Becke line method

The refractive index, n, may be measured using an optical microscope [1,2,23,27,34]. Phase contrast increases the contrast due to differences in n and allows a more accurate determination. Interference contrast in transmission gives the optical path length and the average refractive index through the specimen thickness [1], The Becke line method gives the surface refractive index [1],... 

Birefringence measurements were obtained by the Becke line method, and density measurements were obtained by the sink-float method (26). 

CHARACTERIZATION. Melting points were determined on an E. I. DuPont Series 99 Thermal Analyzer at 20°C/min. Inherent viscosities of polyamic acid solutions were obtained at a concentration of 0.5% (w/w) in DMAc at 35°C. Glass transition temperatures (T ) of the fully cured polymer films were measured by thermomechanical analysis (TMA) on a DuPont 943 Analyzer in air at 5°C/min. Films fully-cured at 300°C were tested for solubility at 3-5% (w/w) solids concentration in DMAc,N,N-dimethylformamide (DMF), and chloroform (CHCl-j). Solubilities at room temperature were noted after periods of 3 hours, 1 day and 5 days. Refractive indices of 1 mil thick films were obtained at ambient temperature by the Becke line method (11) using a polarizing microscope and standard immersion liquids obtained from R. P. Cargille Labs. 

ISO R489 suggests two methods based on Abbe refractometer and the Becke line methods. It does not recommend any specimen conditioning procedure prior to the test. DIN 53491 provides practical details relevant to refractometer measurements. It recommends test temperature... 

Figure 14.11 The Becke line method of determining a material s relative refractive index. When the focus is moved upward, the Becke line moves into the medium with the higher RI.

Becke line method Compensator (wedge) Crossed polars... 

Becke line method A method of determining relative refractive index using mounting media of known refractive index and adjustment of focus. 

Measurement of Refractive Indexes. Some of the techniques used to measure refractive indexes are the Becke line method (94), interference microscopy (95,96), Abbe refractometer (40,94,97-99), and ellipsometry (100). The Abbe re-fractometer (Fig. 5) can be used to determine the refractive indexes of materials belonging to all three optical symmetries. Its principle is based on the total reflection phenomenon occurring between a known and unknown isotropic media ... 

One of the more useful methods is the Becke test. Becke noticed that when the microscope is focused up and down a bright halo near the boundary of a particle moves in and out. The halo will always move toward the higher refractive index as the focus is raised and toward the lower refractive index as the focus is lowered. Thus if the particle has a refractive index higher than the liquid, the Becke line will move from outside the boundary to the inside as the focus is raised and vice versa. Figures 4 and 5 show the bright Becke line outside and inside the boundary, respectively. 

Brewster s law. Reflected light in general is more polarised than the incident light and according to Brewster s law, the polarisation is maximum when i + r = 90 where i and r are the angles of incidence and refraction. There are several methods described in the literature such as those based on the use of prisms and fluid immersion (in fluids of known refractive indices) and index matching. The so-called Becke line technique for index matching uses observations under microscope. 

General methods of measuring refractive index, not confined to plastics, can be found in older textbooks on optics [3]. Well-known methods arc those depending on matching the unknown solid with known liquids, using the Becke line technique [4], dispersion staining [5], techniques based on the Abbe refractometer [3], and the measurement of the ratio of real depth to apparent depth [3]. 

The two methods recommended in ISO 489 [6] are the Abbe refractometer technique and the Becke line technique. 

The two measurement methods recommended by IS0489 are the Abbe refractometer technique and the Becke line technique. The latter is more useful with powdered or granulated transparent material or even with small chips of material taken from a larger sample. ASTM D542 includes only the Abbe method and it is widely used for characterization of refractive index of polymers. 

Becke test n. The method for determining refractive index of a transparent particle by noting the direction in which the Becke line moves. The halo (Becke lines) will always move to the higher refractive index medium as the focus is raised. The halo crosses the boundary into the lower refractive index medium when the microscope is focused down. The particle must be illuminated with a narrow cone of axial light obtained by closing the aperture diaphragm of the condenser to a small aperture. Nesse WD (2003) Introduction to optical mineralogy. Oxford University Press, New York. 

If we adopt the assumption tliat the expectation value curve in Figure 8.2 is a line, simple geometry allows us to determine that z = 0.5. Use of this value defines a so-called half-and-half (H H) metliod. Using LDA exchange-correlation, Becke showed that the H H approach had an error of 6.5 kcal mol" over a subset of the G3 enthalpy of formation test set mentioned in Chapter 7. This compared quite favorably with the GGA method BPW91, which had an error of 5.7 kcal mol" over the same set. 

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