Wave-front coded optical readout for the MEMS-based uncooled infrared imaging system

Wavefront coding technique can extend the depth of filed (DOF) of the incoherent imaging system. Several rectangular separable phase masks (such as cubic type, exponential type, logarithmic type, sinusoidal type, rational type, et al) have been proposed and discussed, because they can extend the DOF up to ten times of the DOF of ordinary imaging system. But according to the research on them, researchers have pointed out that the images are damaged by the artifacts, which usually come from the non-linear phase transfer function (PTF) differences between the PTF used in the image restoration filter and the PTF related to real imaging condition. In order to alleviate the image artifacts in imaging systems with wavefront coding, an optimization model based on the PTF was proposed to make the PTF invariance with the defocus. Thereafter, an image restoration filter based on the average PTF in the designed depth of field was introduced along with the PTF-based optimization. The combination of the optimization and the image restoration proposed can alleviate the artifacts, which was confirmed by the imaging simulation of spoke target. The cubic phase mask (CPM) and exponential phase mask (EPM) were discussed as example.

Wavefront coding extended the depth of field to a great extent with simpler structure compared to confocal microscope. With cubic phase mask (CPM) employed in the STOP of the objective lens, blurred images will be obtained in charge coupled device (CCD), which will be restored to sharp images by Wiener filter. We proposed that one CPM is used in one microscope although there are different objective lenses with different power indices. The microscope proposed here is the wavefront coding one when the CPM is used in the STOP; while it is the traditional one when a plane plate is used in the STOP. Firstly, make the STOP in the last surface of the lens, and then add a plane plate at the STOP with the same material and the same center thickness of the CPM. Traditional objective lenses are designed, based on which wavefront coding system will be designed with the plane plate replaced by a CPM. Secondly, the parameters of CPMs in different objective lenses are optimized to certain ranges based on metric function of stability of modulation transfer function (MTF). The optimal parameter is chosen from these ranges. A set of objective lenses is designed as an example with one CPM. The simulation results shows that the depth of field of 4X, 10X, 40X, 60X and 100X objective lenses with the same CPM can reach to 400um, 40um, 24um, 16um and 2um respectively, which is much larger than 55.5um, 8.5um, 1um, 0.4um and 0.19um of the traditional ones.

Wavefront coding involves the insertion of an asymmetric refractive mask close to the pupil plane of an imaging system so as to encode the image with a specific point spread function that, when combined with decoding of the recorded image, can enable greatly reduced sensitivity to imaging aberrations. The application of wavefront coding has potential in the fields of microscopy, where increased instantaneous depth of field is advantageous and in thermal imaging where it can enable the use of simple, low-cost, light-weight lens systems. It has been previously shown that wavefront coding can alleviate optical aberrations and extend the depth of field of incoherent imaging systems whilst maintaining diffraction-limited resolution. It is particularly useful in controlling thermally induced defocus aberrations in infrared imaging systems. These improvements in performance are subject to a range of constraints including the difficulty in manufacturing an asymmetrical phase mask and significant noise amplification in the digitally restored image. We describe the relation between the optical path difference (OPD) introduced by the phase mask and the magnitude of noise amplification in the restored image. In particular there is a trade between the increased tolerance to optical aberrations and reduced signal-to-noise ratio in the recovered image. We present numerical and experimental studies based of noise amplification with the specific consideration of a simple refractive infrared imaging system operated in an ambient temperature varying from 0°C to +50 C. These results are used to delineate the design and application envelope for which infrared imaging can benefit from wavefront coding.

A wavefront coding (WFC) technique provides an extension of the depth of field for a microscopy imaging system with slight loss of image spatial resolution. Through the analysis of the relationship between the incidence angle of light at the phase mask and the system pupil function, a mixing symmetrical cubic phase mask (CPM) applied to 5X-40X micro-objectives is optimized simultaneously based on point-spread function (PSF) invariance and nonzero mean values of the modulation transfer function (MTF) near the spatial cut-off frequency. Optimization results of the CPM show that the depth of field of these micro-objectives is extended 3-10 times respectively while keeping their resolution. Further imaging simulations also prove its ability in enhancing the defocus imaging.

