A retroreflector (sometimes called a retroflector or cataphote) is a device or surface that reflects light back to its source with a minimum of scattering. In a retroreflector an electromagnetic wave front is reflected back along a vector that is parallel to but opposite in direction from the wave's source. The angle of incidence at which the device or surface reflects light in this way is greater than zero, unlike a planar mirror, which does this only if the mirror is exactly perpendicular to the wave front, having a zero angle of incidence.