Geometry symmetrical transmission

Equation (7.4) is also valid in symmetrical transmission geometry (Alexander [7] p. 71-72), which is a classical geometry for goniometers equipped with zero-dimensional detectors... 

Figure 2.24 Geometry in the calculation of the absorption factor in the (a) symmetrical reflection and (b) symmetrical transmission mode with a sheet sample.

Figure 2.26 The thick broken curve gives the single scattering intensity obtained with amorphous bulk polystyrene. The thin solid curves show the calculated double scattering intensities for samples of the indicated thickness placed in a symmetric transmission geometry. The single scattering intensity is referred to the scale on the left axis the double scattering intensities are referred to the expanded scale on the right axis.

Figure 3.19 Geometry of sample orientation in the diffractometer in (a) symmetric transmission technique and (b) symmetric reflection technique.

The twist constant K2 does not appear in this simple expression, valid for reasonably small tilt, and to first order the deformation occurs at constant helical pitch k = const). In a practical device like a display, however, the tilt has to be at least 22.5° to provide 100% modulation in geometry 1 of Fig. 7.4, or about 30-35° in geometry 2 to give an acceptable transmission in the preferred, symmetric drive. 

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