Specular opaque

For opaque materials, the reflectance p is the complement of the absorptance. The directional distribution of the reflected radiation depends on the material, its degree of roughness or grain size, and, if a metal, its state of oxidation. Polished surfaces of homogeneous materials reflect speciilarly. In contrast, the intensity of the radiation reflected from a perfectly diffuse, or Lambert, surface is independent of direction. The directional distribution of reflectance of many oxidized metals, refractoiy materials, and natural products approximates that of a perfectly diffuse reflector. A better model, adequate for many calculational purposes, is achieved by assuming that the total reflectance p is the sum of diffuse and specular components p i and p. ... 

As illustrated by the examples discussed here, the use of FT spectrometers for the observation of surface structures is favored by situations in which the flux of radiation coming from the sample is very low or the data acquisition time is limited. Such cases arise in transmission spectroscopy using strongly absorbing or scattering samples, specular and diffuse reflectance spectroscopy from opaque samples, and emission spectroscopy from low temperature sources. FT spectroscopy is also well suited for observing the dynamics of surface species during adsorption, desorption, and reaction. 

The metallic lustre of the elemental substances formed by the heavier Group 14 elements in the diamond structure can be interpreted in terms of the valence band/conduction band picture. The spectrum of excited states which can arise from promotion of an electron from the valence band to the conduction band covers the whole of the visible region, leading to opaqueness and specular reflectance. In the case of diamond itself, the lowest electronic excited state lies well into the ultraviolet. 

Consider, for example, the arrangement in Fig. 8-61. The two diffuse opaque surfaces are separated by a specular-diffuse transmitting and reflecting plane. For this example all planes are assumed to be infinite in extent. The specular-transmitted exchange between surfaces 1 and 3 may be calculated immediately with... 

The network method which we have used to analyze radiation problems is an effective artifice for visualizing radiant exchange between surfaces. For simple problems which do not involve too many surfaces the network method affords a solution that can be obtained quite easily. When many heat-transfer surfaces are involved, it is to our advantage to formalize the procedure for writing the nodal equations. For this procedure we consider only opaque, gray, diffuse surfaces. The reader should consult Ref. 10 for information on transmitting and specular surfaces. The radiant-energy balance on a particular opaque surface can be written... 

Given the limitations of Kubelka-Munk, a more complex equation was developed by J. L. Saunderson that contrasted the refractive index of the sample to that of air. With the addition of surface or specular (K,) and internal (K2) correction factors the equation became more practical for use in opaque systems. 

Light (or near-ir and uv radiation) that is incident on opaque minerals is partly absorbed and partly reflected by them. There are two kinds of reflection processes that occurring when light is reflected from a flat polished surface of the mineral (specular reflectance) and that occurring when the light is reflected from the mineral after it has been finely powdered (diffuse reflectance). The latter arises from radiation that has penetrated the crystals (as in an electronic absorption spectrum) and reappeared at the surface after multiple scatterings in this case there will also be a specular component to the reflectance from light that is reflected from the surfaces of the particles. The specular reflectance of a flat polished surface of an opaque mineral measured at normal incidence can be related to the n and k terms of the complex refractive index (N) in which ... 

Caron offers four different models, 6510, 6515, 6530, and 6535 as photostability test chambers. (24) The 6510 and 6530 have no humidity control, whereas the 6515 and 6535 do have humidity control. They offer extra UVA and VIS detectors as an option. The interior is constructed using specular reflective aluminum. The lamps are mounted in a "U" shaped opaque cover. 

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. 

Hence K/S can be calculated directly from measurement of infinite thickness but to calculate K and S separately it is necessary to measure the reflectance of thin layers mounted on white and black backgrounds. If K and S are required for prediction purposes, the accuracy of their measurement can be improved by appropriate correction factors for surface reflection (Saunderson 1942). Colour calculated fromi oo, with separate estimation of the specular component as gloss is usually sufficient information to describe opaque objects, but for translucent or layered materials K and S are also necessary. 

One of the major sample-handling problems in FTIR analysis of carbonaceous materials is that many of them are effective blackbody absorbers and thus are too opaque for direct transmission analysis in the midinfrared spectral region. Addition of KBr intensifies the signal to obtain transmission infrared spectra. It is time consuming, and grinding conditions and moisture are known to affect the spectrum of the sample [238]. Alternative techniques such as specular reflectance, diffuse reflectance (DRIFT), photoacustic spectroscopy (FTIR-PAS), and total... 

ASTM D2457 Standard Test Method for Specular Gloss of Plastic Films and Solid Plastics. Used for the measurement of gloss of opaque and transparent plastic films and solid plastics. D2475 contains three separate gloss angles 60°, recommended for intermediate-gloss films,... 

In the classical polishing process the surface is smoothed to a mirror level, so that specular reflections can occur on opaque surfaces and be perceived as luster or gloss. In case of transparent materials, such as lenses, polishing minimizes the amount of incident light physically scattered by asperities and small defects. In both mentioned... 

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