Reflection mirrorlike

Specular reflectance infrared involves a mirrorlike reflection producing reflection measurements of a reflective material or a reflection-absorption spectrum of a film on a reflective surface. This technique is used to look at thin (from nanometers to micrometers thick) films. 

In practice, for simplicity, surfaces are assumed to reflect in a perfectly specular or diffiise manner. In specular (or mirrorlike) reflection, the angle of reflection equals the angle of incidence of the radiation beam. In diffuse reflection, radiation is reflected equally in all directions, as shown in Fig. 12-32. Reflection... 

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...

Surfaces with low emissivities often exhibit approximately mirrorlike or specular reflection rather than diffuse reflection. We want to investigate how the assumption of mirrorlike reflection affects the heat transfer. The assumptions regarding the emission of diffuse and grey radiation remain unaltered. Grey Lambert radiators with mirrorlike reflection are therefore assumed. 

As the pathways between two large, flat plates drawn schematically in Fig. 5.67 show, the radiation emitted by plate 1 always strikes plate 2 and continues to be reflected between the plates until it has been completely absorbed. The same is true for the radiation emitted by plate 2. The heat transfer is not different from that by diffuse reflection and equation (5.169) for the heat flux holds, regardless of whether one or both walls reflect diffusely or mirrorlike. The relevant relationship (5.170) for the heat flux with N radiation shields is applied without any changes for mirrorlike reflection. It is also valid when only the shields reflect mirrorlike and the plates diffusely. 

This is different for the radiation emitted by the outer area 2 it either strikes the inner surface or bypasses it and falls back on the outer surface 2. This part, pathway b in Fig. 5.68, is specularly reflected in such a way that it never leaves area 2. It does not participate in the radiative exchange between the surfaces. The other part, given by the view factor F21 = A1Fl2/A2 = Al/A2, (pathway c), strikes area 1 and is mirrorlike reflected between the surfaces until it is completely absorbed. Therefore, the outer surface only contributes to the radiative exchange by mirrorlike reflection, on the scale as if its surface A2 was reduced by the factor F21 it has the effective surface area A2F2l = Al. 

Fig. 5.68 Ray pathways in mirrorlike reflection between concentric spheres or very long cylinders 1 and 2...

N concentric, thin radiation shields, with the same emissivity s, are placed between the (diffuse or mirrorlike reflecting) inner area 1 and the diffuse reflecting outer area 2. The shields reflect mirrorlike. According to (5.173), the following balance equations hold... 

If the outer area 2 also reflects mirrorlike then A2 in the second term of the denominator should be replaced by Asjv because this is the size of the effective surface area of 2 for mirrorlike reflection. 

Example 5.13 A tube, with liquid nitrogen flowing through it, has an external diameter d = 30 mm. It emissivity is i = 0.075 and its temperature T = 80K. The tube is surrounded by a second concentric tube with internal diameter d2 = 60mm with 2 = 0.12 and T2 = 295 K. The space between them has been evacuated. Determine the heat flow per tube length L that is transferred by radiation. The limiting cases of diffuse and mirrorlike reflection of the outer tube should be investigated. 

The minus sign indicates that the heat is transferred from outside to inside. The inner tube is cooled by the liquid nitrogen. In the case of mirrorlike reflection, from (5.173), the smaller value Q/L = —1.948 W/m is obtained. As only a part of the outer area contributes to radiative exchange the insulation effect is greater. 

This holds for the diffuse reflecting outer cylinder. The heat flow has decreased significantly. The ratio Q(N = 1 )/Q(N = 0) has the value 0.487/2.368 = 0.206. With a mirrorlike reflecting outer cylinder d2 is replaced by dg. This then gives Q(N = 1 )/L = —0.480W/m and Q(N = 1 )/Q(N = 0) = 0.480/1.948 = 0.246. In specular reflection of the outer cylinder, the relative decrease of the heat flow caused by the protective shield is somewhat lower than that for a diffuse reflecting outer cylinder. However, the smallest absolute value of Q/L is yielded when both the shield and the outer cylinder reflect mirrorlike. 

