Hongzhen Jin

Numerical reconstruction of digital holograms for three-dimensional shape measurement

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.

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.

Three-dimensional shape measurement using binary spatio-temporal encoded illumination

A new approach for three-dimensional shape measurement is proposed. The corresponding point pairs between the projector and camera are identified by projecting a special pseudorandom sequence onto the surface of measured objects. The important properties of this sequence are expressed as follows: (1) any subsequence with a length of four symbols is unique; (2) there are no repeated symbols in any subsequence. The pseudorandom sequence is constructed with an alphabet of six symbols. These symbols are encoded with local spatial and temporal information by pixels in vertical strips of the projector plane. During measurement, the patterns for encoding are projected in turn and are captured with a camera. The projected pseudorandom sequence is retrieved by analysing the captured images. Then, the corresponding point pairs are worked out. Finally, the shapes of objects are reconstructed with triangulation. The experimental results reveal that a dense depth image with high resolution can be obtained. However, texture can be acquired simultaneously. The proposed method is robust.

Fast algorithm for reliability-guided phase unwrapping in digital holographic microscopy

A fast reliability-guided phase unwrapping algorithm, using an optimized quality map and combining it with look-up table operation, is proposed for digital holographic microscopy. First, by detecting the residues in the wrapped phase map, an intensity threshold is calculated in the normalized intensity image and the measured region is distinguished into the reliable region and the doubtful region. An optimized quality map is derived by the method in which the intensity values in the reliable region are set to 1 and those in the doubtful region remain unchanged. Then the flood fill algorithm by look-up table is implemented with the optimized quality map to retrieve true phase map. The experimental results demonstrate that not only does the proposed algorithm perform well, but also the speed is significantly faster than that of the conventional flood fill algorithm using insert sorting.

Three-dimensional visualization of shape measurement data based on a computer generated hologram

In this paper, we present a technique to reconstruct a 3D object by its measurement data using a computer generated hologram (CGH). First, both the range image and the intensity image of a 3D object are acquired simultaneously using a 3D shape measurement system. Then, the Fresnel CGH is designed and fabricated using these data. Finally, by combining CGH with optical holography, an image hologram is recorded. This technique resolves how to obtain a 3D image in a CGH as well as how to reproduce the measurements of object images in three-dimensional object shapes. In this paper the principle behind the proposed method, the technology for designing and fabricating the CGH and optical hologram, and the experimental results are given.

Practical method for color computer-generated rainbow holograms of real-existing objects

A novel method for computer-generated rainbow holograms (CGRHs) of full-color objects is proposed. First, a new algorithm for fabricating full-color CGRHs of real-existing objects is proposed based on the interrelationship between coding of a CGRH and reconstruction of the hologram. Second, a color rainbow hologram for a real-existing object is generated by combining the proposed algorithm and computer-generated hologram generating system. Finally, the hologram is outputted by an auto-microfilming system. The principle of the algorithm, the process of hologram calculation, and the hologram generating system for real-existing objects and experimental results are presented. The experimental results demonstrate that the new method is feasible.

Spatially induced spatiotemporally nonspreading Airy–Bessel wave packets

By studying the effect of spatially induced group velocity dispersion (SIGVD) during the propagation of ultrashort pulsed Bessel beams in free space, we numerically prove that third-order SIGVD can temporally cause Gaussian distribution of pulsed Bessel beams to gradually evolve as unsymmetrical trailing oscillatory structures. The pulse shape is confirmed to be temporal Airy distributions on the basis of the cross-correlation function. Therefore, it is demonstrated that the scheme of generating spatiotemporally nonspreading Airy-Bessel wave packets in free space is possible by using a precompensating second-order SIGVD. The results of numerical simulation show that the quasi-Airy pulses induced by third-order SIGVD are temporally nonspreading during propagation in dispersive media. The reasons for nonspreading of such Airy distribution pulses are phenomenologically analyzed by a time-frequency Wigner distribution function of the pulse.

Digital holographic microtomography with few angle data-sets

In this paper, we report the experimental implementation of handling the reconstruction problem from few angle data-sets for digital holographic microtomography. First, the digital holographic microscopy with sample-rotating scheme is established and few holograms with regularly spaced angle steps are recorded. Then, an algebraic iterative reconstruction algorithm with non-positivity constraint and a smoothing operator is applied to reconstruct three-dimensional refractive index distribution of the measured sample from the few angle data-sets. The experimental results demonstrate that the algebraic iterative technique can accurately reconstruct refractive index distribution from few angle data-sets in digital holographic microtomography. The technique is easy to implement and reduces greatly the required recording times.

Partition calculation for zero-order and conjugate image removal in digital in-line holography

Conventional digital in-line holography requires at least two phase-shifting holograms to reconstruct an original object without zero-order and conjugate image noise. We present a novel approach in which only one in-line hologram and two intensity values (namely the object wave intensity and the reference wave intensity) are required. First, by subtracting the two intensity values the zero-order diffraction can be completely eliminated. Then, an algorithm, called partition calculation, is proposed to numerically remove the conjugate image. A preliminary experimental result is given to confirm the proposed method. The method can simplify the procedure of phase-shifting digital holography and improve the practical feasibility for digital in-line holography.

High‐resolution, High‐speed 3D Measurement Based on Absolute Phase Measurement

A method for 3D measurement with high‐resolution and high‐speed is proposed. Three patterns are projected for 3D measurement. The two patterns of them are sinusoidal fringe patterns whose phases are shifted π each other. A modified method of Fourier Transforming Profilometry is used for phase retrieve. The other pattern is used for identifying the order of sinusoidal fringe pattern. It consists of vertical slits corresponding to the period of sinusoidal fringe. Three grey levels are used to form the slits of pattern. Six symbols are encoded with these three grey levels. Then, a pseudorandom sequence is constructed with an alphabet of these six symbols. The slits are arranged according to the sequence to form the pattern. In the procedure of phase unwrapping, the position of the string which consists of symbols corresponding to three neighbor periods is worked out by string matching in the sequence. The order of sinusoidal fringe pattern is identified. So, the absolute phase can be measured. A system consi...