Light transport in inhomogeneous scattering media : perturbation theory and biomedical application

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Light dosimetry in photodynamic therapy using optical fiber delivery

This paper considers rapid calculations of light distributions in a tissue for photodynamic therapy. The delivery system considered is a single optical fiber, although other configurations are easily modeled. The influence of optical heterogeneities in the tissue is considered. The optical distributions surrounding an optical fiber within a tissue can be well approximated by a point source of light placed one 'reduced mean free path' (MFP) in front of the fiber tip. One MFP equals 1/(µ_a + µ_s(1-g)). Then the simple diffusion theory expression for 3D diffusion from a point source predicts the light distribution in the tissue. The influence of optical heterogeneities in the tissue can also be well approximated. A method is described for representing regions of increased absorption (such as a region with increased vascularity) as virtual sources of 'negative radiant power' such that linear superposition of the true primary source and the virtual sources yields the net light distribution in the heterogeneous tissue. The method is rapid, flexible, and generally accurate to within about 15% error. The method yields light distributions in optically complex issues

Towards agent-based building stock modeling: Bottom-up modeling of long-term stock dynamics affecting the energy and climate impact of building stocks

Buildings are responsible for a large share of the energy demand and greenhouse gas (GHG) emissions in Europe and Switzerland. Bottom-up building stock models (BSMs) can be used to assess policy measures and strategies based on a quantitative assessment of energy demand and GHG emissions in the building stock over time. Recent developments in BSM-related research have focused on modeling the status quo of the stock and comparatively little focus has been given to improving the modeling methods in terms of stock dynamics. This paper presents a BSM based on an agent-based modeling approach (ABBSM) that models stock development in terms of new construction, retrofit and replacement by modeling individual decisions on the building level. The model was implemented for the residential building stock of Switzerland and results show that it can effectively reproduce the past development of the stock from 2000 to 2017 based on the changes in policy, energy prices, and costs. ABBSM improves on current modeling practice by accounting for heterogeneity in the building stock and its effect on uptake of retrofit and renewable heating systems and by incorporating both regulatory or financial policy measures as well as other driving and restricting factors (costs, energy prices)

Polarized light transmission through skin using video reflectometry: toward optical tomography of superficial tissue layers

The movement of polarized light through the superficial layers of the skin was visualized using a video camera with a polarizing filter. This study constitutes a description of the impulse response to a point source of incident collimated linearly polarized light. Polarization images reject unwanted diffusely backscattered light from deeper in the tissue and the specular reflectance from the air/tissue interface. Two experiments were conducted: (1) Video polarization reflectometry used a polarized HeNe laser (633 nm) pointing perpendicularly down onto a phantom medium (0.900-µm dia. polystyrene spheres in water). The video camera was oriented 10° off the vertical axis and viewed the irradiation site where the laser beam met the phantom. Video images were acquired through a polarizing filter that was either parallel or perpendicular with the reference plane defined by the source, camera, and irradiation site on the phantom medium's surface. The source polarization was parallel to the reference plane. The two images (parallel and perpendicular) were used to calculate a polarization image which indicated the attenuation of polarization as a function of distance between the source and point of photon escape from the phantom. Results indicated a strong polarization pattern within approximately 0.35 cm (approximately 2.2 mfp') from source. [mfp' = 1/(µ_a + µ_s')]. (2) Optical fiber reflectometry using a polarized diode laser (792 nm) coupled to a polarization-maintaining single-mode fiber, and a multi-mode fiber collector to collect regardless of polarization. Reflectance as a function of fiber separation was measured for the source fiber oriented parallel and perpendicular with the reference plane. Results indicated that the strongest polarization propagated within approximately 0.43 cm (2.2 mfp') from source. The polarization survived ~2.2 mfp', which for skin at 630 - 800 nm (mfp' ≈ 0.066 cm) corresponds to 1.5 mm (or 6.4 ps of travel at the speed of light). Using 6.4 ps as a maximum time of survival, classical paths of photon transport (Feynman paths) were calculated to illustrate the expected depth of interrogation by polarized imaging. The expected mean depth of photons is about 0.36 mm at these longer wavelengths. Shorter wavelengths would result in a shorter mfp' and therefore more superficial imaging of the skin. Polarization images offer an inexpensive approach toward 2-D acquisition of time- gated images based on the early light escaping the tissue. Polarization imaging is an opportunity for a new form of optical image especially useful for dermatology

