Yong Li

Bio: Yong Li is an academic researcher from Zhejiang Normal University. The author has contributed to research in topics: Holography & Digital holography. The author has an hindex of 12, co-authored 50 publications receiving 467 citations.

Numerical reconstruction of digital holograms for three-dimensional shape measurement

Abstract: In this paper, we present a new technique for 3D shape measurement by recovering the 3D numerical reconstruction image of a digital hologram. Firstly, we fabricate a Fresnel digital hologram. Then, a number of 2D light field intensity distributions, which are in the reconstructed field of the digital hologram, are computed on the different depth planes. Finally, the focus measure evaluation of the grey level variance is applied. By finding the maximum focus measure, we decide on the depth information of each small image patch. The experiment confirms that the technique can materialize the 3D rebuilding of the reconstruction image and obtain 3D profile information on the object recorded. So digital holography can be a very promising technology for non-destructive 3D shape measurement.

High-speed and dense three-dimensional surface acquisition using defocused binary patterns for spatially isolated objects.

Abstract: The three-dimensional (3-D) shape measurement using defocused Ronchi grating is advantageous for the high contrast of fringe. This paper presents a method for measuring spatially isolated objects using defocused binary patterns. Two Ronchi grating with horizontal position difference of one-third of a period and an encoded pattern are adopted. The phase distribution of fringe pattern is obtained by Fourier analysis method. The measurement depth and range is enlarged because the third harmonic component and background illumination is eliminated with proposed method. The fringe order is identified by the encoded pattern. Three gray levels are used and the pattern is converted to binary image with error diffusion algorithm. The tolerance of encoded pattern is large. It is suited for defocused optical system. We also present a measurement system with a modified DLP projector and a high-speed camera. The 3-D surface acquisition speed of 60 frames per second (fps), with resolution of 640 × 480 points and that of 120 fps, with resolution of 320 × 240 points are archived. If the control logic of DMD was modified and a camera with higher speed was employed, the measurement speed would reach thousands fps. This makes it possible to analyze dynamic objects.

Phase-shift extraction for phase-shifting interferometry by histogram of phase difference

Abstract: We propose a non-iterative approach to extract the unknown phase shift in phase shifting interferometry without the assumption of equal distribution of measured phase in [0,2π]. According to the histogram of the phase difference between two adjacent frames, the phase shift can be accurately extracted by finding the bin of histogram with the highest frequency. The main factors that influence the accuracy of the proposed method are analyzed and discussed, such as the random noise, the quantization bit of CCD, the number of fringe patterns used and the bin width of histogram. Numerical simulations and optical experiments are also implemented to verify the effectiveness of this method.

Direct phase extraction from interferograms with random phase shifts

Abstract: A new method that extracts phase directly from randomly phase-shifted interferograms is proposed. In this method, the background intensity and the modulation amplitude are determined by a temporal method and used as the system parameters and then the phase is extracted from one single interferogram by arc cosine function with range of (0, π). By partitioning the extracted phase distribution into watershed regions and determining the sign of phase in each region, the extracted phase is recovered to its principle phase with range of (-π, π). Numerical simulation and experiment are implemented to verify the effectiveness of this method. The method has applications in both static and dynamic phase measurement.

Focus plane detection criteria in digital holography microscopy by amplitude analysis

Abstract: We propose and test a focus plane determination method that computes the digital refocus distance of an object investigated by digital holographic microscopy working in transmission. For this purpose we analyze the integrated amplitude modulus as a function of the digital holographic reconstruction distance. It is shown that when the focus distance is reached, the integrated amplitude is minimum for pure amplitude object and maximum for pure phase object. After a theoretical analysis, the method is demonstrated on actual digital holograms for the refocusing of pure amplitude and of pure phase microscopic samples.

Comparative study of iterative reconstruction algorithms for missing cone problems in optical diffraction tomography.

Abstract: In optical tomography, there exist certain spatial frequency components that cannot be measured due to the limited projection angles imposed by the numerical aperture of objective lenses. This limitation, often called as the missing cone problem, causes the under-estimation of refractive index (RI) values in tomograms and results in severe elongations of RI distributions along the optical axis. To address this missing cone problem, several iterative reconstruction algorithms have been introduced exploiting prior knowledge such as positivity in RI differences or edges of samples. In this paper, various existing iterative reconstruction algorithms are systematically compared for mitigating the missing cone problem in optical diffraction tomography. In particular, three representative regularization schemes, edge preserving, total variation regularization, and the Gerchberg-Papoulis algorithm, were numerically and experimentally evaluated using spherical beads as well as real biological samples; human red blood cells and hepatocyte cells. Our work will provide important guidelines for choosing the appropriate regularization in ODT.

High-speed three-dimensional profilometry for multiple objects with complex shapes.

Abstract: This paper describes an easy-to-implement three-dimensional (3-D) real-time shape measurement technique using our newly developed high-speed 3-D vision system. It employs only four projection fringes to realize full-field phase unwrapping in the presence of discontinuous or isolated objects. With our self-designed pattern generation hardware and a modified low-cost DLP projector, the four designed patterns can be generated and projected at a switching speed of 360 Hz. Using a properly synchronized high-speed camera, the high-speed fringe patterns distorted by measured objects can be acquired and processed in real-time. The resulting system can capture and display high-quality textured 3-D data at a speed of 120 frames per second, with the resolution of 640 × 480 points. The speed can be trebled if a camera with a higher frame rate is employed. We detail our shape measurement technique, including the four-pattern decoding algorithm as well as the hardware design. Some evaluation experiments have been carried out to demonstrate the validity and practicability of the proposed technique.

Pixel-wise absolute phase unwrapping using geometric constraints of structured light system

Abstract: This paper presents a method to unwrap phase pixel by pixel by solely using geometric constraints of the structured light system without requiring additional image acquisition or another camera. Specifically, an artificial absolute phase map, Φmin, at a given virtual depth plane z = zmin, is created from geometric constraints of the calibrated structured light system; the wrapped phase is pixel-by-pixel unwrapped by referring to Φmin. Since Φmin is defined in the projector space, the unwrapped phase obtained from this method is absolute for each pixel. Experimental results demonstrate the success of this proposed novel absolute phase unwrapping method.