Multiple reflections radiation

Since radiation arriving at a black surface is completely absorbed, no problems arise from multiple reflections. Radiation is emitted from a diffuse surface in all directions and therefore only a proportion of the radiation leaving a surface arrives at any other given surface. This proportion depends on the relative geometry of the surfaces and this may be taken into account by the view factor, shape factor or configuration F, which is normally written as F, for radiation arriving at surface j from surface i. In this way, F,y, which is, of course, completely independent of the surface temperature, is the fraction of radiation leaving i which is directly intercepted by j. 

For single glazing, the determination of the absorbed and transmitted radiation and of the heat transfer is quite straightforward, but for a window with multipane glazing, the calculation is more complex. Besides conduction in the panes, convection in the gaps as well as multiple reflections between the individual panes must be considered. 

For grey walls, the correction factor allowing for multiple reflection of incident radiation is ... 

Absorption of infrared radiation by characteristic vibrations of a surface can be used to obtain information about that surface, by comparison with known absorption frequencies in molecules of known structure. Surface sensitivity is obtained by using small particles ) and thin films or, better, a multiple-reflection arrangement with optimized angles of incidence and reflection in particular making work on single-... 

The calculation of the radiation heat transfer between black surfaces is relatively easy because all the radiant energy which strikes a surface is absorbed. The main problem is one of determining the geometric shape factor, but once this is accomplished, the calculation of the heat exchange is very simple. When nonblackbodies are involved, the situation is much more complex, for all the energy striking a surface will not be absorbed part will be reflected back to another heat-transfer surface, and part may be reflected out of the system entirely. The problem can become complicated because the radiant energy can be reflected back and forth between the heat-transfer surfaces several times. The analysis of the problem must take into consideration these multiple reflections if correct conclusions are to be drawn. 

Sin e-Gas-Zone/Two-Surface-Zone Systems An enclosure consisting of but one isothermal gas zone and two gray surface zones can, properly specified, model so many industrially important radiation problems as to merit detailed presentation. One can evaluate the total radiation flux between any two of the three zones, including multiple reflection at all surfaces. 

This is a useful prerequisite for the optimization of sample arrangements the low intensity of the Raman radiation can be considerably enhanced by utilizing multiple reflections of the exciting and the emerging Raman radiation at the sample and an external spherical mirror. 

For nonabsorbing crystal powders, the 0° multiple reflection arrangement shown in Fig. 3.5-8 g has proven to be superior to other arrangements, because it combines a high intensity of the Raman radiation with the maximum ratio of Ram an/exciting radiation, Ir/Ip- All of the Raman spectra of crystal powders reproduced in the Raman/ Infrared Atlas (Schrader, 1989) have been recorded with an arrangement according to this principle. 

Fig. 3.5-10 c shows another sample arrangement which makes use of a fiber-optical connection from the laser to the sample and back to the spectrometer. It is specially designed for the scanning of surface layers, e.g., of precious prints or paintings. The half spheric concave mirror reflects the portion of exciting radiation and Raman radiation back to the sample which has been. scattered by the sample and is not collected by the optical fiber. Thus the mirror as a component of a multiple reflection system enhances the observed intensity of the Raman lines by a factor of 2 to 8, depending on the properties of the sample. 

A solid compact sample or a tablet produced from powder may be placed directly in the sample holder. The spectrum is often improved by irradiating a fine hole within the sample with the laser beam and by analyzing the Raman radiation emerging from the hole. The multiple reflection of exciting and Raman radiation inside the hole enhances the efficiency of the transformation of laser radiation into Raman scattered radiation. 

The particular sensitivity of diffuse-reflection spectroscopy to surface structures, is due to the usually multiple reflection and diffraction of radiation at the surfaces of the scattering particles. Therefore catalytic processes and chemically modified surfaces are frequently studied this way. 

Another type of body that closely resembles a blackbody is a large cavity with a small opening, as sirown in l ig. 12-8. Radiation coming in through the opening of area A undergoes multiple reflections, and thus it has several chances to be absorbed by the interior surfaces of the cavity before any part of... 

The analysis of radiation transfer in enclosures consisting of black surfaces is relatively easy, as we have seen, but most enclosures encountered in practice involve nonblack surfaces, which allow multiple reflections to occur. Radiation analysis of such enclosures becoroe.s very complicated unless some simplifying assumptions are made. 

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