Ballistic Photons Research Articles

Correlation gating quantifies the optical properties of dynamic media in transmission.

Quantifying light transport in turbid media is a long-standing challenge. This challenge arises from the difficulty in experimentally separating unscattered, ballistic light from forward scattered light. Correlation gating is a new approach that numerically separates light paths based on statistical dynamics of the optical field. Here we apply correlation gating with interferometric near-infrared spectroscopy (iNIRS) to separate and independently quantify ballistic and scattered light transmitted through thick samples. First, we present evidence that correlation gating improves the isolation of ballistic light in a thick, intrinsically dynamic medium with Brownian motion. Then, from a single set of iNIRS transmission measurements, we determine the ballistic attenuation coefficient and group refractive index from the time-of-flight (TOF) resolved static intensity, and we determine the reduced scattering and absorption coefficients from the diffusive part of the TOF resolved dynamic intensity. Finally, we show that correlation gating is applicable in intrinsically static media in which motion is induced externally. Thus, for the first time, to the best of our knowledge, the key optical properties of a turbid medium can be derived from a single set of transmission measurements.

Light Scattering by Structurally Anisotropic Media: A Benchmark with Transparent Wood

AbstractTransparent wood (TW) is a biocomposite material with hierarchical structure, which exhibits high optical transmittance and anisotropic light scattering. Here, the relation between anisotropic scattering and the internal structure of transparent wood is experimentally studied and the dependence of scattering anisotropy on material thickness, which characterizes the fraction of ballistic photons in the propagating light, is shown. The limitations of the conventional haze, as it is implemented to isotropic materials, are discussed, and a modified characteristic parameter of light scattering—the degree of anisotropic scattering is defined. This parameter together with the transport mean free path value is more practical and convenient for characterization of the material scattering properties. It is believed that the generic routine described in this paper can be applied for scattering characterization and comparison of other TW materials of either different thickness, optical quality or based on various wood species.

Ballistic Injury Imaging: The Basics

As of 2007, there were estimated to be at least 750 million firearms in worldwide circulation, of which 650 million of them were owned by civilians (Weiss et al. in Severe lead toxicity attributed to bullet fragments retained in soft tissue. BMJ Case Reports, 2017). Of these, approximately 270 million are in the United States, equating to 84 guns per 100 Americans [based on 2016 population statistics (assuming the number of firearms remained stable over the intervening 9 years)] and resulting in 84 997 nonfatal injuries and 36 252 fatalities in the United States in 2015. With statistics like these, it stands to reason that victims of gunshot wounds (GSW) will be imaged by most radiologists at least once in their careers. This article seeks to increase radiologists’ knowledge of the pathophysiology of GSW and will review the mechanism of ballistic injury and relate these to commonly encountered imaging findings. Ballistic injuries are a combination of the direct injury caused by the bullet along its path through the tissues and the shockwave created around that path as the bullet expends its energy. CT is the gold standard in ballistic injury assessment. MRI is not contraindicated in patients with retained ballistic fragments, but should be used with caution. The number of entry/exit wound and the number of retained ballistic fragments should be an even number, or there is a missing surface wound or a missing bullet. Retained lead in joints can result in plumbism and arthropathy. As most radiologists will encounter a ballistic injury in the course of their careers, an understanding of this unique mechanism of injury and its complications will aid in both imaging interpretation and patient care.

Imaging through extreme scattering in extended dynamic media.

Critical to navigation, situational awareness, and object identification is the ability to image through turbid water and fog. To date, the longest imaging ranges in such environments rely on active illumination and selection of ballistic photons by means of time gating. Here we show that the imaging range can be extended by using time-gated holography in combination with multi-frame processing. Instead of simply summing the intensity of the frames, we use the complex fields retrieved through digital holographic processing and coherently add the frames. We demonstrate imaging through extended bodies of turbid water and fog at one-way attenuation lengths of 13 and 13.6, respectively. Compared to equivalent traditional time-gated systems, gated holography and coherent processing require 20× less laser illumination power for the same imaging range.

Ballistic Hole Emission Spectroscopy of Self-Assembled GeSi/Si(001) Nanoislands

Ballistic hole emission microscopy/spectroscopy has been applied to study the electronic structure of the hole states in the self-assembled GeSi/Si(001) nanoislands. The ballistic hole emission microscopic images demonstrated the spots of increased collector current related to the ballistic electron tunnelling via the confined valence band states in the GeSi/Si(001) nanoislands. In the ballistic hole emission spectra of the Ge hut clusters the stepwise features attributed to the quantum confined hole states have been observed. The results of present study demonstrate the capabilities of the ballistic hole emission microscopy/spectroscopy in the characterization of the electronic structures of the valence band states in the GeSi/Si nanostructures.

