Types of Reflectance

FIGURE 4.35 An example of diffuse reflectance from a rough surface. The angle of incidence is fixed, but the angle of reflectance has many values. 

FIGURE 4.36 An example of total internal reflectance. The refractive index of the crystal through which the beam is passing, n, is greater than the refractive index of the sample above it, n,. 

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In some situations measurement of the reflected, rather than the transmitted, radiation may be made to assess the amount of radiation that has been absorbed by the sample. There are two main ways by which radiation might be reflected. Specular reflection is similar to the reflection by a mirror and, for quantitative work, the angles of the incident and the reflected radiation are important. Diffuse reflection is from within the layers of the material and the reflected light is disbursed over a range of 180°. This type of reflection is measured in the thin films used in dry chemistry systems. The term reflectance density is often used, which is defined in a manner comparable to absorbance the logarithm of the ratio of incident to reflected light. 

Reflectivity is the act of becoming aware of ourselves, behaviour, our habits, etc. Types of reflectivity include ... 

Shape-symmetry may tell us that a particular crystal has a fourfold axis, but it cannot tell us whether this axis is a simple rotation axis or a screw axis. Nor is it possible by examining the shape of a crystal to distinguish between a reflection plane and a glide plane. But X-ray diffraction patterns do make such distinctions, and in a very straightforward manner just as it is possible to detect compound ( centred ) lattices by noticing the absence of certain types of reflections (p. 233), so also it is possible to detect screw axes and glide planes, for the presence of atoms or groups of atoms related by translations which... 

Thus, while screw axes and glide planes can be detected and distinguished from each other by observing which types of reflections are absent, ordinary rotation axes and reflection planes cannot be detected in this way, since neither type leads to any systematic absences of reflections. 

Diffraction symmetry in relation to point-group symmetry. So far, in our consideration of the intensities of X-ray reflections in the process of discovering the general arrangement in a crystal, we hare dealt only with reflections of zero intensity, and we have seen that when certain types of reflections have zero intensity the presence... 

The type of arrangement of pattern-units is called the space-lattice . Secondly, the group of atoms forming a pattern-unit—the group of atoms associated with each lattice point—may have certain symmetries, and some of these symmetries cause further systematic absences of certain types of reflections from the diffraction pattern. The complex of symmetry elements displayed by the complete arrangement is known as the space-group. ... 

Brightness Meter. A special type of reflection meter for evaluating the brightness of paper and similar products by measuring the diffuse... 

Two types of reflection phenomena may be observed when radiation strikes a surface. If the angle of incidence is equal to the angle of reflection, the reflection is called specular. On the other hand, when an incident beam is distributed uniformly in all directions after reflection, the reflection is called... 

All the preceding discussions have considered radiation exchange between diffuse surfaces. In fact, the radiation shape factors defined by Eq. (8-21) hold only for diffuse radiation because the radiation was assumed to have no preferred direction in the derivation of this relation. In this section we extend the analysis to take into account some simple geometries containing surfaces that may have a specular type of reflection. No real surface is completely diffuse or completely specular. We shall assume, however, that all the surfaces to be considered emit radiation diffusely but that they may reflect radiation partly in a specular manner and partly in a diffuse manner. We therefore take the reflectivity to be the sum of a specular component and a diffuse component ... 

Different features are stressed to specify the most basic type of reflection when referring to it as... 

The three types of reflections in the planes containing both axial vertices and one of the three equatorial vertices (cr ) form the class 3cr of order 3. 

These equations indicate that the intensities of the two types of reflections, Pq and Pf, can be compared on the same scale. Employing the probability functions for the random stack-ing, - the intensity arising from the random stacking is expressed by equation (20), where h and k are integers (h — k =... 

Different types of reflection from a surface (a) actual or irregular. ib) diffuse, and (c) specular or mirrorlike. 

Another important difference between the LQL approximation and LN approximation is that for the former one can choose different types of reflectivity tensors. For example, one can introduce a scalar or diagonal reflectivity tensor. 

Different types of reflectance spectroscopy depend upon the reflecting behavior of the radiation on the solid. Fig. 8 illustrates various categories used to distinguish techniques for reflecting radiation off solids. Specular reflection spectroscopy is used to measure the reflectance spectrum of a smooth, glossy surface. In reflection-absorption spectroscopy, the radiation passes through a thin surface film on a reflective... 

As shown in section 2.12.3, the presence of translational symmetry causes extinctions of certain types of reflections. This property of infinite symmetry elements finds use in the determination of possible space group(s) symmetry from diffraction data by analyzing Miller indices of the observed Bragg peaks. It is worth noting that only infinite symmetry elements cause systematic absences, and therefore, may be detected from this analysis. Finite symmetry elements, such as simple rotation and inversion axes, mirror plane and center of inversion, produce no systematic absences and therefore, are not distinguishable using this approach. 

As is often the case in diffraction analysis, when the space group symmetry is known, it is quite easy to predict which types of reflections can and which cannot be observed. For example, when the space group symmetry is Pnma, only the following types of Bragg peaks may have nonzero intensity (as established by analyzing Table 2.8 to Table 2.10) ... 

There are three types of reflectance techniques specular, diffuse and reflection-absorption as illustrated in Figure 9.22. Specular reflectance is applied to samples with smooth and polished surfaces, diffuse reflectance is applied to samples with rough surfaces, and reflection-absorption is applied to IR-transparent thin films on IR opaque substrates. The specular and diffuse techniques are more widely used and are introduced in more detail in the following text. 

The pattern we have just indexed is for a primitive lattice where all reflections are observed. In the other types of cubic lattice, certain types of reflection are absent and these are called systematic absences. 

Figure 10.18 Spectra by reflection, (a) From a sample of plexiglass, three types of reflection are displayed. Left, crude spectra and right, spectra after correction. Above, crude signal of specular reflection and the result in units of K following application of the Kramers-Kronig (transformation of the reflectance) calculation middle, spectrum obtained by diffused hght comparison of the crude spectrum with the Kubelka-Munk correction below, spectrum obtained by ATR, the latter requiring a fine correction to reduce the absorbance at higher wavelengths which would be overestimated (b) comparison of two spectra of benzoic acid, one obtained through transmission, the other by diffused reflection and subsequent K-M correction.

Besides affecting the plant s structure, the type of reflected light influences the nature of the waxy coating on the leaves and the taste of the plant product. Surprisingly, the color of the reflected light also seems to affect the plant s response to insect damage. 

In principle in Section 4.2.5 it was discussed that reflectance at an interface usually disturbs the absorption measurement. In the present applications, however, this reflectance allows reactions at the surfaces to be monitored. Since nowadays solid samples are exhibited to photo-induced processes, such measurements are a convenient application. In principle two types of reflectance take place [42,96] ... 

The regular or specular reflectance at a mirror surface for which the geometric laws are valid which results in the same angle of exit as for the incidence. Examples of this type of reflectance are metals, glasses or crystals. 

There are quite a few cases where these two types of reflectance appear in combination. 

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