Scattering absorption ratios

Figure 11. Schematics of light distributions in optically-homogeneous, large-volume tissues (a,b) surface irradiation, (c,d) interstitial irradiation, (a,c) high absorption/scatter ratio, (b,d) high scatter/absorption ratio.

The smaller absorptivities and larger scattering coefficients at the shorter NIR wavelengths lead to larger scattering/absorption ratios and allow for the development of effective diffuse-reflection analytical techniques. 

The alternating component of the applied voltage causes a continuous variation in the efficiency of the x-ray excitation (Step II) during each cycle of operation. A polychromatic x-ray beam has a different total absorption coefficient and a different ratio of photoelectric to scattering absorption coefficient for each wavelength (Step V). It is very difficult to take account of these factors exactly. Fortunately, reasonable estimates will suffice in the making of the calculations for Table 4-4 and Figure 4-15. The efficiency has been taken as that for the root-mean-... 

Carbon-13 NMR was used as the reference method to enable comparison of FT-Raman and FTIR spectroscopies as isotacticity determination methods. Calibration curves relating Raman scattering and IR absorption ratios to the carbon-13 NMR results are presented. The repeatability for both FT-Raman and FTIR measurements was comparable to that of carbon-13 NMR analysis and there was good correlation between between carbon-13 NMR and FT-Raman/FTIR results. The reliabilities of the three methods are statistically evaluated and compared. 9 refs. 

The scattering intensity / (s) is measured in absolute units as defined in Equation 9.12. In addition to scattering, absorption takes place in the sample, following the Lambert-Beer law [3] as it does for visible light. For SAXS, the transmitted and the scattered beams are attenuated by the same amount. The transmission measured for the primary beam can therefore be used to correct the scattering signal /exp( y) for absorption effects. The absorption factorzl is the ratio of the primary beam intensity with and without the sample. 

The sensitivity of the luminescence IP s in the systems employed here decreases with increasing x-ray energy more strongly than in the case of x-ray film. Therefore, this phenomenon must be compensated by using thicker lead front and back screens. The specific contrast c,p [1,3] is an appropriate parameter for a comparison between IP s and film, since it may be measured independently of the spatial resolution. Since the absorption coefficient p remains roughly constant for constant tube voltage and the same material, it suffices to measure and compare the scatter ratio k. Fig. 2 shows k as a function of the front and back screen thickness for the IP s for 400 keV and different wall thicknesses. The corresponding measured scatter ratios for x-ray films with 0,1 mm front and back screens of lead are likewise shown. The equivalent value for the front and back screen thicknesses is found from the intersection of the curves for the IP s and the film value. 

The most ubiquitous method of transmission spectroscopy, in which the amount of light passing through a sample is determined. Very often the influence of reflection and scattering is neglected and the ratio of incident and transmitted intensity ( / ) is linked to the absorption coefficient (a) and the sample thickness (d) by Lambert-Beer s law (see Eq. (9.11)). 

Interpretation of the chart utilizes the observation of Andermann and Kemp24 that scattered background and analytical-line intensity decrease in constant ratio as the absorption coefficient of the sample increases. 

Before going further, it may be noted that the flipping ratio does not depend either on the Lorentz factor or on absorption in the sample. Certain instrumental parameters such as the polarisation of the neutron beam for the two spin states, the half wavelength contamination of the neutron beam and the dead-time detector can readily be taken into account when analysing the data. On the other hand, the extinction which may occur in the scattering process is not so easy to assess, but must also be included [14]. Sometimes, it is even possible to determine the magnetisation density of twinned crystals [15]. 

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