|
Light Diffusion
Photon diffusion is a situation where photons travel through a material without being absorbed, but rather undergoing repeated scattering events which change the direction of their path. The path of any given photon is then effectively a random walk. A large ensemble of such photons can be said to exhibit diffusion in the material, and can be described with a diffusion equation. Astrophysics In astrophysics, photon diffusion occurs inside a stellar atmosphere. To describe this phenomenon, one should develop the transfer equation in moments and use the Eddington approximation to radiative transfer (i.e. the diffusion approximation). In 3D the results are two equations for the photon energy flux: :\vec F = -\frac\vec \nabla U, :\vec \nabla \cdot \vec F = 0, where \sigma is the opacity. By substituting the first equation into the second, one obtains the diffusion equation for the photon energy density: :\nabla^2 U -\frac\vec \nabla U \cdot \vec \nabla \sigma=0. Medical science In medi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always move at the speed of light in vacuum, (or about ). The photon belongs to the class of bosons. As with other elementary particles, photons are best explained by quantum mechanics and exhibit wave–particle duality, their behavior featuring properties of both waves and particles. The modern photon concept originated during the first two decades of the 20th century with the work of Albert Einstein, who built upon the research of Max Planck. While trying to explain how matter and electromagnetic radiation could be in thermal equilibrium with one another, Planck proposed that the energy stored within a material object should be regarded as composed of an integer number of discrete, equal-sized parts. To explain the photoelectric effect, Eins ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Diffuse Optical Imaging
Diffuse optical imaging (DOI) is a method of imaging using near-infrared spectroscopy (NIRS) or fluorescence-based methods. When used to create 3D volumetric models of the imaged material DOI is referred to as diffuse optical tomography, whereas 2D imaging methods are classified as diffuse optical imaging. The technique has many applications to neuroscience, sports medicine, wound monitoring, and cancer detection. Typically DOI techniques monitor changes in concentrations of oxygenated and deoxygenated hemoglobin and may additionally measure redox states of cytochromes. The technique may also be referred to as diffuse optical tomography (DOT), near infrared optical tomography (NIROT) or fluorescence diffuse optical tomography (FDOT), depending on the usage. In neuroscience, functional measurements made using NIR wavelengths, DOI techniques may classify as functional near infrared spectroscopy fNIRS. Physical mechanism Biological tissues can be considered strongly diffusive me ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Radiative Transfer Equation And Diffusion Theory For Photon Transport In Biological Tissue
Photon transport in biological tissue can be equivalently modeled numerically with Monte Carlo simulations or analytically by the radiative transfer equation (RTE). However, the RTE is difficult to solve without introducing approximations. A common approximation summarized here is the diffusion approximation. Overall, solutions to the diffusion equation for photon transport are more computationally efficient, but less accurate than Monte Carlo simulations. Definitions The RTE can mathematically model the transfer of energy as photons move inside a tissue. The flow of radiation energy through a small area element in the radiation field can be characterized by radiance L(\vec,\hat,t) (\frac). Radiance is defined as energy flow per unit normal area per unit solid angle per unit time. Here, \vec denotes position, \hat denotes unit direction vector and t denotes time (Figure 1). Several other important physical quantities are based on the definition of radiance: *Fluence rate or inte ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Optical Tomography
Optical tomography is a form of computed tomography that creates a digital volumetric model of an object by reconstructing images made from light transmitted and scattered through an object. Optical tomography is used mostly in medical imaging research. Optical tomography in industry is used as a sensor of thickness and internal structure of semiconductors. Principle Optical tomography relies on the object under study being at least partially light-transmitting or translucent, so it works best on soft tissue, such as breast and brain tissue. The high scatter-based attenuation involved is generally dealt with by using intense, often pulsed or intensity modulated, light sources, and highly sensitive light sensors, and the use of infrared light at frequencies where body tissues are most transmissive. Soft tissues are highly scattering but weakly absorbing in the near-infrared and red parts of the spectrum, so that this is the wavelength range usually used. Types Diffuse optical t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Medical Optical Imaging
Medical optical imaging is the use of light as an investigational imaging technique for medical applications. Examples include optical microscopy, spectroscopy, endoscopy, scanning laser ophthalmoscopy, laser Doppler imaging, and optical coherence tomography. Because light is an electromagnetic wave, similar phenomena occur in X-rays, microwaves, and radio waves. Optical imaging systems may be divided into diffusive and ballistic imaging systems. A model for photon migration in turbid biological media has been developed by Bonner et al. Such a model can be applied for interpretation data obtained from laser Doppler blood-flow monitors and for designing protocols for therapeutic excitation of tissue chromophores. Diffusive optical imaging Diffuse optical imaging (DOI) is a method of imaging using near-infrared spectroscopy (NIRS) or fluorescence-based methods. When used to create 3D volumetric models of the imaged material DOI is referred to as diffuse optical tomography, whereas ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Global Dimming
