Publisher Summary This chapter explains a mechanism that carries a sample and detector, both of which can be maneuvered to permit the measurement of the radiant intensity specularly reflected or scattered from the sample. The mechanism is supplied by a radiation source, for example, a monochromator. Generally, measurements of reflectance require the knowledge of the angular position of both the sample and detector with respect to the incident radiation; hence, a reflectometer might also be classified as a goniometer. In an ideal reflectometer, one has a parallel radiation beam of uniform radiance and a detector of uniform sensitivity. Monochromatic vacuum-ultraviolet (VUV)-radiation beams usually diverge from a source, the exit slit of a monochromator. Thus, the angle of incidence is uncertain by an amount equal to the divergence angle. Not only do monochromatic VUV beams diverge, they are seldom uniform in intensity over their cross-sections because the optical element dispersing the radiation is not uniformly efficient across its surface. To compound the problem, detectors are seldom uniform in their response over their sensitive area. Consequently, the angle of incidence spread caused by the divergence can be weighted by both beam and detector nonuniformity. Furthermore, both the divergence and direction of the beam may change with wavelength because some monochromators move the grating with respect to the exit slit when changing wavelength. Generally, this change is small, but it can be large enough to invalidate angle-of-incidence settings.