Birefringence windows

The just described compensation method for the parasitic birefringence of windows and polarizing prisms is also useful with respect to the parasitic birefringence created by reflections from the cylinder walls. As has been pointed out, the disturbance caused by the windows is compensated at its maximum. This means, as the test fluid possesses an extinction angle of 45°, that the extinction positions of the window birefringence are oriented parallel and perpendicular to the direction of flow (i.e. the... 

Here we have to optimize Au , to achieve the maximum S/N ratio. The differentiation of (2.63) with respect to Auw gives for Aa L the optimum window birefringence... 

The signal contains a constant background term + 0 + Ao /4 that is independent of the frequency co. The quantity = h/h (0 = 0, Ao w = AZ r = 0, Aa = Ano = 0) gives the residual transmission of the completely crossed analyzer P2. With normal Glan-Thomson polarizers < 10 can be realized, with selected samples even < 10" can be obtained. The third term is due to the absorption part of the window birefringence. All these three terms are approximately independent of the frequency co. 

In the absence of window birefringence (that is, AZ r = AZ / = 0, O — 0) the signal-to-noise (S/N) ratio, which is, besides a constant factor a, equal to the signal-to-background ratio, becomes with (7.50) and (7.51a) for the dispersion signals... 

Circular dicliroism has been a useful servant to tire biophysical chemist since it allows tire non-invasive detennination of secondary stmcture (a-helices and P-sheets) in dissolved biopolymers. Due to tire dissymmetry of tliese stmctures (containing chiral centres) tliey are biaxial and show circular birefringence. Circular dicliroism is tlie Kramers-Kronig transfonnation of tlie resulting optical rotatory dispersion. The spectral window useful for distinguishing between a-helices and so on lies in tlie region 200-250 nm and hence is masked by certain salts. The metliod as usually applied is only semi-quantitative, since tlie measured optical rotations also depend on tlie exact amino acid sequence. 

Fig. 27. Abrupt contraction cell for flow visualization, birefringence and degradation measurements A inlet (from a peristaltic pump of a pressurized reservoir B outlet (atmospheric pressure or partial vacuum) C interchangeable metallic nozzle with a sapphire tip D capillary flow meter E glass window for flow visualization AP pressure drop (from pressure transducers)...

As, without the use of one of the photo-electric methods, it becomes completely impossible to measure extinction positions of such small parasitic birefringences, the present author relied on an indirect method by the use of revolving windows (217). As pointed out in Section 6.1.5, the windows of the rotor unit were put in a most suitable fixed position. This position is found in the following way ... 

The windows are always rotated together, so that they form one effective birefringent medium. This is permitted as the effects are so small. 

Fig. 6.7. Slit-apparatus for the measurement of flow birefringence in the 1—3 plane. (A) outer body, (V) conical member enclosing the slit with a rectangular cross-section (this part is split into pieces), (E) entrance, (Wt, IV2) windows, (Hlt Ht, Ha)...

Also, the heterogeneity of the ruby power density cross section in the sample interaction volume caused a large shot to shot variation in the non-linear signal intensity. In the RIKES experiments, all optical elements were placed outside the crossed polarizers only the sample cell windows remained. This arrangement prevented strain birefringence from interfering with the RIKES spectrum. 

Birefringence setups can be designed to characterize molten materials undergoing isothermal homogeneous flow. The ranges of strains and strain rates also often coincide with those of rheometers, and consequently may be limited relative to those used in fabrication. Similarly, time-temperature superposition approaches may be used to expand the rate window. State-of-the-art setups suitable for rapid screening of new materials with research-scale quantities (5-20 g) are available for shear flow [72] and startup of uniaxial extensional flow [73,74]. 

VCD is even more sensitive than ECD to any polarization distortion that can happen in a spectrometer during the light beam throughput. Birefringence in the optical material of optical lenses, filters, cells and other windows, and reflections may cause artifacts that may be comparable in size to the sample signals. As a consequence, the baseline differs from the zero line and a baseline correction should be applied for each VCD measurement. In the case of well optically tuned instruments, satisfactory baseline can be obtained as the VCD spectrum of the pure solvent measured under the same condition in the same sample cell [44, 45, 68]. 

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