  It has recently been shown that periodic layered media can reflect strongly
for all incident angles and polarizations in a given frequency range. The
standard treatment gets these band gaps from an eigenvalue equation for the
Bloch factor in an infinite periodic structure. We argue that such a procedure
may become meaningless when dealing with structures with not very many periods.
We propose an alternative approach based on a factorization of the multilayer
transfer matrix in terms of three fundamental matrices of simple
interpretation. We show that the trace of the transfer matrix sorts the
periodic structures into three types with properties closely related to one
(and only one) of the three fundamental matrices. We present the reflectance
associated to each one of these types, which can be considered as universal
features of the reflection in these media.
