An omni-directional dielectric mirror (also known as a one-dimensional photonic bandgap crystal) exhibits 100% reflectivity at all angles of incidence and for all states of incident polarization. Unlike metallic mirrors, which absorb a small fraction of the incident optical power, dielectric reflectors are lossless. These properties make omni-directional dielectric mirrors ideal candidates for applications in which a beam of light in an unknown or unpredictable polarization state is likely to arrive at the mirror from any direction, and in which loss of light at the mirror, no matter how small, is deemed intolerable. A good example is provided by the walls of an optical waveguide. Since there are numerous reflections from the wall as a beam of light travels through, even small losses at each encounter with a wall rapidly deplete the beam's energy.

A typical omni-directional reflector is a periodic stack of bilayers, each bilayer consisting of a high-index and a low-index dielectric layer. The larger the refractive indices of the available dielectrics (and also the larger the difference between these indices), the easier it is to design the reflector. For example, if the two materials available for fabricating a stack of bilayers have indices n1 = 1.5 and n2 = 2.0, it is impossible to obtain omni-directionality for both p- and s-polarized light. However, with n1 = 1.5 and n2 ≥ 2.3, an omni-directional reflector can be designed.