Policies to decarbonize the Swiss residential building stock: An agent-based building stock modeling assessment

In light of the Swiss government\u27s reduction targets for greenhouse gas (GHG) emissions under the Paris Agreement, this article investigates how and with which policy measures these reduction targets can be met for the Swiss residential building sector. The paper applies an agent-based building stock model to simulate the development of the Swiss residential building stock under three different policy scenarios. The scenario results until 2050 are compared against the reduction targets set by the Swiss government and with each other. The results indicate that while the current state of Swiss climate policy is effective in reducing energy demand and GHG emissions, it will not be enough to reach the ambitious emission-reduction targets. These targets can be reached only through an almost complete phase-out of fossil-fuel heating systems by 2050, which can be achieved through the introduction of further financial and/or regulatory measures. The results indicate that while financial measures such as an increase in the CO2 tax as well as subsidies are effective in speeding up the transition in the beginning, a complete phase-out of oil and gas by 2050 is reached only through additional regulatory measures such as a CO2 limit for new and existing buildings

Laser optoacoustic imaging of turbid media: determination of optical properties by comparison with diffusion theory and Monte Carlo simulation

Laser optoacoustic imaging based on time-resolved detection of laser-induced thermoelastic pressure waves can be potentially used in medicine as a diagnostic tool (tomography) and as means to measure tissue optical properties in vivo. Information on tissue optical properties in vivo is important for laser dosimetry and tissue characterization. Analysis of the profile and amplitude of laser-induced transient stress can provide direct information about absorbed energy distribution in irradiated volume. Pressure wave profiles exactly correspond to temperature distributions generated in tissues under irradiation conditions of temporal stress confinement. In this study we experimentally measured pressure wave profile and amplitude upon irradiation of highly scattering gel phantoms and aqueous solutions colored with potassium chromate and made turbid with polystyrene spheres. A wide-band lithium niobate acoustic transducer was used to detect laser-induced pressure waves. The experimentally measured laser-induced stress profiles and the results of a Monte Carlo simulation were in excellent agreement. Computer code was developed to calculate separately absorption and scattering coefficients from transient stress profiles. Our study has demonstrated feasibility of time-resolved detection of laser-induced acoustic transients as a method for tissue optical properties measurements in vivo

Polarized light transmission through skin using video reflectometry: toward optical tomography of superficial tissue layers

The movement of polarized light through the superficial layers of the skin was visualized using a video camera with a polarizing filter. This study constitutes a description of the impulse response to a point source of incident collimated linearly polarized light. Polarization images reject unwanted diffusely backscattered light from deeper in the tissue and the specular reflectance from the air/tissue interface. Two experiments were conducted: (1) Video polarization reflectometry used a polarized HeNe laser (633 nm) pointing perpendicularly down onto a phantom medium (0.900-µm dia. polystyrene spheres in water). The video camera was oriented 10° off the vertical axis and viewed the irradiation site where the laser beam met the phantom. Video images were acquired through a polarizing filter that was either parallel or perpendicular with the reference plane defined by the source, camera, and irradiation site on the phantom medium's surface. The source polarization was parallel to the reference plane. The two images (parallel and perpendicular) were used to calculate a polarization image which indicated the attenuation of polarization as a function of distance between the source and point of photon escape from the phantom. Results indicated a strong polarization pattern within approximately 0.35 cm (approximately 2.2 mfp') from source. [mfp' = 1/(µ_a + µ_s')]. (2) Optical fiber reflectometry using a polarized diode laser (792 nm) coupled to a polarization-maintaining single-mode fiber, and a multi-mode fiber collector to collect regardless of polarization. Reflectance as a function of fiber separation was measured for the source fiber oriented parallel and perpendicular with the reference plane. Results indicated that the strongest polarization propagated within approximately 0.43 cm (2.2 mfp') from source. The polarization survived ~2.2 mfp', which for skin at 630 - 800 nm (mfp' ≈ 0.066 cm) corresponds to 1.5 mm (or 6.4 ps of travel at the speed of light). Using 6.4 ps as a maximum time of survival, classical paths of photon transport (Feynman paths) were calculated to illustrate the expected depth of interrogation by polarized imaging. The expected mean depth of photons is about 0.36 mm at these longer wavelengths. Shorter wavelengths would result in a shorter mfp' and therefore more superficial imaging of the skin. Polarization images offer an inexpensive approach toward 2-D acquisition of time- gated images based on the early light escaping the tissue. Polarization imaging is an opportunity for a new form of optical image especially useful for dermatology