Carbon Nano-particle Synthesized by Pulsed Arc Discharge Method as a Light Emitting Device

Owing to the specific properties such as high mobility, ballistic carrier transport and light emission, carbon nano-particles (CNPs) have been employed in nanotechnology applications. In the presented work, the CNPs are synthesized by using the pulsed arc discharge method between two copper electrodes. The rectifying behaviour of produced CNPs is explored by assuming an Ohmic contact between the CNPs and the electrodes. The synthesized sample is characterized by electrical investigation and modelling. The current–voltage (I–V) relationship is investigated and bright visible light emission from the produced CNPs was measured. The electroluminescence (EL) intensity was explored by changing the distance between two electrodes. An incremental behaviour on EL by a resistance gradient and distance reduction is identified.

Absorption-enhanced imaging through scattering media using carbon black nano-particles: from visible to near infrared wavelengths

Imaging through scattering media can be improved with the addition of absorbers, since multiply-scattered photons, with their longer path length, are absorbed with a higher probability than ballistic photons. The image resolution enhancement is substantially greater when imaging through isotropic scatterers than when imaging through an ensemble of strongly forward-scattering particles. However, since the angular scattering distribution is determined by the size of the scatterers with respect to the wavelength of incident light, particles that are forward scatterers at visible wavelengths can be isotropic scatterers at infrared (IR) wavelengths. Here, we show that substantial image resolution enhancement can be achieved in the near-infrared wavelength regime for particles that are forward scattering at visible wavelengths using carbon black nanoparticles as a broadband absorber. This observation provides a new strategy for image enhancement through scattering media: by selecting the appropriate wavelength range for imaging, in this case the near-IR, the addition of absorbers more effectively enhances the image resolution.

A frequency-domain non-contact photoacoustic microscope based on an adaptive interferometer.

A frequency-domain, non-contact approach to photoacoustic microscopy (PAM) that employs amplitude-modulated (0.1-1 MHz) laser for excitation (638-nm pump) in conjunction with a 2-wave mixing interferometer (532-nm probe) for non-contact detection of photoacoustic waves at the specimen surface is presented. A lock-in amplifier is employed to detect the photoacoustic signal. Illustrative images of tissue-mimicking phantoms, red-blood cells and retinal vasculature are presented. Single-frequency modulation of the pump beam directly provides an image that is equivalent to the 2-dimensional projection of the image volume. Targets located superficially produce phase modulations in the surface-reflected probe beam due to surface vibrations as well as direct intensity modulation in the backscattered probe light due to local changes in pressure and/or temperature. In comparison, the observed modulations in the probe beam due to targets located deeper in the specimen, for example, beyond the ballistic photon regime, predominantly consist of phase modulation.

Heuristic model for ballistic photon detection in collimated transmittance measurements.

An heuristic model for ballistic photon detection in continuous-wave measurements of collimated transmittance through a slab is presented. The model is based on the small angle approximation and the diffusion equation and covers all the ranges of optical thicknesses of the slab from the ballistic to the diffusive regime. The performances of the model have been studied by means of comparisons with the results of gold standard Monte Carlo simulations for a wide range of optical thicknesses and two types of scattering functions. For a non-absorbing slab and field of view of the receiver less than 3° the model shows errors less than 15% for any value of the optical thickness. Even for an albedo value of 0.9, and field of view of the receiver less than 3° the model shows errors less than 20%. These results have been verified for a large set of scattering functions based on the Henyey-Greenstein model and Mie theory for spherical scatterers. The latter has also been used to simulate the scattering function of Intralipid, a diffusive material widely used as reference standard for tissue simulating phantoms. The proposed model represents an effective improvement compared to the existing literature.

Ballistic hole emission spectroscopy of self-assembled GeSi/Si(001) nanoislands

AbstractBallistic hole emission microscopy/spectroscopy has been applied to study the electronic structure of the hole states in the self-assembled GeSi/Si(001) nanoislands. The ballistic hole emission microscopic images demonstrated the spots of increased collector current related to the ballistic electron tunnelling via the confined valence band states in the GeSi/Si(001) nanoislands. In the ballistic hole emission spectra of the Ge hut clusters the stepwise features attributed to the quantum confined hole states have been observed. The results of present study demonstrate the capabilities of the ballistic hole emission microscopy/spectroscopy in the characterization of the electronic structures of the valence band states in the GeSi/Si nanostructures.