Global dimming is the reduction in the amount of global direct irradiance at the Earth's surface that has been observed since systematic measurements began in the 1950s. The effect varies by location, but worldwide it has been estimated to be of the order of a 4–20% reduction. However, after discounting an anomaly caused by the eruption of Mount Pinatubo in 1991, a very slight reversal in the overall trend has been observed. Global dimming is thought to have been caused by an increase in particulates or aerosols, such as sulfate aerosols in the atmosphere due to human action. It has interfered with the hydrological cycle by reducing evaporation and may have reduced rainfall in some areas. Global dimming has been attributed as the leading factor in the 1984 Ethiopian famine by reducing heating at the tropics which drives the annual monsoon, or wet season. Causes It is thought that global dimming is probably due to the increased presence of aerosol particles in Earth's atmos ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Diffusion Damping
In modern cosmological theory, diffusion damping, also called photon diffusion damping, is a physical process which reduced density inequalities (anisotropies) in the early universe, making the universe itself and the cosmic microwave background radiation (CMB) more uniform. Around 300,000 years after the Big Bang, during the epoch of '' recombination'', diffusing photons travelled from hot regions of space to cold ones, equalising the temperatures of these regions. This effect is responsible, along with baryon acoustic oscillations, the Doppler effect, and the effects of gravity on electromagnetic radiation, for the eventual formation of galaxies and galaxy clusters, these being the dominant large scale structures which are observed in the universe. It is a damping ''by'' diffusion, not ''of'' diffusion.Hu, Sugiyama & Silk (1996-04-28), p. 2 The strength of diffusion damping is calculated by a mathematical expression for the ''damping factor'', which figures into the Boltzm ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Diffuse Reflection
Diffuse reflection is the reflection (physics), reflection of light or other radiation, waves or particles from a surface such that a ray (optics), ray incident on the surface is scattering, scattered at many angles rather than at just one angle as in the case of specular reflection. An ''ideal'' diffuse reflecting surface is said to exhibit Lambertian reflection, meaning that there is equal luminance when viewed from all directions lying in the half-space (geometry), half-space adjacent to the surface. A surface built from a non-absorbing powder such as plaster, or from fibers such as paper, or from a polycrystalline material such as white marble, reflects light diffusely with great efficiency. Many common materials exhibit a mixture of specular and diffuse reflection. The visibility of objects, excluding light-emitting ones, is primarily caused by diffuse reflection of light: it is diffusely-scattered light that forms the image of the object in the observer's eye. Mechanism ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Medicine
Medicine is the science and practice of caring for a patient, managing the diagnosis, prognosis, prevention, treatment, palliation of their injury or disease, and promoting their health. Medicine encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness. Contemporary medicine applies biomedical sciences, biomedical research, genetics, and medical technology to diagnose, treat, and prevent injury and disease, typically through pharmaceuticals or surgery, but also through therapies as diverse as psychotherapy, external splints and traction, medical devices, biologics, and ionizing radiation, amongst others. Medicine has been practiced since prehistoric times, and for most of this time it was an art (an area of skill and knowledge), frequently having connections to the religious and philosophical beliefs of local culture. For example, a medicine man would apply herbs and say prayers for healing, o ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Scattering
Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including particles and radiation) in the medium through which they pass. In conventional use, this also includes deviation of reflected radiation from the angle predicted by the law of reflection. Reflections of radiation that undergo scattering are often called ''diffuse reflections'' and unscattered reflections are called ''specular'' (mirror-like) reflections. Originally, the term was confined to light scattering (going back at least as far as Isaac Newton in the 17th century). As more "ray"-like phenomena were discovered, the idea of scattering was extended to them, so that William Herschel could refer to the scattering of "heat rays" (not then recognized as electromagnetic in nature) in 1800. John Tyndall, a pioneer in light scattering researc ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Radiative Transfer
Radiative transfer is the physical phenomenon of energy transfer in the form of electromagnetic radiation. The propagation of radiation through a medium is affected by absorption, emission, and scattering processes. The equation of radiative transfer describes these interactions mathematically. Equations of radiative transfer have application in a wide variety of subjects including optics, astrophysics, atmospheric science, and remote sensing. Analytic solutions to the radiative transfer equation (RTE) exist for simple cases but for more realistic media, with complex multiple scattering effects, numerical methods are required. The present article is largely focused on the condition of radiative equilibrium. Definitions The fundamental quantity that describes a field of radiation is called spectral radiance in radiometric terms (in other fields it is often called specific intensity). For a very small area element in the radiation field, there can be electromagnetic radiat ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Stellar Atmosphere
The stellar atmosphere is the outer region of the volume of a star, lying above the stellar core, radiation zone and convection zone. Overview The stellar atmosphere is divided into several regions of distinct character: * The photosphere, which is the atmosphere's lowest and coolest layer, is normally its only visible part. Light escaping from the surface of the star stems from this region and passes through the higher layers. The Sun's photosphere has a temperature in the 5,770 K to 5,780 K range. Starspots, cool regions of disrupted magnetic field lie on the photosphere. * Above the photosphere lies the chromosphere. This part of the atmosphere first cools down and then starts to heat up to about 10 times the temperature of the photosphere. * Above the chromosphere lies the transition region, where the temperature increases rapidly on a distance of only around 100 km. * The outermost part of the stellar atmosphere is the corona, a tenuous plasma which has a tem ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
