Binary sampling ghost imaging: add random noise to fight quantization caused image quality decline

When the sampling data of ghost imaging is recorded with less bits, i.e., experiencing quantization, decline of image quality is observed. The less bits used, the worse image one gets. Dithering, which adds suitable random noise to the raw data before quantization, is proved to be capable of compensating image quality decline effectively, even for the extreme binary sampling case. A brief explanation and parameter optimization of dithering are given.Comment: 8 pages, 7 figure

Effect of Pump Dithering at Each Stage of Cascaded Fiber Optical Parametric Amplifier

Cascaded fiber optical parametric amplifier (FOPA) can enhance gain and bandwidth. The gain and bandwidth can be further enhanced by dithering the FOPA pump. However, to our knowledge, the effects of a pump dithering at every stage of cascaded FOPA have not been discussed. The study of performance at every stage of cascaded FOPA is quite interesting and beneficial in designing the system. Here, we analyzed, using OptiSystem software, each stage of a cascaded FOPA, when there was a pump dithering and not. The results showed that the pump dithering enhanced the gain and broaden the bandwidth at every stage. The gain and bandwidth obtained with the pump dithering were 27 dB and 20 nm, respectively. On the other hand, when there was no pump dithering, the gain and bandwidth were 9 dB and 12 nm, respectively

A partial-dithering strategy for edge-illumination X-ray phase-contrast tomography enabled by a joint reconstruction method

Edge-illumination X-ray phase-contrast tomography (EIXPCT) is a promising imaging technology where partially opaque masks are utilized with laboratory-based X-ray sources to estimate the distribution of the complex-valued refractive index. EIXPCT resolution is mainly determined by the period of a sample mask, but can be significantly improved by a dithering technique. Here, dithering means that multiple images per tomographic view angle are acquired as the object is moved over sub-pixel distances. Drawbacks of dithering include increased data-acquisition times and radiation doses. Motivated by the flexibility in data-acquisition designs enabled by a recently developed joint reconstruction (JR) method, a novel partial-dithering strategy for EIXPCT data-acquisition is proposed. In this strategy, dithering is implemented at only a subset of the tomographic view angles. The strategy can result in spatial resolution comparable to that of the conventional full-dithering strategy, where dithering is performed at every view angle, but the acquisition time is substantially decreased. Here, the effect of dithering parameters on image resolution are explored

Evaluation of advanced air-fuel ratio control strategies and their effects on three-way catalysts in a stoichiometric, spark ignited, natural gas engine

2021 Spring.Includes bibliographical references.Engine emissions are a growing concern in the 21st century. As the world works to combat rising pollution levels, engine emissions are under scrutiny. Natural gas engines are increasing in popularity over diesel engines, due to the high availability of fuel and fewer pollutant emissions than comparable diesel engines. Pollutants such as NOx, CO, and THCs (total hydrocarbons) are harmful to the environment and are currently regulated, and limits for these pollutants are expected to decrease further in the future. A three-way catalyst (TWC) is a cost-effective exhaust after treatment system can be used to reduce pollutant emissions through a series of reactions that are catalyzed by special conditions within the catalyst. Using TWCs, emissions can be drastically reduced using simple chemical reactions, without affecting engine performance. Air-fuel ratio dithering is a strategy that can be used to increase catalyst reduction efficiency by utilizing the oxygen storing properties of ceria, a material in the catalyst washcoat. Dithering is a method of periodically varying the air-fuel ratio of the engine around an optimum point. The focus of this work is understanding how dithering affects oxygen storage in a catalyst, as well as how dithering amplitude and frequency can be tuned to maximize catalyst efficiency. Experiments were performed on a CAT CG137-8, a stationary natural gas engine used for gas compression. Three different catalysts were tested, including the standard catalyst for the test engine, a custom catalyst with one half of the oxygen storage capability of the standard catalyst, and the standard catalyst artificially aged to 16,000 hours. Emissions data were collected across a dithering parameter sweep where a large number of amplitude and frequency combinations were tested. Additionally, steady state and dithering air-fuel ratio sweeps were performed to investigate the emissions window of compliance across a wide range of air-fuel ratios. It was found that dithering with optimized amplitude and frequency can significantly reduce pollutant emissions with a fresh catalyst. However, dithering does not have a large effect on aged catalysts. Additionally, dithering was shown to improve the window of emissions compliance on a standard catalyst by 100% but showed a smaller improvement on a catalyst with ½ oxygen storage capability. The window of compliance with an aged catalyst was unimproved by dithering. Optimized dithering has the potential to significantly reduce engine emissions, allowing for compliance with more stringent emissions requirements or for less expensive catalysts to be used