Organizational and environmental determinants of ballistics imaging productivity in United States crime laboratories

This study employs a structural contingency theory framework to assess the influence of the formal organizational structure of crime laboratories and their environments, on the productivity of labs’ ballistics imaging systems. We contribute to the nascent evolution of organizational focus in forensics while exploring different ways in which overall lab performance can be conceptualized. We analyze data provided by the Bureau of Alcohol, Tobacco, Firearms and Explosives’ (ATF) National Integrated Ballistics Information Network (NIBIN), and data from the Bureau of Justice Statistics, Census of Publicly Funded Forensic Crime Laboratories, 2009. Three Negative Binomial models regressed the organizational and environmental variables on the productivity of crime labs, operationalized as their ability to produce bullet, brass, and total hits. Results indicate that in addition to the number of bullet and brass inputs, local labs are more productive in identifying ballistics imaging hits. Additionally, larger labs with a greater proportion of firearms requests and the ability to process those requests produce more hits. Although the current research did not find that organizational structure significantly predicted labs’ productivity, the importance of the population of bullet and brass inputs was clearly evident. Additionally, jurisdiction plays a vital role in the production of hits.

Imaging Transparent Objects in a Turbid Medium Using a Femtosecond Optical Kerr Gate**Supported by the National Natural Science Foundation of China under Grant Nos 61427816 and 61690221, and the Collaborative Innovation Center of Suzhou Nano Science and Technology.

A femtosecond optical Kerr gate time-gated ballistic imaging method is demonstrated to image a transparent object in a turbid medium. The shape features of the object are obtained by time-resolved selection of the ballistic photons with different optical path lengths, the thickness distribution of the object is mapped, and the maximum is less than 3.6%. This time-resolved ballistic imaging has potential applications in studying properties of the liquid core in the near field of the fuel spray.

A new Monte Carlo code for light transport in biological tissue.

The aim of this work was to develop an event-by-event Monte Carlo code for light transport (called MCLTmx) to identify and quantify ballistic, diffuse, and absorbed photons, as well as their interaction coordinates inside the biological tissue. The mean free path length was computed between two interactions for scattering or absorption processes, and if necessary scatter angles were calculated, until the photon disappeared or went out of region of interest. A three-layer array (air-tissue-air) was used, forming a semi-infinite sandwich. The light source was placed at (0,0,0), emitting towards (0,0,1). The input data were: refractive indices, target thickness (0.02, 0.05, 0.1, 0.5, and 1cm), number of particle histories, and λ from which the code calculated: anisotropy, scattering, and absorption coefficients. Validation presents differences less than 0.1% compared with that reported in the literature. The MCLTmx code discriminates between ballistic and diffuse photons, and inside of biological tissue, it calculates: specular reflection, diffuse reflection, ballistics transmission, diffuse transmission and absorption, and all parameters dependent on wavelength and thickness. The MCLTmx code can be useful for light transport inside any medium by changing the parameters that describe the new medium: anisotropy, dispersion and attenuation coefficients, and refractive indices for specific wavelength.

Ballistic imaging through an intense scattering medium using a supercontinuum with a roundabout spatial gate.

We propose a new ballistic imaging method that is capable of imaging an object through an intense scattering medium. In this method, a femtosecond supercontinuum and a roundabout spatial gate were used to suppress speckles and filter background noise, respectively. The roundabout spatial gate extracts ballistic light and avoids low-pass spatial filtering to ensure the high resolution of images. The experimental results showed that even when the optical depth of the scattering medium reached 17, the images extracted by the method had improved identifiability and contrast.

Ballistic and snake photon imaging for locating optical endomicroscopy fibres.

We demonstrate determination of the location of the distal-end of a fibre-optic device deep in tissue through the imaging of ballistic and snake photons using a time resolved single-photon detector array. The fibre was imaged with centimetre resolution, within clinically relevant settings and models. This technique can overcome the limitations imposed by tissue scattering in optically determining the in vivo location of fibre-optic medical instruments.

Determining the light scattering and absorption parameters from forward-directed flux measurements in cardiac tissue.

We describe a method to accurately measure the light scattering model parameters from forward-directed flux (FDF) measurements carried out with a fiber-optic probe (optrode). Improved determination of light scattering parameters will, in turn, permit better modeling and interpretation of optical mapping in the heart using voltage-sensitive dyes. Using our optrode-based system, we carried out high spatial resolution measurements of FDF in intact and homogenized cardiac tissue, as well as in intralipid-based tissue phantoms. The samples were illuminated with a broad collimated beam at 660 and 532 nm. Measurements were performed with a plunge fiber-optic probe (NA=0.22) at a spatial resolution of up to 10 μm. In the vicinity of the illuminated surface, the FDF consistently manifested a fast decaying exponent with a space constant comparable with the decay rate of ballistic photons. Using a Monte Carlo model, we obtained a simple empirical formula linking the rate of the fast exponent to the scattering coefficient, the anisotropy parameter g, and the numerical aperture of the probe. The estimates of scattering coefficient based on this formula were validated in tissue phantoms. Potential applications of optical fiber-based FDF measurements for the evaluation of optical parameters in turbid media are discussed.

