New single-pixel camera design, but this time using multicore fibers (MCF) and a photonic lantern instead of a spatial light modulator. Cool!
The fundamental idea is to excite one of the cores of a MCF. Then, light propagates through the fiber, which has a photonic lantern at the tip that generates a random-like light pattern at its tip. Exciting different cores of the MCF generates different light patterns at the end of the fiber, which can be used to obtain images using the single-pixel imaging formalism.
There is more cool stuff in the paper, for example the Compressive Sensing algorithm the authors are using, using positivity constraints. This is indeed quite relevant if you want to get high quality images, because of the reduced number of cores present in the MCF (remember, 1 core = 1 pattern, and the number of patterns determines the spatial resolution of the image in a single-pixel camera). It is also nice that there is available code from the authors here.
Compressive optical imaging with a photonic lantern
The thin and flexible nature of optical fibres often makes them the ideal technology to view biological processes in-vivo, but current microendoscopic approaches are limited in spatial resolution. Here, we demonstrate a new route to high resolution microendoscopy using a multicore fibre (MCF) with an adiabatic multimode-to-singlemode photonic lantern transition formed at the distal end by tapering. We show that distinct multimode patterns of light can be projected from the output of the lantern by individually exciting the single-mode MCF cores, and that these patterns are highly stable to fibre movement. This capability is then exploited to demonstrate a form of single-pixel imaging, where a single pixel detector is used to detect the fraction of light transmitted through the object for each multimode pattern. A custom compressive imaging algorithm we call SARA-COIL is used to reconstruct the object using only the pre-measured multimode patterns themselves and the detector signals.
Last week I posted a recently uploaded paper on using positive-only patterns in a single-pixel imaging system.
Today I just found another implementation looking for the same objective. This time the authors (from University of Warsaw, leaded by Rafał Kotyński) introduce the idea of simplexes, or how any point in some N-dimensional space can be located using only positive coordinates if you choose the correct coordinate system. Cool concept!
Single-pixel imaging with sampling distributed over simplex vertices
We propose a method of reduction of experimental noise in single-pixel imaging by expressing the subsets of sampling patterns as linear combinations of vertices of a multidimensional regular simplex. This method also may be directly extended to complementary sampling. The modified measurement matrix contains nonnegative elements with patterns that may be directly displayed on intensity spatial light modulators. The measurement becomes theoretically independent of the ambient illumination, and in practice becomes more robust to the varying conditions of the experiment. We show how the optimal dimension of the simplex depends on the level of measurement noise. We present experimental results of single-pixel imaging using binarized sampling and real-time reconstruction with the Fourier domain regularized inversion method.
A group of researchers working in France and USA, leaded by N. Ducros, has uploaded an interesting paper this week.
When doing single-pixel imaging, one of the most important aspects you need to take into account is the kind of structured patters (functions) you are going to use. This is quite relevant because it is greatly connected with the speed you are going to achieve (as the number of total measurements needed for obtaining good images strongly depends on the set of functions you choose). Usually, the go-to solution for single-pixel cameras is to either choose random functions, or a set (family) of orthogonal functions (Fourier, DCT, Hadamard, etc.).
The problem with random functions is that they are not orthogonal (it is very hard to distinguish between two different random functions, all of them are similar), so you usually need to project a high number of them (which is time consuming). Orthogonal functions that belong to a basis are a better choice, because you can send the full basis to get “perfect” quality (i.e., without losing information due to undersampling). However, usually these functions have positive and negative values, which is something you cannot directly implement in lots of Spatial Light Modulators (for example, in Digital Micromirror Devices). If you want to implement these patterns, there are multiple workarounds. The most common one is to implement two closely-related patterns sequentially in the SLM to generate one function. This solves the negative-positive problem, but increases the time it takes to obtain an image in a factor two.
What Lorente-Mur et al. show in this paper is a method to generate a new family of positive-only patterns, derived from the original positive-negative family. This makes it possible to obtain images with a reduced number of measurements when compared to the dual or splitting approach I mentioned earlier, but still with high quality. Nice way to tackle one of the most limiting factors of single-pixel architectures.
Handling negative patterns for fast single-pixel lifetime imaging
Pattern generalization was proposed recently as an avenue to increase the acquisition speed of single-pixel imaging setups. This approach consists of designing some positive patterns that reproduce the target patterns with negative values through linear combinations. This avoids the typical burden of acquiring the positive and negative parts of each of the target patterns, which doubles the acquisition time. In this study, we consider the generalization of the Daubechies wavelet patterns and compare images reconstructed using our approach and using the regular splitting approach. Overall, the reduction in the number of illumination patterns should facilitate the implementation of compressive hyperspectral lifetime imaging for fluorescence-guided surgery.
