficient magnitude, based on which an approximate density value is given. The capability for noise reduction and the accuracy of the method are discussed.

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PubMed2005

Phase retrieval with a three-frame phase-shifting algorithm with an unknown phase shift.

A three-frame phase-shifting algorithm with a constant but unknown phase shift is proposed. The algorithm is based on background-intensity removal prior to phase retrieval to eliminate an undetermined factor in a fringe pattern. The proposed method is validated on three-dimensional profilometry by fringe projection and on deformation measurement by means of digital speckle shearing interferometry. For a fringe pattern with slow-varying background intensity, the background removal is achieved in the frequency domain. For a speckle pattern, a background removal technique is integrated with the three-frame algorithm. In this process, manual intervention is minimal, and high computational speed is achieved. In addition, high-frequency phase signals would not be removed in the noise-reduction process as is the case in the bandpass-filtering technique. Accuracy of the method is discussed.

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PubMed2003

Spatial-fringe-modulation-based quality map for phase unwrapping.

The quality-guided algorithm is a method widely used in phase unwrapping. The algorithm uses a quality map to guide its unwrapping process, and its validity depends on whether the quality map can truly reflect phase quality. In fringe projection surface profilometry, discontinuous surface structure, low surface reflectivity, and saturation of the image-recording system are sources of unreliable phase data. To facilitate the unwrapping process, we demonstrate an accurate quality map based on spatial fringe modulation, which is extracted from a single fringe pattern. Compared with temporal fringe modulation, the new criterion is more sensitive to spatial structure changes and less dependent on illumination conditions.

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