A theoretical study on the impact of particle scattering on the channel characteristics of underwater optical communication system

Scattering by water molecules and particulate matters determines the path and distance of photon propagation in underwater medium. Consequently, photon angle of scattering (given by scattering phase function) requires to be considered in addition to the extinction coefficient of the aquatic medium governed by the absorption and scattering coefficients in channel characterization for an underwater wireless optical communication (UWOC) system. This study focuses on analyzing the received signal power and impulse response of UWOC channel based on Monte-Carlo simulations for different water types, link distances, link geometries and transceiver parameters. A newly developed scattering phase function (referred to as SS phase function), which represents the real water types more accurately like the Petzold phase function, is considered for quantification of the channel characteristics along with the effects of absorption and scattering coefficients. A comparison between the results simulated using various phase function models and the experimental measurements of Petzold revealed that the SS phase function model predicts values closely matching with the actual values of the Petzold’s phase function, which further establishes the importance of using a correct scattering phase function model while estimating the channel capacity of UWOC system in terms of the received power and channel impulse response. Results further demonstrate a great advantage of considering the nonzero probability of receiving scattered photons in estimating channel capacity rather than considering the reception of only ballistic photons as in Beer’s Law, which severely underestimates the received power and affects the range of communication especially in the scattering water column. The received power computed based on the Monte-Carlo method by considering the receiver aperture sizes and field of views in different water types are further analyzed and discussed. These results are essential for evaluating the underwater link budget and constructing different system and design parameters for an UWOC system.

Dual-axis optical coherence tomography for deep tissue imaging.

We have developed dual-axis optical coherence tomography (DA-OCT) which enables deep tissue imaging by using a novel off-axis illumination/detection configuration. DA-OCT offers a 100-fold speed increase compared with its predecessor, multispectral multiple-scattering low coherence interferometry (ms2/LCI), by using a new beam scanning mechanism based on a microelectro-mechanical system (MEMS) mirror. The data acquisition scheme was altered to take advantage of this scanning speed, producing tomographic images at a rate of 4 frames (B-scans) per second. DA-OCT differs from ms2/LCI in that the dual axes intersect at a shallower depth (∼1 mm). This difference, coupled with the faster scanning speed, shifts the detection priority from multiply scattered to ballistic light. The utility of this approach was demonstrated by imaging both ex vivo porcine ear skin and in vivo rat skin from a McFarlane flap model. The enhanced penetration depth provided by the DA-OCT system will be beneficial to various clinical applications in dermatology and surgery.

Optimizing Ballistic Imaging Operations.

Ballistic imaging systems can help solve crimes by comparing images of cartridge cases, which are recovered from a crime scene or test-fired from a gun, to a database of images obtained from past crime scenes. Many U.S. municipalities lack the resources to process all of their cartridge cases. Using data from Stockton, CA, we analyze two problems: how to allocate limited capacity to maximize the number of cartridge cases that generate at least one hit, and how to prioritize the cartridge cases that are processed to maximize the usefulness (i.e., obtained before the corresponding criminal case is closed) of hits. The number of hits can be significantly increased by prioritizing crime scene evidence over test-fires, and by ranking calibers by their hit probability and processing only the higher ranking calibers. We also estimate that last-come first-served increases the proportion of hits that are useful by only 0.05 relative to first-come first-served.

Forensic Evidence and Criminal Investigations: The Impact of Ballistics Information on the Investigation of Violent Crime in Nine Cities.

We explore the impact of information from ballistics imaging hit reports on the investigation into violent crimes. Ballistics imaging hits link two crimes involving the same firearm by forensically matching tool marks on the fired bullets or cartridge cases. Interview data collected from detectives who received a hit report were used to explore the relationship between the presence of a hit report and outcomes in 65 gun-related violent crime investigations in nine U.S. police agencies. Findings indicate hit reports rarely contribute to identification, arrest, charging, or sentencing of suspects, because of delays in producing hit reports. On average, hit reports were completed 181.4 days after the focal crime. This delay forces investigations to proceed without the benefit of information from ballistics analysis. Additionally, hit reports rarely contained detailed information that was immediately useful to investigators. Instead, hit reports required additional research by the investigator to unlock useful information.