Passive range measurement through wavefront coding

In the space limited infrared imaging system based MEMS, the adjustment of optical readout part is inconvenient. This paper proposed a method of wave-front coding to extend the depth of focus/field of the optical readout system, to solve the problem above, and to reduce the demanding for precision in processing and assemblage of the optical readout system itself as well. The wave-front coded imaging system consists of optical coding and digital decoding. By adding a CPM (Cubic Phase Mask) on the pupil plane, it becomes non-sensitive to defocussing within an extended range. The system has similar PSFs and almost equally blurred intermediate images can be obtained. Sharp images are supposed to be acquired based on image restoration algorithms, with the same PSF as a decoding core.We studied the conventional optical imaging system, which had the same optical performance with the wave-front coding one for comparing. Analogue imaging experiments were carried out. And one PSF was used as a simple direct inverse filter, for imaging restoration. Relatively sharp restored images were obtained. Comparatively, the analogue defocussing images of the conventional system were badly destroyed. Using the decrease of the MTF as a standard, we found the depth of focus/field of the wave-front coding system had been extended significantly.

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.

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.

<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>

Real‐time terahertz passive imaging systems are highly demanded in security checking application. In this paper, two four‐pixel receiver arrays for 340 and 250 GHz passive imaging systems are presented. As the core device of the front‐end of the receiving array, the low noise terahertz mixer is realized based on the enhanced Schottky diode model and low‐loss circuit structure. The minimum double side band (DSB) noise temperature of the separated 340 and 250 GHz mixers is 900 and 580 K, respectively, within the operational band‐width. Without considering fluctuation in the power gain, the calculated temperature sensitivity of 340 GHz receiver is 0.8 K with 40 GHz RF band. Finally, based on all‐solid‐state millimeter‐wave drive front end and reflector optics, a passive terahertz imaging system is completed. Less than 1 cm space resolution is achieved for 1–3 m object distances, and the distinct imaging to hidden threats is produced and made contrast among the two receiver arrays. The perfect imaging performances of the system verify the low noise characteristic of terahertz mixer. The Schottky solid‐state receiver array provides a cheap and feasible option for personal security screening.

Much research effort has been spent in the 21st century on superresolution imaging techniques, methods which can beat the diffraction limit. Subwavelength composite structures called ``metamaterials" had initially shown great promise in superresolution imaging applications in the early 2000s, owing to their potential for nearly arbitrary capabilities in controlling light. However, for optical frequencies they are often plagued by absorption and scattering losses which can decay or destroy their interesting properties. Similar issues limit the application of other superresolution devices operating as effective media, or metal films that can transfer waves with large momentum by supporting surface plasmon polaritons. In this dissertation, new methods of mitigating the loss of object information in lossy and noisy optical imaging systems are presented. The result is an improvement in the upper bound on lateral spatial resolution. A concentration is placed on metamaterial and plasmonic imaging systems, and the same methods are subsequently adapted to more conventional far-field imaging systems. First, through numerical simulation it is shown that a lossy metamaterial lens has degraded imaging performance which can be partially compensated by deconvolution post-processing of the resultant image. This post-processing procedure is then shown to emulate a physical process called plasmon injection, which has been previously implemented to effectively remove the losses in a plasmonic metamaterial. Next, a more realistic scenario is considered; a thin film of silver acting as a near-field plasmonic ``superlens." In this case, methods are implemented to model incoherent light propagation so that the image can be reconstructed using only intensity data, removing the need for phase measurement. The same procedure from above is followed, and the resolution is enhanced. To push the resolution further, a spatial filtering method called active convolved illumination is developed to overcome the resolution limit set by the noise floor of the system. Finally, the spatial filtering methods are applied to more a more conventional far-field imaging system. Supported by experiment, the lateral resolution of a low numerical aperture imaging system is improved by blocking photons at the Fourier plane. For coherent light, a diffractive superlens is designed which uses the same principles from the above theory, except it encodes the high spatial frequency waves into propagating waves via a diffraction grating. The result is lateral resolution performance that surpasses similar previously published devices by 10 nm at a wavelength more than 80 nm longer.

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.

