  We show that under proper conditions thin films structures behave like
thermal sources that are able to emit radiation in well defined directions over
a broad spectral band for both polarization states of light. This effect
results from the quantization of modes inside the structure as in a Fabry-Perot
resonator. A theoretical demonstration of this effect is given using the matrix
transfer method. This result is similar to the best efficient reported results
with gratings but using a completely different physical principle [J. J.
Greffet et al., Nature 416, 61 (2002)].
