Several variations are given of a relatively simple thin film optical cavity construction which can be designed either to induce absorption in a weak absorber or to induce thermal emission from a weak emitter. In both cases the cavity is very selective with respect to polarization and wavelength.
N. J. Harrick and A. F. Turner, "A Thin Film Optical Cavity to Induce Absorption or Thermal Emission," Appl. Opt. 9, 2111-2114 (1970)