Suppose you prepared two samples consisting of a luminescent layer on a silicon substrate. The thicknesses differ by a factor of 2. The observed PL spectra would be the following:

What would you conclude from this? Did you prepare two very different luminescent systems with two broad, overlapping emission peaks in one case (the blue curve) and a single emission mechanism in the other (red)?

You should be careful with interpretations unless you checked the influence of the layer thickness on the shape of the PL spectrum! In fact, the two curves are once more porous silicon spectra computed with the same internal efficiency (see above). The layer thickness is the only difference between the two cases.

Here are some more spectra of this kind, including the two presented above:

Interpretation problems arise more likely in the case of thin films where the external efficiency variations lead to spectral shifts and modifications which are not easily identified. In thicker layers the rich structure of the interference patterns clearly (more or less) show their origin.