Attenuation coefficient | EPFL Graph Search

The linear attenuation coefficient, attenuation coefficient, or narrow-beam attenuation coefficient characterizes how easily a volume of material can be penetrated by a beam of light, sound, particles, or other energy or matter. A coefficient value that is large represents a beam becoming 'attenuated' as it passes through a given medium, while a small value represents that the medium had little effect on loss. The SI unit of attenuation coefficient is the reciprocal metre (m−1). Extinction coefficient is another term for this quantity, often used in meteorology and climatology. Most commonly, the quantity measures the exponential decay of intensity, that is, the value of downward e-folding distance of the original intensity as the energy of the intensity passes through a unit (e.g. one meter) thickness of material, so that an attenuation coefficient of 1 m−1 means that after passing through 1 metre, the radiation will be reduced by a factor of e, and for material

Local optical characterization of biological tissues in vitro and in vivo

Two methods for measuring the optical properties of tissue, i.e. reduced scattering and absorption coefficient, were developed. The first one was designed for in vitro investigations. It is based on the measurement of the spatially resolved transmittance through a tissue slab (typically 5 mm thick). The second one was designed for local in vivo investigations. It is based on the measurement of the spatially-resolved diffuse reflectance, close to the source (< 2 mm). A sterilizable probe, of diameter of 2.2 mm was realized for measurements in clinic, during surgery or endoscopy. A model of the light propagation in turbid media was developed based on Monte Carlo simulations. It allows us to establish the correspondence between the optical coefficients to the experimental data (spatially-resolved transmittance or reflectance). The influence of the scattering phase function on the reflectance and transmittance was carefully studied. In particular, the role of the first and the second moment of the phase function were shown for short distances measurements (close to one transport mean free path). Similarity relations, taking into account the first and second moment of the phase function were proposed and assessed. The model was tested extensively by measurements on tissue-like phantoms, with known optical properties. Further assessments of the model were performed by comparison with time-resolved measurements. Investigations on human brain, breast and cervix tissues were performed. Tissue structure (i.e. heterogeneities not taken into account by the model) were found to play a significant role in the light propagation. Normal and malignant brain tissue were investigated in vitro, and in vivo during surgery. Significant differences were found between different type of tissues, and were explained in terms of chromophore content (hemoglobin, water) and specific features of scattering properties.

EPFL1998

Sunlight-mediated inactivation of health-relevant microorganisms in water: A review of mechanisms and modeling approaches

Michael Cameron Dodd, Tamar Kohn, Yang Liu, Krista Rule Wigginton

Health-relevant microorganisms present in natural surface waters and engineered treatment systems that are exposed to sunlight can be inactivated by a complex set of interacting mechanisms. The net impact of sunlight depends on the solar spectral irradiance, the susceptibility of the specific microorganism to each mechanism, and the water quality; inactivation rates can vary by orders of magnitude depending on the organism and environmental conditions. Natural organic matter (NOM) has a large influence, as it can attenuate radiation and thus decrease inactivation by endogenous mechanisms. Simultaneously NOM sensitizes the formation of reactive intermediates that can damage microorganisms via exogenous mechanisms. To accurately predict inactivation and design engineered systems that enhance solar inactivation, it is necessary to model these processes, although some details are not yet sufficiently well understood. In this critical review, we summarize the photo-physics, -chemistry, and -biology that underpin sunlight-mediated inactivation, as well as the targets of damage and cellular responses to sunlight exposure. Viruses that are not susceptible to exogenous inactivation are only inactivated if UVB wavelengths (280 – 320 nm) are present, such as in very clear, open waters or in containers that are transparent to UVB. Bacteria are susceptible to slightly longer wavelengths. Some viruses and bacteria (especially Gram-positive) are susceptible to exogenous inactivation, which can be initiated by visible as well as UV wavelengths. We review approaches to model sunlight-mediated inactivation and illustrate how the environmental conditions can dramatically shift the inactivation rate of organisms. The implications of this mechanistic understanding of solar inactivation are discussed for a range of applications, including recreational water quality, natural treatment systems, solar disinfection of drinking water (SODIS), and enhanced inactivation via the use of sensitizers and photocatalysts. Finally, priorities for future research are identified that will further our understanding of the key role that sunlight disinfection plays in natural systems and the potential to enhance this process in engineered systems.

2018

Sunlight-mediated inactivation of health-relevant microorganisms in water: A review of mechanisms and modeling approaches

Michael Cameron Dodd, Tamar Kohn, Yang Liu, Krista Rule Wigginton

2018

Local optical characterization of biological tissues in vitro and in vivo

EPFL1998