The focal point of the parabola is removed, allowing light close to the optical axis to pass through untouched. Photons off axis, but still below the critical angle, are reflected inward, creating a parallel beam. During WDS, x-ray photons transmitted or reflected by an object are directed at a rotating diffracting crystal. As the crystal is rotated, it diffracts x rays of a particular energy, providing spectroscopic information on the sample under study. According to Gibson, when the beam is collimated, the system can use a flat crystal, which reduces the computational burden.
Similar arguments apply to x-ray fluorescence in material science.
This technique is often used in silicon wafer inspection for contaminates. By focusing a lower energy x-ray beam on a wafer, the system can avoid exciting the silicon layer while revealing the contaminants underneath. As a result, the mirrors maintained the same high levels of reflectivity while showing better spectral purity. By changing the layers to Ni-based materials, the mirrors showed an intensity gain of 16 compared with gains of 6 of previous work. Conventional multilayer mirrors are created by bending the underlying silicon substrates into the optical shape after deposition, or by pressing the deposited silicon wafer into a negative shape and then glueing the mirror onto a stiff backing.