Specular transmission

A number of transparent polymers have been examined and tested for this purpose prototype domes have been fabricated from polyvlnylfluorlde which was later determined to be too expensive and not sufficiently stable. The dominant requirement for this application Is good specular transmission. Several polyesters... 

All the components mentioned interact with the powder and, therefore, contain information about its absorption coefficient. However, only the specular transmission and volume KM components give the absorptionlike spectra of the powder directly. The Fresnel components produce specular reflection (first derivative or inverted) specha [which can be converted into the spectra of the absorption coefficient using the KK transformation (1.1.13°)]. Therefore, to obtain the absorption spectrum of a powder, the Fresnel components must be eliminated from the final spectrum. In practice, this can be achieved by immersion of the sample in a hansparent matrix with a refractive index close to that of the powder, selection of appropriate powder size, or special construction of reflection accessories (Section 4.2). 

The principles and standard procedures used in the determination of color can be found in ASTM E308. Equipment is conunercially available for measuring optical properties of polymers for these purposes. Major suppliers include Gardner (gloss meter for specular gloss, haze meter for haze and luminous transmittance, and clarity meter for specular transmission) and Hunter Lab (spectrocolorimeter). 

Transparent plastic refractive index test methods are described in ASTM D0542 (ISO 489), Index of Refraction of Transparent Organic Plastics. Test methods for transmission through clear plastic sheet are described in ASTM D1746, Regular Transparency of Clear Sheet, Ratio of Undiffused Transmission Flux to Incident Flux. Specular transmission is a property of transparent plastic sheet to show clearly a relatively distant object. ASTM D1044 is entitled Resistance of Transparent Plastics to Surface Abrasion. ... 

Fig. IZ Specular transmission refers to light passing through an object without diffusion. Specular transmission measurements are widely used in the chemical analyses and color measurements of liquids. Potentially, appearance attributes important for specular transmission should be roughly analogous with gloss attributes associated with specular reflection (from HunterLab).

The interface properties can usually be independently measured by a number of spectroscopic and surface analysis techniques such as secondary ion mass spectroscopy (SIMS), X-ray photoelectron spectroscopy (XPS), specular neutron reflection (SNR), forward recoil spectroscopy (FRES), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), infrared (IR) and several other methods. Theoretical and computer simulation methods can also be used to evaluate H t). Thus, we assume for each interface that we have the ability to measure H t) at different times and that the function is well defined in terms of microscopic properties. 

Reflection-Absorption IR spectroscopy (RAIRS) where the linearly polarized IR beam is specularly reflected from the front face of a highly reflective sample, such as a metal single crystal surface (Figure 3.1(a)). This is also sometimes referred to as IRAS (IR reflection absorption). The IR beam comes in at grazing angle (i.e. almost parallel to the surface), and although absorption bands in RAIRS have intensities that are some two orders of magnitude weaker than in transmission studies on... 

It is important to stress that ATR absorbance is strongly affected by the sample/crystal contact. Quantitative results are thus difficult to obtain even if the contact is maintained during the sample rotation that is required to record all four polarized spectra. A reference band that does not show significant dichroism is thus most often used to normalize the polarized absorbances in order to obtain quantitative data. For instance, the 1,410 cm-1 band of PET has often been chosen for that purpose, not only for ATR studies but also for specular reflectance (see below) and even transmission studies when the sample thickness is not uniform. It was shown that an appropriate normalization is possible even if no such reference band is available, by using a combination of two bands with orthogonal dichroism [34]. When performing ATR experiments, one should also make certain that the applied pressure does not create artifacts by affecting the structure of the sample. 

Specular reflection IR spectroscopy has been used by Cole and coworkers to study the orientation and structure in PET films [36,37]. It has allowed characterizing directly very highly absorbing bands in thick samples, in particular the carbonyl band that can show saturation in transmission spectra for thickness as low as 2 pm. The orientation of different conformers could be determined independently. Specular reflection is normally limited to uniaxial samples because the near-normal incident light does not allow measuring Ay. However, it was shown that the orientation parameter along the ND can be indirectly determined for PET by using the ratio of specifically selected bands [38]. This approach was applied to the study of biaxially oriented PET bottles [39]. 

