Electromagnetic radiation (and particle beam radiation) are used in 4 main categories of characterization techniques:
Imaging Techniques
Imaging techniques are the most familiar to the average person and present data that are the easiest to interpret. A typical imaging technique results in a 2-dimensional photograph of the material with some limited depth of field. It is generally not possible to make firm conclusions concerning 3-d structure using microscopy alone and it is generally coupled with scattering techniques for a full analysis of structure. Microscopy techniques are not quantitative, that is it is generally difficult to obtain numerical results on phase composition using microscopy. Scattering, absorption and fluoresence techniques are generally the source of quantitative information.
Fluoresence Techniques
From the perspective of microscopy and scattering fluoresence is generally either an additional feature that can be detected, i.e. x-ray fluoresence in TEM/SEM for phase analysis or a problem that can corrupt data, Fe fluoresence in XRD using copper radiation. Generally fluoresence relies on certain chemical groups or elements being present in a material, so has a more limited scope than the other analytic techniques listed, although in certain cases it can be an extremely important characterization method.
Absorption Techniques
Transmission microscopy could be considered an absoption technique in the most general sense, however, generally the term refers to spectroscopic techniques where a scan of the wavelength, energy or momentum of a radiation is made for a fixed thickness sample in transmission. Characteristic absorption peaks are correlated with specific chemical or elemental groups. Absorption spectroscopy is one of the simpilest and easiest techniques so is widely used in all fields of materials, especially where organic materials are encountered.
Scattering
Scattering encompases any technique where a beam of radiation impinges on a sample and the sample re-radiates the same wavelength radiation with interference between radiation emitted by different parts of the structure resulting in a spatially descriminated intensity profile. All of the listed techniques have a similar mathematical and theoretical basis.
Scattering yields 3-dimensional information on a material, however, scattering data can not directly yield the structure of a material. Inorder to interpret scattering data a model must be used. The model, for XRD, is the crystal unit cell structure that has been previously studied.
Why Study Diffraction?
Since a model is required to understand diffraction, most structral models for materials have been developed in the framework of the diffraction measurement. Concepts such as Miller-indicies, symmetry operations, and the definitions of unit cells all have a basis directly lined to the diffraction measurement. From a physics perspective, diffraction offers a direct link between theory and measurement through structural correlation functions (not dealt with in this course). Additionally, use of diffraction is wide spread across all materials disciplines and a fundamental understanding of diffraction has come to be a basic expectation of materials engineers and scientists.