Your question itself has the answer. We typically think of a window as some medium through which light may pass to a varying degree, i.e. a stained glass window. If you are talking about a 'one way mirror' like one you might find in some Hollywood interrogation scene, then by definition some light should pass through; how else would you 'see through' the non-mirror side? The point of such a 'one-way-mirror' isn't to reflect all light- rather, it is to reflect most light while still allowing outside (or inside, in your case) viewers to see the other side.
Therefore, by definition light should be passed through from the lighter room (and vice versa.)