Refractive index reflection type

Chemical modification of the wax can improve smear resistance (5). Sihcones, which do not harm furniture finishes (6), are incorporated as film-forming ingredients in furniture pohshes. The lubricant properties of sihcones improve ease of apphcation of the pohsh and removal of insoluble soil particles. In addition, sihcones make dry films easier to buff and more water-repeUent, and provide depth of gloss, ie, abihty to reflect a coherent image as a result of a high refractive index (7). Wax-free pohshes, which have sihcones as the only film former, can be formulated to dehver smear resistance (8). Another type of film former commonly used in oil-base furniture pohshes is a mineral or vegetable oil, eg, linseed oil. 

Deviation refractometers are the most commonly used. This version of the DRI measures the deflection in the location of a light beam on the surface of a photodiode by the difference in refractive index between the polymer solution and pure solvent. The Fresnel-type refractometers operate on the principle that the intensity of light reflected from a glass-liquid interface is dependent on the incident angle and the RI difference between the two phases. The deviation and Fresnel detectors typically have cell volumes of 5 to 10 pi, detection limits of about 5 x 10-6 refractive index units (RIU), and a range of 10 7 to 10 3 RIU.156 The deflection-type DRI is relatively insensitive to the buildup of contaminants on the sample cell and is therefore of special utility in laboratories that process large numbers of samples, such as industrial laboratories. 

Automatic instruments are usually reflection-type instruments that measure the deflection of a beam of light as it passes from one medium, into the sample and then is reflected back to a detector. The angle at which the light beam exits the medium is related to the refractive index of the sample. Automated instruments are calibrated with standard substances of precisely known refractive index prior to use. 

Many techniques are based on this principle and can be used for the analysis of all types of samples. The spectrum obtained from reflected light is not identical to that obtained by transmittance. The spectral composition of the reflected beam depends on the variation of the refractive index of the compound with wavelength. This can lead to specular reflection, diffuse reflection or attenuated total reflection. Each device is designed to favour only one of the above. The recorded spectrum must be corrected using computer software. 

The Waters Assoc. R401 differential refractometer is based on an optical-deflection design rather than on light reflection. This permits the use of a single cell throughout the refractive-index range of 1.00-1.75. The system also has a wider dynamic range of linearity for quantitation than the Fresnel-type refractometers. 

Optical microscopy is another method that has been used to determine the distribution of minerals in coal. This method is based on the detailed microscopic examination of polished or thin sections of coal in transmitted and/or reflected light. In principle, observing several of its optical properties, such as morphology, reflectance, refractive index, and anisotropy, makes identification of a mineral type possible. 

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