<p>\n\tWave-front coding, proposed by Dowski and Cathey in 1995, is widely known to be capable of extending the depth of focus (DOF) of incoherent imaging systems. However, benefiting from its very large point spread function (PSF) generated by a suitably designed phase mask that is added to the aperture plane, wave-front coding could also be used to achieve super-resolution without replacing the current sensor with one of smaller pitch size. An image amplification based super-resolution reconstruction procedure has been specifically designed for wave-front coded imaging systems and its effectiveness has been tested by experiment. For instance, for a focal length of 50 mm and f-number 4.5, objects within the range [5 m, infinity] are clearly imaged with the help of wave-front coding, which indicates a DOF extension ratio of approximately 20. The proposed super-resolution reconstruction procedure produces at least 3x resolution improvement, with the quality of the reconstructed super-resolution image approaching the diffraction limit.</p>

A novel anti-cat-eye effect imaging technique based on wavefront coding is proposed as a solution to the problem of previous anti-cat-eye effect imaging techniques where imaging quality was sacrificed to reduce the retroreflection from the photoelectric imaging equipment. With the application of the Fresnel–Kirchhoff diffraction theory, and the definition of generalized pupil function combining both phase modulation and defocus factors, the cat-eye echo formation of the wavefront coded imaging system is theoretically modeled. Based on the physical model, the diffracted spot profile distribution and the light intensity distribution on the observation plane are further simulated with the changes in the defocus parameter and the phase modulation coefficient. A verification test on the cat-eye laser echo power of the wavefront coded imaging system and that of the conventional imaging system at a 20 m distance are conducted, respectively. Simulations and experiment results show that compared with conventional imaging systems, the wavefront coding imaging system can reduce the retroreflection echo by two orders of magnitude while maintaining better imaging quality through defocusing.

Since some previous methods sacrifice too much imaging quality to achieve the valid retroreflection reduction, a novel image plane tilted system combined with the wavefront coding is proposed in this paper, which could improve the privacy and security of the imaging equipment by eliminating the retroreflection without suffering significant degradation in imaging quality. First, the theoretical model of the tilted wavefront coding system is established based on the wave-optics theory. According to the derived point spread function and the modulation transfer function, the maximum tilt angle of the image plane is obtained. Second, the target system is designed and optimized by Zemax, and the later simulation analyzes the effects of the tilting on both the retroreflection and the imaging property. Finally, the experiments verify the simulation results. It shows that when the tilt angle equals 5°, there is no longer significant retroreflection on the observation plane with the total receiving power decreasing to nearly 4 orders of magnitude compared with the state without tilting. Moreover, within the maximum tilt angle, the final decoded images of the wavefront coding system still maintain superior clarity and uniformity for both the artificial and the natural objects. Consequently, the BRISQUE index is reduced by 15.92% and 28.19%, respectively, compared with the conventional imaging system, which indicates that the image quality is significantly improved by the proposed system. In conclusion, the technique of wavefront coding could effectively compensate for the high-order aberrations induced by the tilting and preserve a large field of view for precise imaging with no retroreflection, which shows us a broad prospect.

Wavefront coding technology can extend the depth of field of the iris imaging system, but the iris image obtained through the system is coded and blurred and can't be used for the recognition algorithm directly. The paper presents a fuzzy iris image restoration method used in the wavefront coding system. After the restoration, the images can be used for the following processing. Firstly, the wavefront coded imaging system is simulated and the optical parameter is analyzed, through the simulation we can get the system's point spread function (PSF). Secondly, using the blurred iris image and PSF to do a blind restoration to estimate a appropriate PSFe. Finally, based on the return value PSFe of PSF, applying the regularization filter on the blurred image. Experimental results show that the proposed method is simple and has fast processing speed. Compared with the traditional restoration algorithms of Wiener filtering and Lucy-Richardson filtering, the recovery image that got through the regularization filtering is the most similar with the original iris image.

Imaging systems using aspheric imaging lenses with complementary computation can deliver performance unobtainable in conventional imaging systems. These new imaging systems, termed Wavefront coded imaging systems, use specialized optics to capture a coded image of the scene. Decoding the intermediate image provides the "human-usable" image expected of an imaging system. Computation for the decoding step can be made completely transparent to the user with today's technology. Real-time Wavefront coded systems are feasible and cost-effective. This "computational imaging" technology can be adapted to solve a wide range of imaging problems. Solutions include the ability to provide focus-free imaging, to increase the field of view, to increase the depth of read, to correct for aberrations (even in single lens systems), and to account for assembly and temperature induced misalignment. Wavefront coded imaging has been demonstrated across a wide range of applications, including microscopy, miniature cameras, machine vision systems, infrared imaging systems and telescopes.

Underwater laser imaging is of great significance in underwater search and marine science, <i>etc</i>. However, traditional underwater laser imaging is often of poor quality with noises and blurs, moreover, the resolution of the image is also low. In order to obtain clear underwater images with high resolution and quality, here, we have designed a range gated imaging underwater imaging system and realized an image restoration approach. In this paper, based on the introduction to the imaging system and image restoration algorithm, the experiment is established by setting the imaging system under water in the lake to capture the underwater targets. With the proposed underwater image restoration approach, images of high quality could be retrieved which proves that the method is able to identify the target ~10 meters away underwater.

