Chir-Weei Chang

Digital Decoding Design for Phase Coded Imaging System

This paper develops a digital decoding design for the imaging system with phase coded lens. The phase coded lens is employed to extend the depth of filed (DoF), and the proposed design is used to restore the special-purpose blur caused by the lens. Since in practice the imaging system inevitably contains manufacturing inaccuracy, it is often difficult to obtain precise point spread function (PSF) for image restoration. To deal with this problem, we develop a flow for designing filters without PSF information. The imaging system first takes a shot of a well-designed test chart to have a blur image of the chart. This blur image is then corrected by using the perspective transformation. We use both of the image of the test chart and the corrected blur image to calculate a minimum mean square error (MMSE) filter, so that the blur image processed by the filter can be very alike to the test chart image. The filter is applied to other images captured by the imaging system in order to verify its effectiveness in reducing the blur and for showing the capability of extending the DoF of the integrated system.

Tolerance analysis of lenses with high zoom ratio

Axial and lateral compensators are used for zoom systems tolerancing to improve the manufacturability of high zoom ratio lenses. An investigation into the tolerance distribution for various zoom ratios, and compensation conditions and corresponding performance is presented. A flexible method is applied to a rear focusing four-groups 10X zoom to demonstrate the effectiveness of different compensators.

Single shot depth camera lens design optimization based on a blur metric

Computational imaging technology can capture extra information at the sensor and can be used for various photographic applications, including imaging with extended depth of field or depth extraction for 3D applications. The depth estimation from a single captured photograph can be achieved through a phase coded lens and image processing. In this paper, we propose a new method to design a phase coded lens, using a blur metric (BM) as the design criterion. Matlab and Zemax are used for the co-optimization of optical coding and digital image process. The purpose of the design is to find a curve for which the BM changes continuously and seriously within a distance range. We verified our approach by simulation, and got a axial symmetric phase mask as the coded lens. By using a pseudo-random pattern which contains uniform black and white patches as the input image, and the on-axis point spread function (PSF) calculated from Zemax, we can evaluate the BM of the simulated image which is convoluted by the pseudo-random pattern and PSF. In order to ensure the BM curve evaluated from the on-axis PSF represents the result of the whole field of view, the PSF is also optimized to get high off-axis similarity.

Depth perception with a rotationally symmetric coded camera

A novel design of a phase coded depth-sensing camera is presented. A rotational symmetric phase mask is designed to discriminate the point spread functions (PSF) from different scene distances. The depth information can then be computationally obtained from a single captured photograph through a phase coded lens. The PSF must be carefully optimized at off-axis angles in order to create a restored image which is sharp over the required field of view. In this paper, a phase coded depth camera with a focal length 10.82mm, sensor size 2mm and F-number 5 is designed. Simulation data is exchanged between Matlab and Zemax for co-optimization of optical coding and digital decoding process. The simulation result shows that coarse depth information is investigated for object distance from 513 mm to 1000 mm.

Image deblurring using a pyramid-based Richardson-Lucy algorithm

In image deblurring, it is important to reconstruct images with small error, high perception quality, and less computational time. In this paper, a blurred image reconstruction algorithm, which is a combination of the Richardson-Lucy (RL) deconvolution approach and a pyramid structure, is proposed. The RL approach has good performance in image reconstruction. However, it requires an iterative process, which costs a lot of computation time, and the reconstructed image may suffer from a ringing effect. In the proposed algorithm, we decompose a blurred image from a coarse scale to a fine scale and progressively utilize the RL approach with different number of iterations for each scale. Since the number of iterations is smaller for the large scale part, the computation time can be reduced and the ringing effect caused from details can be avoided. Simulation results show that our proposed algorithm requires less computation time and has good performance in blurred image reconstruction.