The properties of UWFC (Ultra Wide-Field Camera) astronomical systems along with specific visual data in astronomical images contribute to a comprehensive evaluation of the acquired image data. These systems contain many different kinds of optical aberrations which have a negatively effect on image quality and imaging system transfer characteristics, and reduce the precision of astronomical measurement. It is very important to figure two main questions out. At first: In which astrometric depend on optical aberrations? And at second: How optical aberrations affect the transfer characteristics of the whole optical system. If we define the PSF (Point Spread Function) [2] of an optical system, we can use some suitable methods for restoring the original image. Optical aberration models for LSI/LSV (Linear Space Invariant/Variant) [2] systems are presented in this paper. These models are based on Seidel and Zernike approximating polynomials [1]. Optical aberration models serve as suitable tool for estimating and fitting the wavefront aberration of a real optical system. Real data from the BOOTES (Burst Observer and Optical Transient Exploring System) experiment is used for our simulations. Problems related to UWFC imaging systems, especially a restoration method in the presence of space variant PSF are described in this paper. A model of the space variant imaging system and partially of the space variant optical system has been implemented in MATLAB. The “brute force” method has been used for restoration of the testing images. The results of different deconvolution algorithms are demonstrated in this paper. This approach could help to improve the precision of astronomic measurements.

Spectral sensing has been widely utilized for detecting foodborne contaminants. The objective of this research was to develop a handheld multispectral instrument for food safety inspection for poultry and other foodstuffs. The prototype system developed in this research consisted of a compact two-band spectral imaging system, LED light, and handheld computer. The two-band spectral imaging system was a two-port imaging system that consisted of two identical digital monochrome cameras, optical system, and two narrow bandpass filters. By incorporating an interchangeable filter design, the imaging system can measure any two spectral bands in the 400 to 1000 nm portion of the spectrum without any complicated manufacturing process. That is a great advantage in terms of the selection of the spectral bands, as compared to conventional multispectral imaging systems that integrate filters and sensors in one unit. Lens distortions and geometric misalignment of the two cameras were mathematically corrected, resulting in co-registered two-band images. The prototype system was tested with poultry carcasses, and the preliminary results showed that the handheld two-band spectral imaging system could effectively detect feces and ingesta on the surface of poultry carcass.

Diagnostics are crucial to the operation of all particle accelerator facilities. They provide a means of optimising beam parameters and monitoring beam quality for users. However, for the next generation of accelerators, operating in extreme parameter spaces, most current beam diagnostics will fail; either through material failure or lack of resolution. This thesis presents three new diagnostic concepts to address this imminent diagnostic need. These novel diagnostics are supported by a simulation-based algorithm, successfully benchmarked throughout this work. All work has been conducted by the Author unless otherwise stated and referenced. The first technique measures sub-micron transverse beam profiles using optical transition radiation (OTR). Simulations were used to design a new imaging system, which was then installed at KEK (Japan). OTR images were then collected for a range of beam sizes. Comparisons were then made with simulations to successfully achieve sub-micron beam size measurements. Validation of these results was found in the agreement across different imaging systems for fixed beam parameters, providing a self-consistency check for the algorithm in lieu of alternate corroboration. The next diagnostic is a bunch length monitor for ∼<100 fs particle bunches; based on imaging the spatial distribution of coherent transition radiation (CTR) in the THz regime. Simulations demonstrated a relationship between image width and intensity, with bunch length. A THz imaging system was then installed at MAX IV (Sweden). CTR images were taken for a range of compressions. Maximum compression was successfully identified by monitoring relative changes in the properties of the images. Bunch lengths were then acquired from these images through comparison with simulations. Finally, the optical performance of a Digital Micro-mirror Device (DMD) was assessed. Image resolution produced by a DMD was quantified by measuring the Point Spread Function (PSF) of an imaging system, which was capable of remotely replacing a DMD with a planar mirror. The PSFs were measured using the sub-micron focus of a laser. Results show no measurable alteration to a PSF with the introduction of a DMD. This provides a foundation for the development of future diagnostics centred around this device. This thesis provides several validated proof-of-principle experimental results and supporting simulations. Future work has been identified for each topic, only possible following the results presented here, which will provide an array of novel next generation diagnostics.

Photoacoustic (PA) imaging systems are spreading in the biomedical community, and the development of new PA contrast agents is an active area of research. However, PA contrast agents are usually characterized with spectrophotometry or uncalibrated PA imaging systems, leading to partial assessment of their PA efficiency. To enable quantitative PA spectroscopy of contrast agents in vitro with conventional PA imaging systems, we have developed an adapted calibration method. Contrast agents in solution are injected in a dedicated non-scattering tube phantom imaged at different optical wavelengths. The calibration method uses a reference solution of cupric sulfate to simultaneously correct for the spectral energy distribution of excitation light at the tube location and perform a conversion of the tube amplitude in the image from arbitrary to spectroscopic units. The method does not require any precise alignment and provides quantitative PA spectra, even with non-uniform illumination and ultrasound sensitivity. It was implemented on a conventional imaging setup based on a tunable laser operating between 680 nm and 980 nm and a 5 MHz clinical ultrasound array. We demonstrated robust calibrated PA spectroscopy with sample volumes as low as 15 μL of known chromophores and commonly used contrast agents. The validated method will be an essential and accessible tool for the development of new and efficient PA contrast agents by improving their quantitative characterization.