For a discussion of reflectance spectroscopy, two types of reflectance must be defined, specular and diffuse. Specular reflectance is simply mirrorlike reflectance from a surface and is sometimes called regular reflectance it has a well-defined reflectance angle. Diffuse reflectance is defined as reflected radiant energy that has been partially absorbed and partially scattered by a surface with no defined angle of reflectance. The diffuse reflectance technique is widely used today for industrial applications involving textiles, plastics, paints, dyestuffs, inks, paper, food, and building materials. In the area of basic research, diffuse reflectance spectroscopy has been used in studies of solid-solid reactions, of species absorbed on metal surfaces, of radiation transfer, and of slightly soluble species. 

One can distinguish the surface and volume components in the diffuse transmission /dt and the diffuse reflection /dr (Fig. 1.22) [224-227]. The surface component, which is referred to as Fresnel diffiise reflectance, is the radiation undergoing mirrorlike reflection and still obeying the Fresnel reflection law but arising from randomly oriented faces. This phenomenon was first described by Lambert in 1760 [228] to account for the colors of opaque materials. The volume, or Kubelka-Munk (KM), component is the radiation transmitted through at least one particle or a bump on the surface (Fig. 1.22). 

The character of reflection of electrons from the evanescent wave strongly depends on the relationship between the duration r of the laser pulse and the time of flight of an electron through the laser wave, rtr. It may be shown that when the laser pulse duration is much longer than the characteristic transit time rtr, the character of reflection of the electrons is close to the mirror. Where the relationship between these times is reversed, the mirrorlike character of reflection is disturbed. Let us make some simple estimates of the laser field and electron beam parameters with which the reflection of electrons is possible. 

One of the advantages of the emission method is that it makes possible studies of molecules adsorbed on massive pieces of metal. This advantage is shared by the reflection method by which spectra of monolayers adsorbed on mirrorlike surfaces are obtained. Work on the development of reflection techniques has been carried out by Pickering and Eckstrom (66) and by Francis (67). 

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

The radiation flow reflected from the surface of a body can be described using dimensionless reflectivities, in the same manner as for the absorbed power with the absorptivities dealt with in the last section. However, this involves further complications if we do not only want to find out what proportion of the radiation from a certain direction is reflected but also in which direction the reflected energy is sent back. The possible reflective behaviour of a surface can be idealised by two limiting cases mirrorlike (or specular) reflection and diffuse reflection. 

Therefore, if the outer surface 2 reflects mirrorlike, in the following equation for the heat flow, from (5.148) with 12 — 12 1)... 

Sometimes it is important to consider the direction of reflected irradiation exitent from a surface. A property called the bidirectional reflectance distribution function (BRDF) is used to specify the directional distribution of the reflected intensity for a specified direction of incident radiation [2-4]. A specular surface is a mirrorlike surface for which the incidence angle is equal to the reflection angle. For a diffusely reflecting surface, the reflected intensity is the same in all directions, and if perfectly reflective, the BRDF is l/n sr. ... 

The application of the ATR method in Raman spectroscopy provides a unique way to study essentially mirrorlike polished metals, in particular single crystalline surfaces. The ATR method is used to excite surface plasmon polaritons (SPPs) effectively at the smooth metal surface, to improve the sensitivity in Raman spectroscopy by electromagnetic field enhancement [19]. The enhancement of the ATR configuration, as shown in Figs. 8(d and e), with respect to the normal external reflection geometry spans one to three orders, depending on the electrodes and their crystallographic orientations. 

Since the front surface of the LGP is mirrorlike (M) and the back surface is a gradation (G) of the MR elements, the LGP is named MRq-LGP. In case of uniform the pattern elements with constant pitch, the LGP is MRu-LGP. To enhance the TIR function of the elements, the tangent of the concave surface or the angle of reflection should be kept constant, i.e., the surface should be as close as to a prism surface. 

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