Matter that does not absorb all incident radiation emits less total energy than a black body. Emissions are reduced by a factor ε ε , where the emissivity, ε ε , is a material property which, for most matter, satisfies 0 ≤ ≤ ε ε ≤ ≤ 1 . Emissivity can in general depend on wavelength, direction, and polarization. However, the emissivity which appears in the non-directional form of the Stefan–Boltzmann law is the hemispherical total emissivity, which reflects emissions as totaled over all wavelengths, directions, and polarizations.