For some applications, especially such involving solid samples or fluids containing suspended particles, reflection spectroscopic systems are better suitable than transmission sensors. Apart from specular reflection, which provides comparatively little information and is of hardly any practical importance for IR sensing, two reflectrometric methods can be used to gain spectroscopic information about a sample diffuse reflection and transflection, a combination of transmission and diffuse reflection. 

The light reflected by a powdered solid will consist of a specular reflection component and of a diffuse reflection component. The specular component represents reflection of the incident light by the surfaces of the component particles, and it is characterized by a complete absence of light transmission through the interiors of the particles. By contrast, diffuse reflectance is associated with the radiation that penetrates into the particles to some extent and that then emerges from the bulk solid. This light will exhibit spectral characteristics that are modified from those of the incident beam by the electronic transitions that took place within the solid phase and at the boundaries of the component particles. 

A majority of traditional NIR measurements are made on solid materials and these involve reflectance measurements, notably via diffuse reflectance. Likewise, in the mid-IR not all spectral measurements involve the transmission of radiation. Such measurements include internal reflectance (also known as attenuated total reflectance, ATR), external reflectance (front surface, mirror -style or specular reflectance), bulk diffuse reflectance (less common in the mid-IR compared to NIR), and photoacoustic determinations. Photoacoustic detection has been applied to trace-level gas measurements and commercial instruments are available based on this mode of detection. It is important to note that the photoacoustic spectrum is a direct measurement of infrared absorption. While most infrared spectra are either directly or indirectly correlated... 

Within the IR spectroscopy arena, the most frequently used techniques are transmission-absorption, diffuse reflectance, ATR, specular reflectance, and photoacoustic spectroscopy. A typical in situ IR system is shown in Fig. 7. Choosing appropriate probe molecules is important because it will influence the obtained characteristics of the probed solid and the observed structure-activity relationship. Thus, the probe molecules cover a range from the very common to the very rare, in order to elucidate the effect of different surfaces to very specific compounds e.g. heavy water and deuter-ated acetonitrile, CDsCN). The design of the IR cell is extremely important and chosen to suit the purposes of each particular study. For catalytic reactions, the exposure of catalytic metals must be eliminated in cell construction, otherwise the observed effect of the catalyst may not be accurate. 

The spectrophotometer measures the transmission and, if an absorption measurement is carried out, converts the transmission into absorbance using these equations. This conversion works fine for samples where there is no reflection, either specular or diffuse, as is the case for nonturbid solutions. However, for films there is invariably some reflection, which is often quite large, particularly for films of high dielectric constant (or refractive index) materials, such as PbS and PbSe. Additionally, if the films are not completely transparent, then scattering introduces an extra element of reflection. Therefore, to measure the real absorption of a film, a reflection measurement must also be carried out and correction for this reflection made. The correction will be approximate and depends on the nature of the film itself. However, that most commonly used is... 

Specular reflection is encountered when the reflecting medium is a smooth polished surface. The angle of reflection is identical to the incident angle of the radiation beam. If the surface is IR absorbent, the relative intensity of reflection is less for wavelengths that are absorbed than for wavelengths that are not. Thus, the plot of reflectance R, defined as the fraction of reflected incident radiant energy versus the wavelength (or wavenumber) appears similar to a transmission spectrum for the sample. 

Figure 3.7 Spectroelectrochemical techniques (A) transmission, (B) internal reflectance, (C) specular reflectance, (D) parallel.

The techniques used in this stage of the work were all IR spectroscopy using a Perkin-Elmer spectrometer 599 and appropriate attachments. Conventional transmission through a sodium chloride cell of path length 0.1 mm, and multiple specular reflectance from aluminium films with a mirror finish were both used. 

To validate the whole technique, a transmission spectrum of the neat liquid silane was compared with one obtained by specular reflectance from an aluminium mirror surface on which a film of silane had been cast from 1% solution in anhydrous methanol. These two spectra were virtually indistinguishable, as regards both position and intensity of all peaks. They contained a considerable number of peaks, most of which could be assigned with complete satisfaction in terms of the known structure of y-glycidoxypropyltrimethoxy-silane. 

Specular and Diffuse Reflectance. Reflectance techniques have been applied primarily to samples which do not permit observation by transmission. Flat surfaces such as those of metal foils and single crystals can he studied by specular reflectance, whereas rough surfaces such as those of powders must be observed by diffuse reflectance. In both cases FT spectroscopy offers strong advantages in terms of the time required to acquire a spectrum. 

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. 

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