Reflectance of radiation

Figure 3.18 Total reflection of radiation in a medium of refractive index 2 by a thin film of refractive index wj, where 2...

Powdered iron phosphates, 26 215 Powdered sugar, 23 482 Powdered surfaces, diffuse reflectance of radiation from, 24 110 Powder glass-ceramic processing,... 

Diffuse-reflectance MIRS has found a number of applications for dealing with hard-to-handle solid samples, such as polymer films, fibers, or solid dosage forms. Reflectance MIR spectra are not identical to the corresponding absorption spectra, but sufficiently close in general appearance to provide the same level of information. Reflectance spectra can be used for both qualitative and quantitative analysis. Basically, reflection of radiation may be of four types specular, diffuse, internal, and attenuated total. 

Typically the source is tuned with the sample in place and then locked to match the cavity resonance frequency so as to achieve maximum energy storage and minimum reflected power. This reflected power is directed through a one-way coupler called a circulator to a crystal diode detector to convey information about sample absorption in the cavity. An iris opening to the cavity is adjusted to match the impedance of the cavity to that of the source so as to produce minimum reflection of radiation from the cavity. This condition gives maximum sensitivity for the impedance mismatch produced when sample absorption occurs in the cavity. 

In order to avoid effects caused by the sample itself, quantitative analyses should be carried out of dilute solutions. The bands which are selected for analysis should be isolated and preferably be free of interference with the solvent or other components. If overlapping cannot be avoided, the bands of the solvent can be compensated for by placing a matching solvent-filled cell into the reference beam, or by subtracting the solvent spectrum by computer. In recording a spectrum of the empty cell, interferences occur due to the reflection of radiation at the surface of the window material inside the cell. The number and wavenumbers of the maxima and minima of the interference fringes in the spectrum (see Fig. 5.1-6) gives the cell thickness ... 

The screeners are interposed as a shield between the radiation and the polymer. They function either (a) by absorbing the radiation before it reaches the photoactive species in the polymer or (b) by limiting the damaging radiation penetration into the polymer matrix. Reflection of radiation can be achieved by selection of suitable paints, coatings, pigments or by metallizing [94, 95] the surface. 

As with X-ray fluorescence, the characteristic X rays are analysed using wavelength dispersive spectrometers (WDS) based on the selective reflection of radiation by a monochromator crystal. The related analytical performance levels are ... 

Fig. 5.17 Mirrorlike reflection of radiation Fig. 5.18 Diffnse reflection of the radiation incident from the polar angle ft incident from the polar angle ft...

As an alternative to lenses and optical fibers so-called light pipes which use total reflection of radiation may also be used. 

Using optical fibers, mid-lR spectroscopy has been used for online analysis and remote sampling [84]. Fibers used in the mid-lR region are produced from oxides, chalcogenides, and halides of various elements. To be useful, the fibers must have IR radiation transmission capability over short distances. Like ATR crystals, fibers are based on the total reflection of radiation inside a material. 

Boundary Conditions for the RTE. The solution of the radiative transfer equation in a given geometry is subject to boundary conditions, which give the radiation intensity distribution on the boundaries. The boundary intensity is comprised of two components (1) contribution due to emission at the boundary surfaces and (2) contribution due to diffuse and specular reflection of radiation intensity incident on the boundaries. The radiation incident on the boundary is due to intensity emitted from all volume and surface elements in the medium. In mathematical terms, the general boundary condition on any surface element is written as [1,6] ... 

Geometric Optics Results with Emission. When the temperature of a semitransparent layer is large, emission of radiation becomes significant, and the problem of radiative transfer becomes more complex. The change in refractive index at each interface causes total internal reflection of radiation in the medium with higher refractive index at the boundary. This effect must be treated in the RTE at the boundary of the medium, and diffuse boundary conditions are no longer correct for the exact solution of this type of problem. Various approaches have been attempted. 

In the next section, we provide equations to calculate the matrix (12) corresponding to the reflection of radiation from a plane-parallel layer. In solving these equations numerically, our main interest is in the dependence of the opposition effects on microphysical properties of the medium such as particle size, refractive index, and concentration. 

Reflection of radiation by planes of atoms furnishes an image which is less abstract than reflection by lattice planes. However, the distribution of the atoms represents both the structural motif as well as the periodicity of the structure. Parallel to the series of lattice planes (HKL), we find planes formed by different atomic species. Figure 3.18 shows the reflection of radiation by such planes. The amplitude is dependent on the atomic species. Beams coming from planes which are equivalent by translation must be in phase, hence Bragg s law. The interference of beams reflected by different types of plane gives rise to the structure factor (3.37). The scattering factors [/J-S)], represent the amplitude of the reflected... 

REFLECTION OF RADIATION AT PLANAR INTERFACE COVERED BY SINGLE LAYER... 

In general, the application of IR spectroscopy to investigate nanolayers located at various interfaces must involve the determination of optimum experimental conditions for each system to be analyzed. These conditions include angle of incidence, polarization of radiation, and the number of reflections of radiation from the layer-substrate interface. 

The walls of the oven are metallic and facilitate the reflection of radiation and, when interfaced with an absorbent material, dissipation results, and the provoking sample heats up. 

A general and more practical case, which is the same as for Eq. (4.11-40) but with the surfaces A, and Aj being gray with emissivities ei and 2, will be considered. Nonconducting reradiating walls are present as before. Since the two surfaces are now gray, there will be some reflection of radiation which will decrease the net radiant exchange between the surfaces below that for black surfaces. The final equations for this case are... 

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