Until the late 1960s, nearly all spectrometers were the so-called wavelengthdispersive spectrometers in which wavelengths are separated by Bragg diffraction from a single crystal. In the early 1970s energy-dispersive spectrometers became available in which a lithium-drifted silicon or germanium detector is used to give a distribution to voltage pulse amplitudes proportional to the distribution of photon energies. Electronic separation of the pulse height distribution then gives a photon energy spectrum. Each of these x-ray photon separation techniques has its own advantages and disadvantages, and a detailed description of each method will be found in the succeeding chapters of this book.