Light dosimetry in photodynamic therapy using optical fiber delivery

This paper considers rapid calculations of light distributions in a tissue for photodynamic therapy. The delivery system considered is a single optical fiber, although other configurations are easily modeled. The influence of optical heterogeneities in the tissue is considered. The optical distributions surrounding an optical fiber within a tissue can be well approximated by a point source of light placed one 'reduced mean free path' (MFP) in front of the fiber tip. One MFP equals 1/(µ_a + µ_s(1-g)). Then the simple diffusion theory expression for 3D diffusion from a point source predicts the light distribution in the tissue. The influence of optical heterogeneities in the tissue can also be well approximated. A method is described for representing regions of increased absorption (such as a region with increased vascularity) as virtual sources of 'negative radiant power' such that linear superposition of the true primary source and the virtual sources yields the net light distribution in the heterogeneous tissue. The method is rapid, flexible, and generally accurate to within about 15% error. The method yields light distributions in optically complex issues

Synthetic building stocks as a way to assess the energy demand and greenhouse gas emissions of national building stocks

In Europe, the final energy demand and greenhouse gas (GHG) emissions of residential and commercial building stocks account for approximately 40% of energy and emissions. A building stock model (BSM) is a method of assessing the energy demand and GHG emissions of building stocks and developing pathways for energy and GHG emission reduction. The most common approach to building stock modeling is to construct archetypes that are taken to representing large segments of the stock. This paper introduces a new method of building stock modeling based on the generation of synthetic building stocks. By drawing on relevant research, the developed methodology uses aggregate national data and combines it with various data sources to generate a disaggregated synthetic building stock. The methodology is implemented and validated for the residential building stock of Switzerland. The results demonstrate that the energy demand and GHG emissions can vary greatly across the stock. These and other indicators vary significantly within common building stock segments that consider only few attributes such as building type and construction period. Furthermore, the results indicate a separation of the stock in terms of GHG emissions between old fossil fuel-heated buildings and new and refurbished buildings that are heated by renewable energy. Generating a disaggregated synthetic building stock allows for a discrete representation of various building states. This enables a more realistic representation of past building stock alterations, such as refurbishment, compared with commonly used archetypes, and not relying on more extensive data sources and being able to accommodate a wide variation of data types. The developed methodology can be extended in numerous manners and lays groundwork for future studies

Laser optoacoustic imaging of turbid media: determination of optical properties by comparison with diffusion theory and Monte Carlo simulation

Laser optoacoustic imaging based on time-resolved detection of laser-induced thermoelastic pressure waves can be potentially used in medicine as a diagnostic tool (tomography) and as means to measure tissue optical properties in vivo. Information on tissue optical properties in vivo is important for laser dosimetry and tissue characterization. Analysis of the profile and amplitude of laser-induced transient stress can provide direct information about absorbed energy distribution in irradiated volume. Pressure wave profiles exactly correspond to temperature distributions generated in tissues under irradiation conditions of temporal stress confinement. In this study we experimentally measured pressure wave profile and amplitude upon irradiation of highly scattering gel phantoms and aqueous solutions colored with potassium chromate and made turbid with polystyrene spheres. A wide-band lithium niobate acoustic transducer was used to detect laser-induced pressure waves. The experimentally measured laser-induced stress profiles and the results of a Monte Carlo simulation were in excellent agreement. Computer code was developed to calculate separately absorption and scattering coefficients from transient stress profiles. Our study has demonstrated feasibility of time-resolved detection of laser-induced acoustic transients as a method for tissue optical properties measurements in vivo