Dithering Strategies and Point-Source Photometry

The accuracy in the photometry of a point source depends on the point-spread function (PSF), detector pixelization, and observing strategy. The PSF and pixel response describe the spatial blurring of the source, the pixel scale describes the spatial sampling of a single exposure, and the observing strategy determines the set of dithered exposures with pointing offsets from which the source flux is inferred. In a wide-field imaging survey, sources of interest are randomly distributed within the field of view and hence are centered randomly within a pixel. A given hardware configuration and observing strategy therefore have a distribution of photometric uncertainty for sources of fixed flux that fall in the field. In this article we explore the ensemble behavior of photometric and position accuracies for different PSFs, pixel scales, and dithering patterns. We find that the average uncertainty in the flux determination depends slightly on dither strategy, whereas the position determination can be strongly dependent on the dithering. For cases with pixels much larger than the PSF, the uncertainty distributions can be non-Gaussian, with rms values that are particularly sensitive to the dither strategy. We also find that for these configurations with large pixels, pointings dithered by a fractional pixel amount do not always give minimal average uncertainties; this is in contrast to image reconstruction for which fractional dithers are optimal. When fractional pixel dithering is favored, a pointing accuracy of better than $\sim 0.15$ pixel width is required to maintain half the advantage over random dithers

Stabilising a nulling interferometer using optical path difference dithering

Context. Nulling interferometry has been suggested as the underlying principle for the Darwin and TPF-I exoplanet research missions. Aims. There are constraints both on the mean value of the nulling ratio, and on its stability. Instrument instability noise is most detrimental to the stability of the nulling performance. Methods. We applied a modified version of the classical dithering technique to the optical path difference in the scientific beam. Results. Using only this method, we repeatedly stabilised the dark fringe for several hours. This method alone sufficed to remove the 1/ f component of the noise in our setup for periods of 10 minutes, typically. These results indicate that performance stability may be maintained throughout the long-duration data acquisitions typical of exoplanet spectroscopy. Conclusions. We suggest that further study of possible stabilisation strategies should be an integral part of Darwin/TPF-I research and developmen

Optimal Compression of Floating-point Astronomical Images Without Significant Loss of Information

We describe a compression method for floating-point astronomical images that gives compression ratios of 6 -- 10 while still preserving the scientifically important information in the image. The pixel values are first preprocessed by quantizing them into scaled integer intensity levels, which removes some of the uncompressible noise in the image. The integers are then losslessly compressed using the fast and efficient Rice algorithm and stored in a portable FITS format file. Quantizing an image more coarsely gives greater image compression, but it also increases the noise and degrades the precision of the photometric and astrometric measurements in the quantized image. Dithering the pixel values during the quantization process can greatly improve the precision of measurements in the images. This is especially important if the analysis algorithm relies on the mode or the median which would be similarly quantized if the pixel values are not dithered. We perform a series of experiments on both synthetic and real astronomical CCD images to quantitatively demonstrate that the magnitudes and positions of stars in the quantized images can be measured with the predicted amount of precision. In order to encourage wider use of these image compression methods, we have made available a pair of general-purpose image compression programs, called fpack and funpack, which can be used to compress any FITS format image.Comment: Accepted PAS

Results on lattice vector quantization with dithering

The statistical properties of the error in uniform scalar quantization have been analyzed by a number of authors in the past, and is a well-understood topic today. The analysis has also been extended to the case of dithered quantizers, and the advantages and limitations of dithering have been studied and well documented in the literature. Lattice vector quantization is a natural extension into multiple dimensions of the uniform scalar quantization. Accordingly, there is a natural extension of the analysis of the quantization error. It is the purpose of this paper to present this extension and to elaborate on some of the new aspects that come with multiple dimensions. We show that, analogous to the one-dimensional case, the quantization error vector can be rendered independent of the input in subtractive vector-dithering. In this case, the total mean square error is a function of only the underlying lattice and there are lattices that minimize this error. We give a necessary condition on such lattices. In nonsubtractive vector dithering, we show how to render moments of the error vector independent of the input by using appropriate dither random vectors. These results can readily be applied for the case of wide sense stationary (WSS) vector random processes, by use of iid dither sequences. We consider the problem of pre- and post-filtering around a dithered lattice quantifier, and show how these filters should be designed in order to minimize the overall quantization error in the mean square sense. For the special case where the WSS vector process is obtained by blocking a WSS scalar process, the optimum prefilter matrix reduces to the blocked version of the well-known scalar half-whitening filter