This paper introduces an expectation-maximization (EM) algorithm for image restoration (deconvolution) based on a penalized likelihood formulated in the wavelet domain. Regularization is achieved by promoting a reconstruction with low-complexity, expressed in the wavelet coefficients, taking advantage of the well known sparsity of wavelet representations. Previous works have investigated wavelet-based restoration but, except for certain special cases, the resulting criteria are solved approximately or require demanding optimization methods. The EM algorithm herein proposed combines the efficient image representation offered by the discrete wavelet transform (DWT) with the diagonalization of the convolution operator obtained in the Fourier domain. Thus, it is a general-purpose approach to wavelet-based image restoration with computational complexity comparable to that of standard wavelet denoising schemes or of frequency domain deconvolution methods. The algorithm alternates between an E-step based on the fast Fourier transform (FFT) and a DWT-based M-step, resulting in an efficient iterative process requiring O(N log N) operations per iteration. The convergence behavior of the algorithm is investigated, and it is shown that under mild conditions the algorithm converges to a globally optimal restoration. Moreover, our new approach performs competitively with, in some cases better than, the best existing methods in benchmark tests.

Through the introduction of a simply designed phase mask at the pupil of a conventional imaging system, the range to objects in view can be precisely determined. The mask allows for roughly 50% light transmission, and is segmented to introduce a 1/4 wave phase shift into half of the transmitted light. The resulting point spread function is highly sensitive to object range and is incorporated into the resultant image. Through comparison of the wavefront coded image with a conventional image of the same scene, the precise range to the object can be determined. The range measurement precision obtained is directly related to image contrast. For objects having a simple linear edge possessing a contrast of 32 gray levels above noise, range can be measured to 1% precision. While based on an interference effect, wide-band polychromatic light can be used to determine object range. No actively moving components in the optical system are required for operation. The resultant approach allows for imaging and range determination simultaneously. Based on a fundamental interference phenomenon, this approach is applicable to all passive optical imaging systems ranging from the UV to the infrared. Both a theoretical analysis and an experimental verification of the approach showing the expected performance is provided.

Iris recognition imaging is attracting considerable interest as a viable alternative for personal identification and verification in many defense and security applications. However current iris recognition systems suffer from limited depth of field, which makes usage of these systems more difficult by an untrained user. Traditionally, the depth of field is increased by reducing the imaging system aperture, which adversely impacts the light capturing power and thus the system signal-to-noise ratio (SNR). In this paper we discuss a computational imaging system, referred to as Wavefront Coded(R) imaging, for increasing the depth of field without sacrificing the SNR or the resolution of the imaging system. This system employs a especially designed Wavefront Coded lens customized for iris recognition. We present experimental results that show the benefits of this technology for biometric identification.

The image deconvolution technique can recover potential sharp images from blurred images affected by aberrations. Obtaining the point spread function (PSF) of the imaging system accurately is a prerequisite for robust deconvolution. In this paper, a computational imaging method based on wavefront coding is proposed to reconstruct the wavefront aberration of a photographic system. Firstly, a group of images affected by local aberration is obtained by applying wavefront coding on the optical system’s spectral plane. Then, the PSF is recovered accurately by pupil function synthesis, and finally, the aberration-affected images are recovered by image deconvolution. After aberration correction, the image’s coefficient of variation and mean relative deviation are improved by 60% and 30%, respectively, and the image can reach the limit of resolution of the sensor, as proved by the resolution test board. Meanwhile, the method’s robust anti-noise capability is confirmed through simulation experiments. Through the conversion of the complexity of optical design to a post-processing algorithm, this method offers an economical and efficient strategy for obtaining high-resolution and high-quality images using a simple large-field lens.

Microscopic study of rapid biological processes often requires both high resolution and high acquisition speed. When the speed requirement precludes acquiring a full 3D focal series at each time point, it can be attractive to sacrifice all axial information and instead record a single, 2D image per time point. This can be done at very high frame rates. High-resolution objectives, however, have a very short depth of focus. There are several established methods to achieve extended depth of focus, including annular pupil masks; mechanical sweeping of the focus and wavefront coding, which uses a pupil-plane optical device to introduce geometric aberrations. We have developed a new pupil plane approach where the light is manipulated chromatically rather than geometrically. A phase mask with circularly symmetric stair steps divides the pupil plane into a series of annular zones. The stair steps are large compared to the coherence length of the observation light, so that images from different zones form independently and combine incoherently into a final image. Each zone carries only a fraction of the objective's axial resolution, but the larger zones still carry the full lateral resolution of the objective. The incoherent addition of the different single-zone images results in a smooth and circularly symmetric point spread function with a depth of focus that is extended by a factor approximately equal to the number of zones in the mask. The method has been demonstrated both on bead samples and on whole cells with a performance that is well in accordance with the theoretical predictions.