Optical methods are finding increasing acceptance in the field of experimental solid mechanics due to the easy availability of novel, compact, and sensitive light sources; detectors; and optical components. This, coupled with the faster data and image acquisition, processing, and display has resulted in optical methods providing information directly relevant to engineers in the form they require. The various methods discussed in the earlier chapters provide a background for the methods described in this chapter. Optical methods principally rely on theories of geometric optics or wave optics to gather information on the deformation, strain, and stress distribution in specimen subject to external loads. Methods based on geometric optics such as moiré and speckle correlation usually have simpler optical systems making them more robust at the expense of resolution. Wave optics based systems are generally very sensitive and hence have a more complex optical system.