Liquid immersion refractive index measurement

To evaluate its capability for refractive index measurement, the fiber FPI device was tested using various liquids including methanol, acetone, and 2-propanol at room temperature. The interference spectra of the device immersed in various liquids are shown in Fig. 7.12 for comparison. The signal intensity dropped when the device was immersed in liquids as a result of the reduced refractive index contrast and thus lowered Fresnel reflections from the cavity endfaces. However, the interference fringes maintained a similar visibility. The spectral distance between the two adjacent valleys also decreased, indicating the increase of refractive index of the medium inside the cavity. Using (7.4), the refractive indices of the liquids were calculated to be nmethanoi = 1 -3283, acetone = 1 -3577, and n2-propanoi = 1.3739, which was close to the commonly accepted values. 

From a knowledge of the adsorption, immersion, and wetting properties of solid particles, we have examined the influence of particle-particle and particle-liquid interactions on the stability and structure formation of suspensions of hydrophobic and hydrophilic Aerosil particles in benzene-n-heptane and methanol-benzene mixtures. For the binary mixtures, the Hamaker constants have been determined by optical dispersion measurements over the entire composition range by calculation of the characteristic frequency (Vk) from refractive index measurements [7,29,36,64], The Hamaker constant of an adsorption layer whose composition is different from that of the bulk has been calculated for several mixture compositions on the basis of the above results. Having the excess isotherms available enabled us to determine the adsorption layer thickness as a function of the mixture composition. For interparticle attractive potentials, calculations were done on the basis of the Vincent model [3-5,39]. In the case of hydrophobic particles dispersed in benzene- -heptane and methanol-benzene mixtures, it was established that the change in the attractive potential was in accordance with the interactions obtained from rheological measurements. 

Where R is the reflectivity and d is the thickness. Very accurate values of R and T are needed when the absorptance, (id, is small. The technique of photothermal deflection spectroscopy (PDS) overcomes this problem by measuring the heat absorbed in the film, which is proportional to ad when ad 1. A laser beam passing just above the surface is deflected by the thermal change in refractive index of a liquid in which the sample is immersed. Another sensitive measurement of ad is from the speetral dependence of the photoconductivity. The constant photocurrent method (CPM) uses a background illumination to ensure that the recombination lifetime does not depend on the photon energy and intensity of the illumination. Both techniques are capable of measuring ad down to values of about 10 and provide a very sensitive measure of the absorption coefficient of thin films. 

If a crystal belongs to the cubic system, the velocity of light through it (and therefore its refractive index) is isotropic (the same in all directions). The refractive index of such an isotropic crystal is measured by observing it when it is immersed in a colorless liquid of matching refractive index (obtained by mixing appropriate liquids of known refractive indices in which the crystal is insoluble). When the refractive index of the surrounding mixture of liquids exactly matches that of the crystal, the latter becomes invisible. The refractive index of the liquid mixture can be measured and is equal to the refractive index of the crystal. 

The refractive index of solid particles can be measured on the microscope by immersing the particle in liquids of known refractive index until a match is found. A match is found when the particle virtually disappears in the liquid. 

One of the major problems with these kind of products is the effect of particle size. In order to minimise grind size effects in ground coffees of different roasts. Little and MacKiimey (1956) standardised on a very fine grind with maximum hghtness value. Seakins (1971) tackled the problem by another means and developed a method to measure the colour of casein by immersing it in a liquid of similar refractive index. Reflectance measurement of dry sample preparations resulted in poor separation of colour grades because reflectance increased as particle size decreased. However, when transmittance values were measured on the liquid suspension, the colour grades were clearly differentiated. 

Using a Berek compensator a value of 0 65 was found. This was felt to be too high and an attempt was made to determine the individual refractive indices using a Becke line assessment with immersion liquids of different refractive indices. The refractive index perpendicular to the fibre axis, was found to be in the range 1-63-1-64. The refractive index /."n parallel to the fibre axis, was very much more difficult to measure but... 

A distinctive feature of adsorption from solution is that the surface is always completely covered and the sorption mechanism involves competition between the components of the liquid phase. Detailed theoretical interpretation of isotherms from liquid mixtures is most often based on binary liquid systems. The isotherms are measured by immersing the solid sample into the liquid phase. The concentration is monitored by UV, refractive index (RI), or other analytical methods. 

Although measurements of refractive index can be inferred by applying the Fresnel relationship to the measured reflectance values, this is an imprecise method. Instead, methods such as immersion in liquids and measurement of the Brewster angles are generally used. 

Medullation, when provided, is also based on an indirect method, in this case via the opacity resulting when the wool is immersed in a liquid of the same refractive index [9]. For both fiber diameter and medullation the primary reference method is projection microscopy. Such measurements are very slow, tedious and incompatible with the classic NIR requirements for large numbers of high-quality measmements. The use of the indirect methods effectively makes NIR cahbration possible. It should be noted, however, that fiber diameter is best measured in other ways if the measurement is important and that the demand for medullation measurements is low. Neither of these morphological characteristics is affected by the scouring process. 

Measurements of 2 have been carried out with the monofilament surrounded by an immersion liquid [33], and the diameter is measured directly with a calibrated eyepiece. The immersion liquid was chosen to have refractive index approximately equal to that of the monofilament, hence reducing diffraction effects without making the monofilament invisible. Very careful focusing of the microscope was necessary in these experiments. Inaccuracy in focusing can cause errors in the diameter measurements of the order of U2 itself. 

However, some polymeric samples contain crystallites or voids comparable in size to the visible wavelengths of the laser. These crystallites or voids scramble the incident laser polarization and thereby prevent any useful measurement of depolarization ratios. Some error in Raman polarization measurements arises because the incident light and Raman scattered light are multiply reflected at the surface of the sample and are also refracted upon entering or leaving the sample. The light-polarization directions are therefore poorly defined. Immersing the sample in a liquid that has a refractive index close to that of the polymer helps to minimize this problem [4],... 

The contribution of surface haze to the total haze of a specimen can be removed if its surfaces are coated with a liquid (such as immersion oil) with a refractive index approximately matching that of polyethylene ( == 1.5). Haze determined on coated specimens is indicative of the internal scattering. The surface haze value can be obtained by subtracting the internal haze from the total haze measurement. 

In the literature, particular methods of measurement of these values are described in detail [24, 27], Among them, let us describe a modified variant of the method using a fme slice [30 - 32], According to this method, a plane-parallel glass plate with known value of kd with accuracy up to 10" (standard) is placed simultaneously with the sample studied in a cuvette filled with an immersion liquid. The standard and the sample are turned to each other by finely polished edges. The path of interference bands on the interferometer output is recorded, and the difference between the refractive index of a selected point of the sample and the standard is determined by displacement of these bands on the standard-immersion liquid and the sample-standard borders. Then the value of the refractive index at any point of the sample is determined [30]. 

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