Zhongsheng Zhai

Extended depth of field through an axicon

One of the main disadvantages of imaging systems is the limited depth of field. We present the ability of a refractive axicon for imaging with an extended depth of field. We design an imaging system with a two-step imaging approach that uses a CCD camera to capture intermediate images and uses a digital process to obtain the final images. The depth of field is analyzed based on the condition of the focal segment. In particular, the point spread functions (PSFs) are discussed in simulation in detail. The performed experiments validate the effect and feasibility of the axicon for imaging with an extending depth of field.

Extended depth of field with the optimal ‘0, π’ binary phase pupil mask

A ‘0, π’ phase pupil mask was developed to extend the depth of the field of a circularly symmetric optical imaging system. A global search algorithm was used to seek an optimal pupil mask which provides the largest spatial frequency band in a certain desired contrast value. The modulation transfer function curves and the normalized point spread function figures of the imaging system with the optimal mask were analyzed. The results show that the imaging system has a high resolution in a long frequency band and can obtain clear images without any post-processing. The experimental results also demonstrate that the depth of field of the imaging system is extended sixfold successfully.

Algorithm for sine wave cure fit based on frequency precise estimation

In the process of curve fitting, the unknown relationship between the data sampling rate and the frequency of the measured signal as well as signals frequency fluctuations can cause remarkable error. In this paper, a sine curve fitting algorithm with frequency precise estimation is proposed to solve this problem. Firstly, ellipse fitting algorithm is used to estimate the measured signal frequency. On the basis of this, the three-parameter sine fitting calculation based on the least squares method is applied to the sampling data, and the accuracy of the sine curve fitting result is improved. The experimental results show that the algorithm proposed in this paper can avoid the problem of frequency fluctuation and error caused by the unknown relationship between the sampling frequencies. Moreover, the fitting has a good randomness after frequency estimation, and the fitting result has nothing to do with the signal amplitude. Compared without frequency accurate estimation, the fitting error is greatly reduced and the fitting precision is higher.

Image classification and recognition method to mechanical parts based on fractal dimension

Complex mechanical parts have characteristics of irregularity and certain statistical self-similarity, which can be described by fractal dimension. And the values of their fractal dimension can be used as an measurement to classify and recognize the mechanical parts. In addition, the values can guide robots to grab parts. However, the image obtained by a vision system, which contains part images main image and image background will affect the calculation of fractal dimension of main images. In order to solve the problem, an improved differential box-counting method is designed in this paper. The fractal dimension of part images which has been cut and rotated can be calculated using this differential box- counting method. The experimental result shows that the improved differential box-counting method can calculate the fractal dimension of different size-length images, and the values are more stable. The improved method solves the problem that traditional algorithm can only calculate the fractal dimension of image which side length is integer power of 2.

Imaging characteristics of the axicon imaging system

The depth of an image system can be extended by an axicon which can generate line focus. According to physical optical theory, the diffracting patterns of the defocus point spread function (PSF) for the imaging system with axicon are analyzed through the generalize pupil function. The expressions of the PSF for the imaging system illuminated by white light are described as the superposition of the intensities in individual monochromatic patterns. Experimental results show that the central portion contains the most energy of the diffraction pattern from the PSF produced by the white light, and the contrast of secondary outside circular rings decreased rapidly. Furthermore, the central spot radius varied slowly with the increase of defocus parameter, and the depth of field of the imaging system is effectively extended with a shortcoming that the images need further processing.

Theoretical analyses of non-diffracting beams interference in a long distance

The intensity distribution of diffraction field of two non-diffracting beams which interfere in a long distance is derived. Non-diffracting beam is generated by an axicon, which is then split into two coherent beams by a beam splitting prism, one of the two beams is regarded as reference beam, which meet the other one through reflector and another beam splitting prism after a long distance. The distribution of interference field is the coherent superposition of each diffraction field. Results show that the locus of interference fringes can be generally assumed hyperbola, the shape of which is affected by interference instance. When the two beams remain parallel, the diffraction pattern is analyzed to be hyperbolic moire fringe,in a case where there is an oblique angle between two beams, the trend of hyperbolic moire fringe change with the interference fringe trend without axicon. Experimental results are compared with theoretical analysis results. Good agreements between them is obtained.

A driving step auto-access method for single-wavelength microscopic interference

Single wavelength microscopic interferometry, driven by the Piezoelectric Transducer (PZT), is a common surface topography measurement method. Its measurement accuracy is directly determined by the original phase acquisition precision of every pixels in area array CCD. Traditional phase identification methods adopt 3 points or 4 points algorithm to obtain the phase. However, they require the displacement step, actuated by the PZT, to strictly keep the same to satisfy the 90° phase condition. Therefore, these methods are only suitable for the strict anti-vibration experimental environment or conditions with high precise closed-loop PZT actuator and strict calibration of interferometric wavelength. An auto-acquisition driving step method for the single wavelength microscopic interferometry is presented in this paper. Firstly, interference sequence diagrams, containing the surface topography information, are gathered by the CCD under open-loop PZT actuating. Next, two pixels whose phase difference is approximate 90° are selected as the calculating center to obtain smoothed gray values with regional gray average algorithm, which can reduce the influence of random noise. Finally, an optimal fitting algorithm for the ellipse, formed by the average gray values, is proposed to obtain the amplitudes and offsets of the two gray values array. According these fitting parameters and gray values, the drive step can be calculated by elliptic equations. Experiments have shown that this method can reduce the requirement conditions of measurement conditions and improve the measurement accuracy.

A high-accuracy signal processing algorithm for frequency scanned interferometry

A high-accuracy signal processing algorithm was designed for the absolute distance measurement system performed with frequency scanned interferometry. The system uses frequency-modulated laser as light source and consists of two interferometers: the reference interferometer is used to compensate the errors and the measurement interferometer is used to measure the displacement. The reference interferometer and the measurement interferometer are used to measure synchronously. The principle of the measuring system and the current modulation circuit were presented. The smoothing convolution was used for processing the signals. The optical path difference of the reference interferometer has been calibrated, so the absolute distance can be measured by acquiring the phase information extracted from interference signals produced while scanning the laser frequency. Finally, measurement results of absolute distances ranging from 0.1m to 0.5m were presented. The experimental results demonstrated that the proposed algorithm had major computing advantages.

Displacement measurement based on TWAIN interface

The standard of technology without an interesting name (TWAIN) was introduced and a displacement measurement method was presented. Based on the theories of image capture and image measurement, a displacement measurement system was developed and subpixel and centroid technique were used to improve the measurement precision. The Experiment results show that the proposed method has low cost and high precision.