Absorption. Reflection. Transmission

The emissivity, S, is the ratio of the radiant emittance of a body to that of a blackbody at the same temperature. Kirchhoff s law requires that a = e for aH bodies at thermal equHibrium. For a blackbody, a = e = 1. Near room temperature, most clean metals have emissivities below 0.1, and most nonmetals have emissivities above 0.9. This description is of the spectraHy integrated (or total) absorptivity, reflectivity, transmissivity, and emissivity. These terms can also be defined as spectral properties, functions of wavelength or wavenumber, and the relations hold for the spectral properties as weH (71,74—76). 

In practice we usually think about the response of a material to oscillatory fields— absorption, reflection, transmission, refraction, etc. We learn to connect the frequencies at which electromagnetic waves are absorbed with the natural motions of the material. If necessary, we can use oscillatory-field responses to know what the material would do in nonoscillatory fields. 

Nuclear. Mass can be determined directly by measuring changes in the absorption, reflection, or transmission of alpha- or beta-rays, which changes in proportion to the amount of material present. This method is primarily used to determine the mass of bulk material moving on a conveyor. The advantages include the following ... 

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. 

In a recent.comprehensive investigation, an optical integrating sphere was used to measure the effects of radiation and its component parts, absorptivity, reflectivity, and transmissivity, of a fabric. Emissivity of the textile wrapped around a heated brass cylinder was measured by thermoprobes in an evacuated environment (3.). The specific heat of textiles is usually measured with adiabatic calorimeters other thermal characteristics such as heat of fusion of absorbed water in fibers were also measured by this technique (19). 

We now consider a simple extension of the presentations in Secs. 8-10 and 8-11 to analyze a medium where reflection, transmission, and absorption modes are all important. As in Sec. 8-10, we shall analyze a system consisting of two parallel diffuse planes with a medium in between which may absorb, transmit, and reflect radiation. For generality we assume that the surface of the transmitting medium may have both a specular and a diffuse component of reflection. The system is shown in Fig. 8-58. 

Some other materials, such as glass and water, allow visible radiation to penetrate to considerable depths before any significant absorption takes place. Radiation through such scmitranspareiu materials obviously cannot be considered to be a surface phenomenon since the entire volume of the material interacts with radiation. On the other hand, both glass and water ace practically opaque to infrared radiation. Therefore, materials can exhibit different behavior at different wavelengths, and the dependence on wavelength is an important consideration in the study of radiative properties such as emissivity, absorptivity, reflectivity, and transmissivity of materials. 

The absorption, reflection, and transmission of incident radiation by a semitransparent material. 

In general the absorptivity, reflectivity, and transmissivity are functions of the wave length, and the energy of each wave length is resolved independently of the others into these three portions. 

Of the radiation that falls on a body, a fraction a is absorbed, a fraction r is reflected, and a fraction t is transmitted. These fractions are called absorptivity, reflectivity, and transitivity, respectively. Most industrial solids are opaque such that the transmissivity is zero and Eq. (35) holds. 

In another direction, words ending in -tion, such as absorption, reflection, radiation, transmission, and diffraction, are process terms. As such, they should not be used for the entity absorbed, radiated, or transmitted, nor for the corresponding measured values, such as absorptance, absorbance, reflectance, and transmittance. 

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

Given a specimen of sufficiently low absorption, a transmission Laue pattern can be obtained and used, in much the same way as a back-reflection Laue pattern, to reveal the orientation of the crystal. 

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