Reflection, Absorption, and Transmission

Figure 6.83 Reflection, absorption and transmission by a sohd. From K. M. Rahs, T. H. Conrtney, and J. Wulff, Introduction to Materials Science and Engineering. Copyright 1976 by John Wiley Sons, Inc. This material is nsed by permission John Wiley Sons, Inc.

Early work using microwaves as a diagnostic tool relied upon measuring a secondary effect of the dielectric properties of the material under interrogation, i.e., reflection, absorption and transmission. The two fundamental microwave parameters, e and e" are related to the food or material composition. These two fundamental parameters also determine the reflection, absorption and transmission of the materials exposed to a microwave signal. Thus by measuring the amplitude and phase of the reflected or transmitted wave, or the characteristics of absorption of a wave through the material, one is able to empirically establish a relationship to the constituency of the product. 

The reflectivity, absorptivity and transmissivity of a translucent body depend in large part on the surface conditions, the wavelength of the radiation, the composition of the material and the thickness of the body. Since the attenuation of radiation within a body should be analyzed as a bulk process, the evaluation of the reflectivity and transmissivity of a translucent object is more involved. 

FIGURE 4.1 Incident radiation is divided into reflection, absorption, and transmission. 

In the first models of incorporation of cyclic peptide stacks in membranes, a perpendicular orientation of the central axis of the nanotubes relative to the bilayer plane was assumed. Subsequent investigations of cyclo[(L-Trp-D-Leu)3-L-Gln-D-Leu-] in functionally relevant lipid membranes by polarized attenuated total reflectance (ATR), grazing angle reflection-absorption and transmission Fourier transform infrared (FT-IR) techniques showed that this is not quite so. In fact, their central... 

Electromagnetic waves Electromagnetic wave reflection, absorption and transmission coefficients Dielectric properties (permittivity, loss factor), moisture absorption, delamination, geometry, flaw size... 

Emission from a body occurs from thermally excited atoms and molecules within the body. The basic principle of thermal emission is described by Kirchhoff s law which states that the ratio between the energy of radiation emitted by a body in a thermal equilibrium and its absorptance is a function of only the temperature of the body and the wavenumber (or wavelength) of the radiation it does not depend on the material constituting the body. The absorptance mentioned above may be defined as follows. When a body is irradiated, the radiation is partly reflected, partly absorbed, and the remainder passes through the body, if scattering by the body is ignored. If the proportions of the reflection, absorption and transmission are expressed, respectively, by reflectance (r), absorptance (a), and transmittance (t), the following relation holds r -i- a -t- t = 1. It is clear that each of the three quantities is a dimensionless constant with a value between 0 and 1. (As each of them is a function of wavenumber v, they are expressed as r(v), a(v), and t(v) when necessary.) Usually, transmittance is denoted by T, but it is not used in this chapter to avoid confusion with temperature T. 

Possible interactive phenomena that may occur as hght radiation passes from one medium to another are refraction, reflection, absorption, and transmission. Regarding degree of light transmissivity, materials are classified as follows ... 

Radiosity J Total thermal radiation energy leaving a surface (emitted and reflected) per unit time per unit area of energy transfer per unit area). The three terms, Absorptivity (a), Reflectivity (p), and Transmissivity (x), are all surface properties... 

When the agitated reactors are used for gas-liquid systems, several correlations have been proposed to evaluate the effective attenuation coefficient. Otake et al. (1981) proposed a simple empirical expression that accounts for the absorption effects produced by the liquid phase and the reflection, refraction, and transmission effects provoked by the gaseous phase. Considering that the latter are proportional to the specific surface area (aj.), /jeff v may be represented by the correlation... 

Fraser DJJ, Norton KL, and Griffiths PR (1988) HPLC/FT-IR measurements by transmission, reflection-absorption, and diffuse reflection microscopy. In Messerschmidt RG and Harthcock MA (eds.) Infrared Microspectroscopy Theory and Applications, pp. 197-210. New York Dekker. 

A schematic layout of an UHV chamber, which is used to study the IR absorption of adsorbed species on an insulator, is shown in Figure 26.13. Essentially, the collimated beam exiting an FTIR spectrometer is focused onto the sample surface, re-collimated and focused onto the detector. Both reflection spectroscopy and transmission spectroscopy allow one to obtain the IR spectrum of adsorbed species, as a function of the gas dosage and polarization of the IR radiation. For example, using this technique it was found that the tilt angle 6 between molecular axis of an adsorbed HBr and the LiF(OOl) surface is 0 = 21 5°. 

Kaynak, A., Unsowrth, J., Glout, R., Mohan, A. S., and Bears, G. R, A study of microwave transmission, reflection, absorption and shielding effectiveness of conducting polypyrrole films, J. Appl, Polym. ScL, 54, 269-278 (1994). 

Light striking a dielectric interface can undergo a number of processes, the most important of which are reflection, absorption, and scattering. In a spectroscopic experiment, the intensity of light incident on a surface, Iq, is compared to that which has been transmitted through the medium, T = Iq/I, scattered from the medium S = IJIq or reflected from the medium, where R = IJI. A material s optical parameters (i.e., reflectivity, transmissivity, and absorbtivity) are described by the Fresnel equations which define these properties in terms of the materials refractive index, n, where n = n, + ik or complex dielectric constant e = + ik. These two parameters are interrelated since n = Ve. Fresnel also... 

Absorption is the property of the material of transferring energy from the photons to its atoms and molecules. The ratio of the energy transferred to the matter from the incident light to the total incident energy is called absorptance (A), which can be expressed in terms of the reflection (R) and transmission (T) as... 

The absorptivity, a, reflectivity, p, and transmissivity, r, are the key radiation properties of a material. The magnitudes of a, p, and r depend on the type of material, its thickness and its surface finish, and also on the wavelength of the radiation. Absorption characteristics are defined in a similar manner to emission characteristics (see Section 10.2). The absorptivity for a nonblack body is the ratio of the nonblack absorption to the black absorption at the same surface temperature. Only the absorbed portion causes heating however, as very few bodies behave as black bodies a more realistic assumption would be to treat those as gray bodies, which have the same absorptivity over the entire wavelength spectrum. 

This subsection describes a method to determine reflection, absorption and scattering properties of a tissue (Niemz, 1996). In a typical transmission experiment, one measures the transmission of a collimated beam (a laser soiuce being the most convenient source)... 

AH bodies above absolute-zero temperature radiate energy. This energy is transmitted as electromagnetic waves. Waves striking the surface of a substance are partially absorbed, partially reflected, and partially transmitted. These portions are measured in terms of absorptivity a, reflectivity p, and transmissivity t, where... 

AH bodies other than blackbodies reflect or transmit some portion of the radiation incident upon them. The fractions of the incident radiation that are absorbed, transmitted, and reflected by a body are caHed the absorptivity, a, the reflectivity, p, and the transmissivity, T, respectively. These must add up to... 

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