As a way to keep posts going, I am starting a short recap about interesting papers being published (or being discovered) every now and then. Probably I will write longer posts about some of them in the future.
Let’s get this thing going:
Two papers using ‘centroid estimation‘ to retrieve interesting information:
Mariko Akutsu, Yasuhiro Oikawa, and Yoshio Yamasaki, at The Journal of the Acoustical Society of America
Conversation is one of the most important channels for human beings. To help communications, speech recognition technologies have been developed. Above all, in a conversation, not only contents of utterances but also intonations and tones include important information regarding a speaker’s intention. To study the sphere of human speech, microphones are typically used to record voices. However, since microphones have to be set around a space, their existences affect a physical behavior of the sound field. To challenge this problem, we have suggested a recording method using a high-speed camera. By using a high-speed camera for recording sound vibrations, it can record two or more points within the range of the camera at the same time and can record from a distance, without interfering with the sound fields. In this study, we extract voice information using high-speed videos, which capture both a face and a cervical part of the subject. This method allows recording skin vibrations, which contain voices with individuality and extrapolating sound waves by using an image processing method. The result of the experiment shows that a high-speed camera is capable of recording voice information.
Dekel Raanan, Liqing Ren, Dan Oron, and Yaron Silberberg, at Optics Letters
Stimulated Raman scattering (SRS) has recently become useful for chemically selective bioimaging. It is usually measured via modulation transfer from the pump beam to the Stokes beam. Impulsive stimulated Raman spectroscopy, on the other hand, relies on the spectral shift of ultrashort pulses as they propagate in a Raman active sample. This method was considered impractical with low energy pulses since the observed shifts are very small compared to the excitation pulse bandwidth, spanning many terahertz. Here we present a new apparatus, using tools borrowed from the field of precision measurement, for the detection of low-frequency Raman lines via stimulated-Raman-scattering-induced spectral shifts. This method does not require any spectral filtration and is therefore an excellent candidate to resolve low-lying Raman lines (<200cm−1<200 cm−1), which are commonly masked by the strong Rayleigh scattering peak. Having the advantage of the high repetition rate of the ultrafast oscillator, we reduce the noise level by implementing a lock-in detection scheme with a wavelength shift sensitivity well below 100 fm. This is demonstrated by the measurement of low-frequency Raman lines of various liquid samples.
Machine learning keeps leaking into photonics. This time with a Compressive Sensing flavor and some holography:
The compressed sensing (CS) has been successfully applied to image compression in the past few years as most image signals are sparse in a certain domain. Several CS reconstruction models have been proposed and obtained superior performance. However, these methods suffer from blocking artifacts or ringing effects at low sampling ratios in most cases. To address this problem, we propose a deep convolutional Laplacian Pyramid Compressed Sensing Network (LapCSNet) for CS, which consists of a sampling sub-network and a reconstruction sub-network. In the sampling sub-network, we utilize a convolutional layer to mimic the sampling operator. In contrast to the fixed sampling matrices used in traditional CS methods, the filters used in our convolutional layer are jointly optimized with the reconstruction sub-network. In the reconstruction sub-network, two branches are designed to reconstruct multi-scale residual images and muti-scale target images progressively using a Laplacian pyramid architecture. The proposed LapCSNet not only integrates multi-scale information to achieve better performance but also reduces computational cost dramatically. Experimental results on benchmark datasets demonstrate that the proposed method is capable of reconstructing more details and sharper edges against the state-of-the-arts methods.
Yair Rivenson, Yibo Zhang, Harun Günaydın, Da Teng & Aydogan Ozcan, at Light: Science & Applications
Phase recovery from intensity-only measurements forms the heart of coherent imaging techniques and holography. In this study, we demonstrate that a neural network can learn to perform phase recovery and holographic image reconstruction after appropriate training. This deep learning-based approach provides an entirely new framework to conduct holographic imaging by rapidly eliminating twin-image and self-interference-related spatial artifacts. This neural network-based method is fast to compute and reconstructs phase and amplitude images of the objects using only one hologram, requiring fewer measurements in addition to being computationally faster. We validated this method by reconstructing the phase and amplitude images of various samples, including blood and Pap smears and tissue sections. These results highlight that challenging problems in imaging science can be overcome through machine learning, providing new avenues to design powerful computational imaging systems.
