Digital Double-pulsed Holographic Interferometry for Vibration Analysis

Abstract

In this paper we describe a digital double-pulsed holographic system. Two separate holograms of an object under test are recorded within a few microseconds using a charge-coupled device camera and stored in a frame grabber. The holograms are digitally reconstructed using a computer, by simulation of the Fresnel diffraction of the hologram illuminated by the reference wave. The phases of the two reconstructed wave fields are calculated from the complex amplitude and the deformation is obtained from the phase difference. Experimental results are presented.     

AN OPTICAL METHOD FOR CONTINUOUSLY MEASURING FLOW VELOCITY AND COMPOSITION OF A MIXTURE OF POWDERS

ABSTRACT

A new method for continuously measuring flow velocity and composition of a mixture of powders in solid mixing equipment is proposed in this paper. This method uses an optical technique combining the image sensor anemometer of reflected light system and an image fiber inserted directly into the flowing powders' bed. From experiments for demonstration of this method, it is found that the flow velocity and the composition of the powder mixture in the mixer can be measured accurately and with high precision by applying this method.     

Multiple fractional fourier transform hologram by use of holographic lens

Abstract

A new method is presented to record multiple fractional Fourier transform holograms by use of a holographic lens. With holographic lenses, several fractional Fourier transform holograms of different objects can be recorded in a simple way, and images of these recorded objects can be reconstructed in different positions and directions in three-dimensional space. In this paper, the theory and characteristics of the hologram recorded using the holographic lens are analysed. Using this method, a multiple fractional Fourier transform hologram was fabricated, with satisfactory results.     

High compressive ghost imaging method based on discrete cosine transform using weight coefficient matching

ABSTRACT

Ghost imaging can capture a scene without directly catching sight of the target, but in the case of high compression ratio, high-quality imaging is challenging at present. Here a ghost imaging method using weight coefficient matching based on discrete cosine transform (DCT) is proposed, in which the high-quality target images can be retrieved by obtaining the larger weight value in one-dimensional (1D) DCT spectrum. In the case of low sampling, the proposed method can not only acquire the spectral coefficients with large weight, but also put them in the correct position; eventually it can obtain the desired image by inverse discrete cosine transform of the spectrum. At the same sampling ratio, both simulation results and optical experiments show that the reconstructed image quality of the proposed method exhibits better performance. In addition, even the sampling ratio is as low as about 3%, the outline of the target image can still be roughly recognized.     

IMAGE COMPRESSION SCHEME WITH EMBEDDED MULTILEVEL BLOCK TRUNCATION CODING BASED ON HUMAN VISUAL EFFECT

ABSTRACT

In this paper, we first propose a new embedded multilevel block truncation coding (BTC) technique. Unlike Differential pulse code modulation (DPCM), Vector quantization (VQ), and general multilevel BTC algorithms which determine the image quality at a time, the embedded multilevel BTC improves the image quality largely and progressively until obtaining an image with excellent quality. In order to reduce the bit rate efficiently, we propose a perception model and utilize it to develop a pruning algorithm. The pruning algorithm removes the useless information, which the human eyes are not sensitive, generated by the embedded multilevel BTC. The simulation results indicate that the bit rate with the proposed method is much less than that with the DPCM and general multilevel BTC under the same objective criterion, PSNR, or subjective criterion. This paper also shows that the computation complexity of the proposed method is much less than that with VQ under the same high quality reconstructed image.     

Pixel expansion-free grey-scale image sharing method

Abstract

In recent years, many visual secret sharing technologies have been proposed to protect the security of secret images (black and white, grey scale or colour images). In 2005, Lukac and Plataniotis used the concept of the Naor–Shamir method to propose a visual secret sharing technology for sharing secret grey-scale images based on bit-plane decomposition. Although their method can avoid pixel-value cutting problem, it still suffers from the pixel expansion problem. In this paper, we propose a new secret grey-scale image sharing method to improve this situation. In the proposed sharing image creation phase, two sharing images (one is grey-scale and the other is binary) are created and later shared by two protectors. The original secret image can be easily reconstructed if both sharing images are obtained. However, one cannot obtain the original secret image from each of the sharing images. Experimental results also show that the proposed method can effectively solve the pixel expansion problem.     

