The Fresnel Method

This method utilizes the relationship between reflectance from an interface between two transparent media and their refractive indices as given by the Fresnel equation  

if the medium 2 represents the liquid eluted from the column, then any change in n i will result in a change in R and thus, a measurement of R could determine changes in the value of 1 1 due to solute being present in the eluent. 

The optical flat was then tube which acted as a 

The disadvantages of this system are that it does not contain a reference cell to compensate for temperature changes or changes in solvent composition, and the relatively large cell volume would not permit the high efficiencies obtained from present day columns to be realized. 

The sample and reference signals from the detector diodes are converted to one voltage output which represents the difference between the two signals. This voltage is then fed to a 

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The Fresnel method of refractive index measurement has also been used in the design of commercially available detectors. The different... 

A Early Deteetor Based on the Fresnel Method of Refraetive Index Measurement... 

However, it was the one of the first refractive index detector to work on the Fresnel method. 

A Diagram of the Optical System of a Refractive Index Detector Operating on the Fresnel Method Courtesy of the Perkin Elmer Corporation... 

TEM combined with electron diffraction has been a key technique for proper characterization of nanoporous materials [48-55,58,60,61,80,92-94] and elucidation of their structures [87,95]. An interesting application of TEM was reported by Chen et al. [80] who used the Fresnel method to determine the thickness of the FSM-16 walls. According to this method the imaged wall thickness decreases with decreasing focus and reaches a minimum which is the real thickness at zero defocus. 

Fig. 1 A diagram of the optical system of a refractive index detector operating on the Fresnel method.

Since the original model of Tiselius there have been many papers published that describe different methods of refractive index monitoring. From these publications, four general methods emerge for measuring refractive index the angle of deviation method, the critical angle method, the Fresnel method and the Christiansen method. The theory behind each of these methods will now be discussed. 

The magnitudes of e i =1, )contam the Fresnel factors from equation Bl.5,34. equation B1,5,35 and equation B 1,5.36. which depend on the incident, reflected and polarization angles. Experimentally, one approach is to fix the input polarization and adjust the analyser to obtain a null in the SFl signal [ ]. By choosing distinct configurations such that the corresponding tliree equations from equation B 1.5.40 are linearly independent, the relative values of Xs lim = inferred. This method has... 

Optics. Good optical properties and low thermal resistance make poly(methyl methacrylate) polymers well suited for use as plastic optical fibers. The manufacturing methods and optical properties of the fibers have been reviewed (124) (see Fiber optics). Methods for the preparation of Fresnel lenses and a Fresnel lens film have been reported (125,126). Compositions and methods for the industrial production of cast plastic eyeglass lenses are available (127). 

When adsorption from solution Is monitored by the depletion method. It Is very difficult to measure changes in bulk concentration over time Intervals down to milliseconds. Perhaps this Is the reason that such systematic studies are not abundant in the literature. Fast measurements require stopped-flow, pressure-jump or temperature-jump techniques. The method used to determine concentrations must also be fast suitable methods include certain spectroscopies and, for charged substances, conductivity. When adsorption on Fresnel surfaces Is studied, say by reflectometry, concentration measurements in the solution are not needed. 

The Fresnel fringe provides a satisfactory criterion for focusing if a suitable edge is available on the specimen. An alternative method of finding exact focus is mentioned in Section 6.3.7. 

Another useful method, especially when only a single wavelength is available, is the different dependence of Y and surface electric field on the polarisation and angle of incidence.72 From the Fresnel equations and the known optical constants of metals the electric field experienced by the adsorbate and the absorbance of the substrate can be calculated. 72.73 por substrate excitation Y should follow (1-R). For adsorbate excitation some knowledge (or model) of the symmetry of the adsorbate layer (orientation of transition dipole) is required to relate the electric field to the excitation probability. The angle of incidence dependence of Y for different input polarisations have been calculated for some typical cases.72.74 Cavanagh s group have... 

A second method utilizes the Fresnel principle (Figure 19-44), which relates the transmittance of a dielectric interface to the refractive indices of the interface materials. Such an interface may be formed between a glass prism of selected optical properties and the liquid whose refractive index is to be measured. These detectors are difficult to use with gradient elution systems, and temperature control of the solvents is critical. 

The interaction between a light wave and a surface plasmon in the ATR method can be investigated using the Fresnel multilayer reflection theory [18]. Herein, we shall present analysis of the reflectivity for the Kretschmann geometry of the ATR method. 

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