Optical diffraction methods structure amplitude

Graphical methods and machines for evaluating structure amplitudes 286 Optical diffraction methods ... 

In electron microscopy as in any field of optics the overall contrast is due to differential absorption of photons or particles (amplitude contrast) or diffraction phenomena (phase contrast). The method provides identification of phases and structural information on crystals, direct images of surfaces and elemental composition and distribution (see Section H below). Routine applications, however, may be hampered by complexities of image interpretation and by constraints on the type and preparation of specimens and on the environment within the microscope. 

Another method for making the sample visible by using its effects on the phase of the transmitted wave comes from the realisation that it is only in the plane immediately behind the sample, or in the image of this plane, that the amplitude is constant. In any plane just outside the sample, interference between the undiffracted and diffracted waves gives rise to an intensity distribution that is not uniform and depends on the structme of the sample. It is possible to show that, if a particular plane just outside the sample is imaged, rather than the sample itself, a visible image showing the structures within the sample is formed (see fig. 2.20(a)). This method is related to, but is not identical to, the method of phase-contrast microscopy used with optical microscopes and is sometimes known by that name. 

---