Digital image encryption and watermarking by phase-shifting interferometry

A method for both image encryption and watermarking by three-step phase-shifting interferometry is proposed. The image to be hidden is stored in three interferograms and then can be reconstructed by use of one random phase mask, several specific geometric parameters, and a certain algorithm. To further increase the security of the hidden image and confuse unauthorized receivers, images with the same or different content can be added to the interferograms, and these images will have no or only a small effect on the retrieval of the hidden image, owing to the specific property of this algorithm. All these features and the utility of this method for image retrieval from parts of interferograms are verified by computer simulations. This technique uses intensity maps as decrypted images for delivery, and both encryption and decryption can be conveniently achieved digitally. It is particularly suitable for the remote transmission of secret information via the Internet.     

A blind proposed 3D mesh watermarking technique for copyright protection

ABSTRACT

Watermarking provides an efficient mechanism for copyright protection of digital media by embedding information in the object to identify the owner. This paper proposed a watermarking technique that can successfully embed a grey-scale image in a 3D mesh object. In our proposed technique, each three vertices of the 3D object can hide one pixel from the secret image. The experimental results showed that the proposed technique has better visual quality on both the watermarked object and the extracted secret image.     

Long-term reading experiment on a photorefractive holographic memory with the hologram sustainment technique by optical feedback

We demonstrate a long-term continuous readout of a two-dimensional image in a photorefractive holographic memory with a BaTiO₃ crystal. A considerable extension of reading time is achieved by use of a hologram sustainment technique with an optical feedback circuit. Hologram rewritings by the simultaneous illumination of the reading beam and the feedback beam, which is incident on a crystal from the opposite direction to the reading beam, contribute to all-optical hologram sustainment without any fixing technique. In this paper, the effectiveness of the hologram sustainment technique is explained by the analysis of the temporal property of the amplitude of the index grating in a crystal. By calculating the temporal properties of the reconstructed beam intensity, we reveal the feedback rate and the coupling strength for high output efficiency. We perform an experiment on two-dimensional image reading and writing with a 45° cut BaTiO₃ crystal and show that a recorded image can be reconstructed over 20?min without critical image degradation whereas a reconstructed image fades away within 20?s in the conventional readout technique.     

High-efficiency hybrid joint transform correlator with pre-processed input image and replicated binary spectrum array

Abstract

The performance of a hybrid joint transform correlator (JTC) is improved by the input image pre-processing method and our proposed spectrum binarizing and replicating technique. The input image pre-processing can enhance the correlation discrimination ability of a JTC system, and the replication of the binary spectrum under an appropriate condition can further substantially increase the light efficiency, the maximum correlation peak intensity, the peak sharpness and the signal-to-noise ratio of the system at the same time by the mechanics of constructive interference. Theoretical analyses are presented. Both the computer simulation and optical experiment verifying the effectiveness of this technique are also given.     

Two-dimensional Phase Locked Loop Demodulation of Interferograms

Abstract

A new technique for continuous phase determination of an interferogram based on a digital phase locked loop is presented. The main advantage of this method, with respect to well established techniques such as Fourier or phase stepping demodulation, is that the traditional approach to phase unwrapping processes by removal of discontinuities is not required. The phase is determined continuously as the phase locked loop scans the two-dimensional interferogram. Because of the sequential nature of the algorithm proposed, this can be implemented using a special purpose video processor for phase determination at video rates. The above mentioned properties makes the presented technique a fast algorithm for phase determination of carrier frequency interferograms modulated by a two-dimensional smooth phase function.     

A Study of Errors due to Reconstruction Filters in Computed Tomography

Abstract

In the summation convolution backprojection method of image reconstruction in computed tomography, the final image accuracy depends on the convolution filter used. Filters are designed to attenuate high spatial frequencies when noisy projection data are used. This paper explores the differences between the images reconstructed using a range of filters, and compares the results with the case of the ramp filter that provides the “best” image for ideal, noise-free, projection data. It is shown that systematic errors between these images and the best image exist, and that these errors are related to the second differential of the reconstruction filter with respect to spatial frequency. This error determination may be used to correct computed tomography images that have been reconstructed using inappropriate filters, and this theory is tested using noise-free projection data from two computer simulated images. It is shown that the corrected images are far closer to the original images.     

