Opal-based photonic crystals are promising materials to engineer complex heterostructures for efficient manipulation of nano-emitters fluorescence. We fabricated and characterized a structure composed of a layer of silica embedded between two silica opals. Thanks to this controlled planar defect which opens a permitted frequency band in the photonic stopband, an increase of emission, depending on angular distribution, is evidenced through photoluminescence spectroscopy. We discuss the use of such a structure as a self-assembled micrometer-sized spectroscopic device and demonstrate that it can be used to point the maximum emission wavelength of an unknown light source up to a certain linewidth. It can as well separate two sources, emitting at different wavelengths, with a resolution given by the Rayleigh criterion.