Opaque substances, optical

It is possible to assess the proportion of stray light by measuring the amount of radiation transmitted by samples that are optically opaque at the wavelength to be assessed but that transmit radiation of other wavelengths. The instrument is set to zero and 100% transmittance in the normal way and the opaque substance introduced into the sample compartment. The amount of light transmitted by the sample, measured in percentage transmittance, is... 

Examples are Laser Differential Microanemometry (LMA) and Total Reflection Microscopy (TMA) (8). Both LMA and TMA measure the velocity profile of the fluid in tube flow. However, such optical techniques are generally not suitable for opaque and/or heterogeneous substances such as foods. Acoustic velocimetry seems to be more promising for determining the velocity profiles of opaque substances. Such an acoustic technique has been applied by Brunn et al (19) as an on-line viscometer for flow of mayonnaises in pipes. 

Use of X-ray diffraction patterns or identification. Even when complete structure determination is not possible, however, much valuable information of a less detailed character may be obtained by X-ray methods. In the first place, the diffracted beams produced when X-rays pass through crystals may be recorded on photographic films or plates, and the patterns thus formed may be used quite empirically, without any attempt at interpretation, to identify crystalline substances, in much the same way as we use optical emission spectra to identify elements, or infra-red absorption spectra to identify molecules. Each crystalline substance gives its own characteristic pattern, which is different from the patterns of all other substances and the pattern is of such complexity (that is, it presents so many measurable quantities) that in most cases it constitutes by far the most certain physical criterion for identification. The X-ray method of identification is of greatest value in cases where microscopic methodsare inadequate for instance, when the crystals are opaque or are too small to be seen as individuals under the microscope. The X-ray diffraction patterns of different substances generally differ so much from each other that visual comparison... 

Some substances, such as alkaline-earth sulphides, emit visible light when irradiated by X-rays for these substances it is essential to have a sheet of optically opaque material (such as black paper) between the specimen and the film. 

Why diamond works so well is because of its two most desirable properties, namely. (I) it is the hardest substance known to science, and t2) it is very- transparent to optical radiation as well as to x-rays. Compared to diamond, tungsten carbide, which was used in older pressure generating devices, has a much lower compressive strength and, further, it is opaque to radiation. 

Similar dependencies on concentration are observed for the osmotic pressure or the electrical conductance of the solution. If we look at the optical turbidity of the solution the trend is opposite. At low concentration the solution is transparent. When the concentration reaches the CMC many solutions become opaque. In parallel, a property, which is of great practical relevance, changes the capacity to solubilize another hydrophobic substance. At concentrations below the CMC of the surfactant, hydrophobic substances are poorly dissolved. At the CMC they start being soluble in aqueous solution. This capability increases with increasing surfactant concentration. There may be small systematic differences in the concentration at which the specific property abruptly changes and the CMC determined by different methods may be different. However, the general trend and the dependency on external parameters such as temperature or salt concentration is always the same. 

As compressed carbon dioxide is a nonpolar molecule with weak van der Waals forces (low polarizability per volume), it is a relatively weak solvent [1], Thus, many interesting separations and chemical reactions involving insoluble substances in CO2 can be expected to take place in heterogeneous systems, for example, microemulsions, emulsions, latexes and suspensions. Microemulsion droplets 2-10 nm in diameter are optically transparent and thermodynamically stable, whereas kinetically stable emulsions and latexes in the range from 200 nm to 10 pm are opaque and thermodynamically unstable. 

Fishman and Erdmann (1971) have recently reviewed the topic of water analysis. Among the infrared methods mentioned were the following the use of internal reflection spectroscopy for the analysis of optically opaque samples the determination of total carbon in waste waters the quantitative determination of total CO2 in sea water the determination of the deuterium content in water the use of spectra of hydrocarbon fractions from the acid part of organic substances in natural waters and the use of infrared for the identification of organic substances in water. 

Optical Properties. When light falls on a nylon fiber, it can be partially transmitted, absorbed, or reflected, depending on the cross-section shape and the nature of any second substance added during polymerization or melt spinning. Additive-free nylon with a standard round cross section has a translucent high sheen luster. The inherent luster of the fiber and the color introduced in dyeing are important optical properties that relate to a fabric s visual quality and its acceptance by the customer. Fibers that need to be opaque or to cover well in... 

Recent studies of micro ATR have defined the best experimental conditions for establishing optical contact between the ATR crystal and the sample [25]. This experimental approach has been applied to the analysis of the failure surfaces of adhesively bonded joints. ATR-microscopic measurements have been used for direct measurement and identification of raw materials in textiles coated and impregnated substances on paper [26]. An ATR microscopic probe has been developed which allows one to examine the sample optically through the probe in the microscope. The hemispheric ATR crystal is mounted at the focus of the Cassegrain objective, below the secondary mirror. One can position the crystal in contact with the sample, and run the spectra [27]. In the survey mode, visible light at nearly normal incidence is selected to locate the area of measurement. In the contact mode, low incident angle visible is used to detect contact of the sample to the ATR crystal surface. In the measurement mode, the ATR crystal is slid into position and the incident beam is optimized for total internal reflection. In the Spectra Tech version, all of the available crystals, i.e. ZnSe, Diamond, Silicon, and Germanium can be used. However, Ge and Si are opaque and cannot be used in the survey or contact mode. An optical contact sensor can be used. 

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