Definition A mirror is an optical device used for beam-forming or imaging based on the directional reflection of electromagnetic radiation. Background Computer vision applications apply mirrors in a twofold manner: for optical imaging and for illumination purposes. Furthermore, mirrors themselves could be test objects in visual inspection systems. This leads, with regard to the 3D shape of the mirror, to the shape-from-specular-reflection problem and, in the context of visual inspection systems, to deflectometry. Theory and Application Mirrors consist of a smooth substrate with a metal coating (e.g., Au, Ag, Al) and/or dielectric layers. In the case of a surface mirror, the reflection takes place at a metal coating on the front side that has to be protected against scratches. The main advantage of a surface mirror is the lack of beam displacement due to the glass substrate. Alternatively, the backside of a glass substrate can be coated.