Mobile phone imaging module with extended depth of focus based on axial irradiance equalization phase coding

This paper presents a mobile phone imaging module with extended depth of focus (EDoF) by using axial irradiance equalization (AIE) phase coding. From radiation energy transfer along optical axis with constant irradiance, the focal depth enhancement solution is acquired. We introduce the axial irradiance equalization phase coding to design a two-element 2-megapixel mobile phone lens for trade off focus-like aberrations such as field curvature, astigmatism and longitudinal chromatic defocus. The design results produce modulation transfer functions (MTF) and phase transfer functions (PTF) with substantially similar characteristics at different field and defocus positions within Nyquist pass band. Besides, the measurement results are shown. Simultaneously, the design results and measurement results are compared. Next, for the EDoF mobile phone camera imaging system, we present a digital decoding design method and calculate a minimum mean square error (MMSE) filter. Then, the filter is applied to correct the substantially similar blur image. Last, the blur and de-blur images are demonstrated.

Digital image restoration for phase-coded imaging systems

This paper proposes a digital image restoration algorithm for phase-coded imaging systems. In order to extend the depth-of- field (Dof), an imaging system equipped with a properly designed phase-coded lens can achieve an approximately constant point spread function (PSF) for a wide range of depths. In general, a phase-coded imaging system produces blurred intermediate images and requires subsequent restoration processing to generate clear images. For low-computational consumer applications, the kernel size of the restoration filter is a major concern. To fit for practical applications, a pyramid-based restoration algorithm is proposed in which we decompose the intermediate image into the form of Laplacian pyramid and perform restoration over each level individually. This approach provides the flexibility in filter design to maintain manufacturing specification. On the other hand, image noise may seriously degrade the performance of the restored images. To deal with this problem, we propose a Pyramid-Based Adaptive Restoration (PBAR) method, which restores the intermediate image with an adaptive noise suppression module to improve the performance of the phase-coded imaging system for Dof extension.

Manufacturing process optimization of phase plates for depth extension microscopy systems

Extended depth of field (EDoF) technology can be applied to imaging systems by merging phase-coding design and digital signal processing. This paper presents an application of EDoF to the microscope platform and shows the capability to capture EDoF images in a single shot. Ultra-precision machining conditions for a phase-coding component were compared the peak-to-valley (P-V) error of the cubic surface with the performance of the EDoF. For the phase variation is very small for this phase plate, determining the optimal cutting condition at which the quality of phase plate is stabilized is very important. Therefore, the single point diamond turning (SPDT) was used to manufacture the optical components for its high precision. And the results are as following, the accuracy of non-symmetric phase plate found between 0.4 μm and 1μm had better performance of the EDoF image. Overall, the depth of field of the new objective could be increased more than five times compared to an objective having no such phase plate. The purpose of this study is to compare the relationship between PV error of phase plate surface and imaging restoration quality, which maybe a good benchmark in this field.

Ablation of silicon by focusing a femtosecond laser through a subwavelength annular aperture structure

We experimentally examined the effect of laser energy fluence on the ablation of a silicon wafer using a Ti:sapphire femtosecond laser system. A femtosecond laser was focused through an oxide-metal-oxide (Al2O3/Al/Al2O3) film engraved with a subwavelength annular aperture (SAA) structure, i.e., a Bessel beam composed of a femtosecond laser created using a SAA. The optical performance, such as depth-of-focus (DOF) and focal spot of the SAA structure, was simulated using finite-difference time domain (FDTD) calculations. We found that a far-field laser beam propagating through the SAA structure possesses a sub-micron focal spot as well as high focus intensity. The experimental results demonstrated that silicon can be ablated using an input ablation threshold of an order of 0.05 J/cm2 with a pulse duration at around 120fs. We found the obtained surface hole to have a diameter smaller than 1μm. Different surface ablation results obtained by using different threshold fluences of input laser energy are shown. Possible applications of this technique includes executing high aspect ratio laser drilling for thin film microfabrication, undertaking thru silicon via (TSV) for 3DIC, etc.

Using liquid lens in wavefront coded imaging system

A novel design of a wavefront coded compact camera system with liquid lens is presented. The point spread function must remain practically constant over a wide range of defocus in order to create an image which is sharp over a large depth of field. Therefore, the trade-off between the effect of extended depth of field and reduction in the signal to noise ratio of the restored image become critical. By combining liquid lens and wavefront coding, the system can deliver much greater depth of field without the restoration problems caused by the similarity of point spread function. This could improve the overall image performance.