CHEN WEN
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elechenw@nus.edu.sg
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Publication Profiling of an object using a time-frequency analysis method in digital holographic interferometry(2009) Chen, W.; Quan, C.; Tay, C.J.; MECHANICAL ENGINEERINGIn recent years, temporal analysis methods have become popular in the processing of fringe patterns. In the conventional temporal phase unwrapping method, each pixel is processed independently of other pixels. Thus good data can be effectively preserved. However, the errors may still accumulate along the time axis for the bad data. In this paper, a timefrequency analysis method i.e., short time Fourier transform (STFT) algorithm, is employed to process wrapped phase maps in digital holographic interferometry (DHI). In the proposed STFT method, phase unwrapping operations are effectively avoided in both spatial and temporal domains and the errors along the time axis can be more effectively suppressed. In addition, the continuous phase distributions obtained can characterize the shape of a test object. An experiment is conducted to show the validity of STFT algorithm in DHI. © 2009 SPIE.Publication Ghost imaging for three-dimensional optical security(2013-11-25) Chen, W.; Chen, X.; ELECTRICAL & COMPUTER ENGINEERINGGhost imaging has become increasingly popular in quantum and optical application fields. Here, we report three-dimensional (3D) optical security using ghost imaging. The series of random phase-only masks are sparsified, which are further converted into particle-like distributions placed in 3D space. We show that either an optical or digital approach can be employed for the encoding. The results illustrate that a larger key space can be generated due to the application of 3D space compared with previous works. © 2013 AIP Publishing LLC.Publication Optical encryption based on 3-D sphere and phase retrieval in gyrator transform domain(2013) Chen, W.; Chen, X.; ELECTRICAL & COMPUTER ENGINEERINGWith the rapid development of modern technologies, information security has attracted more and more current attention. In this paper, we present optical image encryption based on 3-D sphere and phase retrieval in gyrator transform domain. The plaintext is divided into a series of small quadratic blocks, and each block is treated as a particle. The series of particles is distributed in 3-D spheres, and is encrypted into one phase-only mask based on an iterative phase retrieval algorithm. During image decryption, the extracted phase-only mask is rotated and illuminated to sequentially extract a series of focused particles. Subsequently, a decrypted image can be obtained by incorporating all the decoded particles in the transverse domain. Numerical experiment is conducted to demonstrate feasibility and effectiveness of the presented method. © 2013 IEEE.Publication Determination of curvature and twist of deformed object by digital holographic interferometry(2009) Quan, C.; Chen, W.; Tay, C.J.; MECHANICAL ENGINEERINGThis paper describes a feasibility study of digital holographic interferometry for the measurement of curvature and twist of a deformed object. Measurement of curvature and twist is an important aspect in experimental mechanics. Numerous methods have been proposed to determine the curvature and twist by using digital shearography. We proposed a novel method to determine curvature and twist based on digital holography (DH) and complex phasor (CP). In the conventional methods, phase difference between the first and second states is obtained directly by digital phase subtraction (DPS) and Fourier transform is then employed to extract phase maps. In this study, CP method is proposed to improve the quality of phase maps corresponding to second-order derivatives. Subsequently, sine/cosine transformation and short time Fourier transform (STFT) are employed to process the wrapped phase maps. An experiment is conducted on a clamped circular plate under a point load at centre. The experimental results show that the proposed method is valid and able to obtain high quality phase maps corresponding to curvature and twist of a deformed object. © 2009 SPIE.Publication Optical color image encryption based on Arnold transform and interference method(2009-09-15) Chen, W.; Quan, C.; Tay, C.J.; MECHANICAL ENGINEERINGIn this paper, we propose a novel method to encrypt a color image based on Arnold transform (ART) and interference method. A color image is decomposed into three independent channels, i.e., red, green and blue, and each channel is then encrypted into two random phase masks based on the ART and interference method. Light sources with corresponding wavelengths are used to illuminate the retrieved phase-only masks during image decryption. The influence of security parameters on decrypted images is also analyzed. Numerical simulation results are presented to illustrate the feasibility and effectiveness of the proposed method. © 2009 Elsevier B.V. All rights reserved.Publication Optical image encryption with a bit-plane separation method in phase-shifting digital holography(2010) Chen, W.; Quan, C.; Tay, C.J.; ELECTRICAL & COMPUTER ENGINEERING; MECHANICAL ENGINEERINGIn this paper, we propose a method for optical image encryption based on fractional Fourier transform (FRFT) and Arnold transform (ART) in phase-shifting digital holography. An input image is first divided into eight bit planes, and each bit plane is encrypted based on double random-phase masks and FRFT. Complex amplitude for the object is retrieved by phase-shifting digital holography in the hologram plane. The real and imaginary parts of the retrieved complex amplitudes for the 0th-7th bit planes are further encrypted using ART algorithm. Numerical results are shown to demonstrate the feasibility and effectiveness of the proposed technique. The sensitivity of security parameters, such as function orders in FRFT and iteration number in ART method, is also analyzed. © 2010 SPIE.Publication Color image encryption based on interference and virtual optics(2010-03) Tay, C.J.; Quan, C.; Chen, W.; Fu, Y.; ELECTRICAL & COMPUTER ENGINEERING; MECHANICAL ENGINEERINGWe propose two approaches to encrypt color images based on interference and virtual optics. In the first method, a color image is first decomposed into three independent channels, i.e., red, green and blue. Each channel of the input image is encrypted into two random phase-only masks based on interference. In the second method, a color image is first converted into an image matrix and a color map, and only the image matrix is encrypted into random-phase masks based on interference. After the phase masks are retrieved, a concept based on virtual optics is further applied to enhance the security level. Numerical simulations are demonstrated to show the feasibility and effectiveness of the proposed methods. © 2009 Elsevier Ltd. All rights reserved.Publication Image authentication via sparsity-based phase-shifting digital holography(SPIE, 2015) Chen W; Chen X; ELECTRICAL & COMPUTER ENGINEERINGPublication Phase retrieval in digital holographic interferometry based on complex phasor and short time Fourier transform(2008) Wen, C.; Chenggen, Q.; Jui, T.C.; MECHANICAL ENGINEERINGThe determination of phase difference distributions is important in digital holographic interferometry. Many methods can be used to determine phase difference maps, such as Fourier transform, digital phase subtraction and phase-shifting. In recent years, some time-frequency analysis methods such as wavelet transform and short time Fourier transform (STFT) are developed to extract continuous phase maps without phase unwrapping in spatial and temporal domains. In this paper, STFT is investigated for the retrieval of phase difference in digital holographic interferometry. In addition, complex phasor (CP) method is employed in advance so as to make STFT available. An experiment is conducted to show the validity of the method based on STFT.Publication Retrieval of complex object fields in coherent diffractive imaging using position shift of a phase mask(2011-08) Chen, W.; Quan, C.; Tay, C.J.; ELECTRICAL & COMPUTER ENGINEERING; MECHANICAL ENGINEERINGIn this letter, we propose a novel method to quantitatively recover a complex-valued object from diffraction intensity maps. A wavefront modulation is introduced in the optical path, and several diffraction intensity maps are recorded through position shift of a phase mask. A phase retrieval algorithm is further developed, and advantages of the proposed method are discussed. © 2011 Society of Photo-Optical Instrumentation Engineers (SPIE).