Last, single-pixel camera / ghost imaging being applied to x-ray tomography:
Andrew M. Kingston, Glenn R. Myers, Daniele Pelliccia, Imants D. Svalbe, David M. Paganin, at arXiv.org
Ghost imaging has recently been successfully achieved in the X-ray regime; due to the penetrating power of X-rays this immediately opens up the possibility of X-ray ghost tomography. No research into this topic currently exists in the literature. Here we present adaptations of conventional tomography techniques to this new ghost imaging scheme. Several numerical implementations for tomography through X-ray ghost imaging are considered. Specific attention is paid to schemes for denoising of the resulting tomographic reconstruction, issues related to dose fractionation, and considerations regarding the ensemble of illuminating masks used for ghost imaging. Each theme is explored through a series of numerical simulations, and several suggestions offered for practical realisations of X-ray ghost tomography.
Recently I have been reading a lot about Compressive Sensing strategies. One of the things we always want when we work in a single-pixel architecture is to project the lowest possible number of masks, because the projecting process is the longest in all the acquisition procedure (and it gets longer and longer when you increase the spatial resolution of your images).
In the past, several strategies haven been implemented to reduce that number of projections. From going fully random to partially scan a basis at random and at the low frequency region, each approach presents some benefits and more or less speed gain.
In this work by the group of K.F. Kelly, they explored a different approach. Instead of chosing one measurement basis and design a sensing strategy (picking random elements, or centering around the low frequency part of the basis, or a mix), they create a measurement basis by merging different functions. They call it hybrid patterns. The basic idea is to chose a low number of patterns which work well for recovering low frequency content of natural images, and also some other patterns which are good to recover high frequency content. The novel thing here is that they do not require the patterns to belong to the same orthogonal basis, thus being able to carefully design its measurement basis. This provides very good quality results with a low number of projections.
Another thing I liked a lot was the Principal Component Analysis (PCA) part of the paper. Basically, they gathered a collection of natural images and they generated an orthogonal basis by using PCA. This leads me to think of PCA as a way of obtaining orthogonal bases where objects have their sparsest representation (maybe I am wrong about that).
Realization of hybrid compressive imaging strategies,
(featured image exctracted from Fig.2 of the manuscript)
The tendency of natural scenes to cluster around low frequencies is not only useful in image compression, it also can prove advantageous in novel infrared and hyperspectral image acquisition. In this paper, we exploit this signal model with two approaches to enhance the quality of compressive imaging as implemented in a single-pixel compressive camera and compare these results against purely random acquisition. We combine projection patterns that can efficiently extract the model-based information with subsequent random projections to form the hybrid pattern sets. With the first approach, we generate low-frequency patterns via a direct transform. As an alternative, we also used principal component analysis of an image library to identify the low-frequency components. We present the first (to the best of our knowledge) experimental validation of this hybrid signal model on real data. For both methods, we acquire comparable quality of reconstructions while acquiring only half the number of measurements needed by traditional random sequences. The optimal combination of hybrid patterns and the effects of noise on image reconstruction are also discussed.
I just read on ArXiv.org that L. Bian and his colleagues made a cool comparison between several ways of performing single-pixel imaging. They have tested the performance on several recovery procedures, some quite familiar but others not so well stablished. I find both Table 1 and Fig. 7 extremely interesting. One sums up really well the different reconstruction approaches that can be used in single-pixel imaging (with or without using Compressive Sensing). The figure points out one thing that experience has told me: every problem you try to solve usually needs an specific solver if you want to get good and fast results (which is extremely important when you start to work with BIG objects, as I plan to write soon here).
Experimental comparison of single-pixel imaging algorithms,
(featured image extracted from Fig.7 of the manuscript)
Single-pixel imaging (SPI) is a novel technique capturing 2D images using a photodiode, instead of conventional 2D array sensors. SPI owns high signal-to-noise ratio, wide spectrum range, low cost, and robustness to light scattering. Various algorithms have been proposed for SPI reconstruction, including the linear correlation methods, the alternating projection method (AP), and the compressive sensing based methods. However, there has been no comprehensive review discussing respective advantages, which is important for SPI’s further applications and development. In this paper, we reviewed and compared these algorithms in a unified reconstruction framework. Besides, we proposed two other SPI algorithms including a conjugate gradient descent based method (CGD) and a Poisson maximum likelihood based method. Both simulations and experiments validate the following conclusions: to obtain comparable reconstruction accuracy, the compressive sensing based total variation regularization method (TV) requires the least measurements and consumes the least running time for small-scale reconstruction; the CGD and AP methods run fastest in large-scale cases; the TV and AP methods are the most robust to measurement noise. In a word, there are trade-offs between capture efficiency, computational complexity and robustness to noise among different SPI algorithms. We have released our source code for non-commercial use.