Progressive image transmission using a prefilter and a modified difference pyramid structure

Abstract

Pyramid data structures have played an important role in progressive image transmission. Over the years, the reduced difference pyramid (RDP) has been found to be one of the best data structure. The RDP takes the differences between the neighboring nodes at the same level. The new modified difference pyramid (MDP) data structure, developed in this paper, takes the differences between successive levels. Simulation results show that both the bit rate and the complexity of the receiver for the MDP structure are lower than those for the RDP structure at the same quality.

An MDP coding process which incorporates a prefilter is also proposed in this paper. Simulation results show it can provide good quality (in a subjective sense) reconstructed images at a lower bit rate than the unfiltered scheme can. Also, acceptable intermediate images are interpolated via the repetition method and via the cubic convolution method to get images which are the same size as the original. Results are compared.     

Sub‐band coding of digital images

Abstract

An efficient sub‐band coding method for encoding images is presented in this paper. In this method a frequency band decomposition of the image is carried out by means of two‐dimensional separable quadrature mirror filters (QMF's), which split the image spectrum into sixteen uniform sub‐bands. In the coding process, we employ special quantizers and encoding algorithms which are designed according to the characteristics (including the visual perceptual property) of the sub‐bands to quantize and to encode these subimages, separately. The simulation results are presented in terms of average bit rates and the quality (in subjective as well as mean squared senses) of the reconstructed images. These results indicate that a good quality reproduction can be achieved at high compression rates. The paper also shows that a seven‐band decomposition has very high performance in quality, compression rate, and economy.     

Contour finding by means of subjective tests

Abstract

A special method has been found and used for the rapid finding of the contours in a natural image. A reference contour image was thus formed by a subjective test. The reference contour image can be further adopted as an objective criterion for other contour‐finding algorithms.     

Palmprint enhancement using recursive histogram equalisation

Abstract

Palmprint identification system is one of the most powerful personal identification systems in recent years. In order to achieve high identification accuracy, all parts of the palmprint are needed to be enhanced. Histogram equalisation is a very popular image enhancing technique. A novel histogram equalisation technique, called recursive\ histogram equalisation, for brightness preservation and image contrast enhancement, is put forward in this paper. The essence of proposed algorithm is to decompose an input histogram into two or more sub-histograms recursively based on its mean, change the sub-histograms through a weighting process based on a normalised power law function and then equalise the weighted sub-histograms independently. Experiments show that our method preserves the mean brightness of a given image, enhances the contrast and produces more natural looking images than the other histogram equalisation methods.     

Temporal phase evaluation by fourier analysis of fringe patterns with spatial carrier

Abstract

A new method is presented for the temporal evaluation of fringe patterns with spatial carrier. The proposed technique involves recording the irradiance fluctuations obtained when a linear variation of the set-up sensitivity is introduced. In this condition, the use of a spatial carrier introduces a linear temporal carrier frequency. In this way, Fourier analysis can be performed to obtain the phase and, finally, the quantity to be measured. The optimum conditions for the sensitivity variation have been studied in order to minimize the errors associated with the Fourier analysis. The technique has been applied to measure the distribution of ray deflections on the surface of two ophthalmic lenses using a deflectometric set-up.     

Pseudocolour encoding by lensless one-step rainbow holography with a synthesized slit

Abstract

Pseudocolour encoding of holographic images by lensless one-step rainbow holography with a synthesized slit is proposed. When a different portion of the object is recorded the reference wave is incident at a different angle θ and during each exposure the incident angle changes continuously around this angle θ. The pseudocolour holographic image is obtained by white-light reconstruction of the lensless one-step rainbow hologram with synthesized slit. The basic advantage of this technique is the elimination of a narrow slit and a lens in the encoding process. The theoretical analysis and experimental result are presented.     

Microwave diversity imaging system ‐ principle,methodology and experimental results

Abstract

The basic principle, methodology and experimental results of the automated microwave diversity imaging system developed in the Electromagnetic Research Laboratory of National Taiwan University are presented. The microwave image formulation is based on Bojarski's identity. In practice, accessing the object backscattering information using the frequency, angular and polarization diversity techniques is conducted by the developed microwave measurement system with major parts including dual polarization antennas, network analyzer and a MicroVax III. It is shown that images of a scale model of the space shuttle reconstructed from the measured data can be identified visually. The implications for high‐resolution microwave imaging radar networks are also discussed.