Properties of the amplitude and intensity impulse responses of passive optical imaging systems have consequences for the spatial coherence filtering properties of such systems. It is shown that imaging systems that have both space-invariant impulse responses and truncating pupil functions have no capability to block spatially coherent sources while imaging those that are incoherent. However, examples of systems with space-variant impulse responses with approximately such capabilities appear to be realizable for a certain range of parameters. However, even space-variant systems cannot block unresolved coherent sources while retaining the capability of imaging those that are spacially incoherent for the same range of source-plane coordinates.

The cone beam computed tomography (CBCT) guided small animal radiation research platform (SARRP) has been developed for focal tumor irradiation, allowing laboratory researchers to test basic biological hypotheses that can modify radiotherapy outcomes in ways that were not feasible previously. CBCT provides excellent bone to soft tissue contrast, but is incapable of differentiating tumors from surrounding soft tissue. Bioluminescence tomography (BLT), in contrast, allows direct visualization of even subpalpable tumors and quantitative evaluation of tumor response. Integration of BLT with CBCT offers complementary image information, with CBCT delineating anatomic structures and BLT differentiating luminescent tumors. This study is to develop a systematic method to calibrate an integrated CBCT and BLT imaging system which can be adopted onboard the SARRP to guide focal tumor irradiation. The integrated imaging system consists of CBCT, diffuse optical tomography (DOT), and BLT. The anatomy acquired from CBCT and optical properties acquired from DOT serve as a priori information for the subsequent BLT reconstruction. Phantoms were designed and procedures were developed to calibrate the CBCT, DOT/BLT, and the entire integrated system. Geometrical calibration was performed to calibrate the CBCT system. Flat field correction was performed to correct the nonuniform response of the optical imaging system. Absolute emittance calibration was performed to convert the camera readout to the emittance at the phantom or animal surface, which enabled the direct reconstruction of the bioluminescence source strength. Phantom and mouse imaging were performed to validate the calibration. All calibration procedures were successfully performed. Both CBCT of a thin wire and a euthanized mouse revealed no spatial artifact, validating the accuracy of the CBCT calibration. The absolute emittance calibration was validated with a 650 nm laser source, resulting in a 3.0% difference between simulated and measured signal. The calibration of the entire system was confirmed through the CBCT and BLT reconstruction of a bioluminescence source placed inside a tissue-simulating optical phantom. Using a spatial region constraint, the source position was reconstructed with less than 1 mm error and the source strength reconstructed with less than 24% error. A practical and systematic method has been developed to calibrate an integrated x-ray and optical tomography imaging system, including the respective CBCT and optical tomography system calibration and the geometrical calibration of the entire system. The method can be modified and adopted to calibrate CBCT and optical tomography systems that are operated independently or hybrid x-ray and optical tomography imaging systems.

In the field of uncooled Long Wave Infra Red (LWIR) imaging, CMOS compatible bolometers technology is being more and more popular, exhibiting precise temperature measurement at moderate cost. The price of this technology is proportional to the number of components produced per wafer, leading to a shrinkage of the pixel. Enhancing the resolution level of the focal plane array (FPA) requires an improvement of the point spread function (PSF) of the optical system, leading to more and more complex aspheric lenses, and an increased cost of imaging systems. We propose to add a sub-wavelength blade to the existing parts of the imaging system to ease the overall improvement of the image quality in applications with a constraint budget. The main function of such a subwavelength blade should be to control the phase of the light into an optical system to compensate optical aberrations. A cost effective solution consists to make such devices using microelectronics based collective fabrication process. The main difficulty is to predict the subwavelength blade behavior within an optical system that is to say combining millimeter sized optical components that are modeled using ray-tracing or electromagnetic simulations. In this paper we present the results obtained from an effort to simulate, fabricate and characterize all-dielectric subwavelength blade. In an imaging system, our devices will have to deal with non-flat wavefronts. Our method is based on Fourier Modal Method and Angular Spectrum Method to simulate subwavelength optics into such an optical system. Finally, we have compared our simulations results to experiments on basic examples, like spherical aberration correction of a commercial lens.

In this paper, a simple method of passive range measurement is discussed. The receiver system can be either hydrophone or horizontal array. Unlike earlier methods of passive source range measurement, we mainly consider the phase information of the passive source, rather than the amplitude. The method only uses the simplest principle range=velocity time. The key of the method is how to acquire the partial deviation of the phase with respect to radial frequency and measurement variables. In physics, the former is equal to time, which can be obtained from the spectrum of the source. The latter is equal to group velocity, which can be evaluated according to the environmental variable, such as water depth, sound profiles, bottom quality, etc. In fact, measurement variables can be substituted by the sound speed in water over the bottom; the error is no more than 8 percent.