We considered luminescent films whose surface was imprinted with a two-dimensional (2-D) square shaped photonic crystal with different pattern depths (from 20 to 61 nm). The aim of this work is to develop a straightforward method to characterize the PhC efficiency on light extraction. Transmission spectra of the patterned areas as obtained using a routine spectrometer exhibit peaks evidencing the coupling by the photonic crystal structure of free-space light with guided modes within the film, in good agreement with 2-D rigorous-coupled wave analysis (RCWA) simulations. As expected, the deeper the pattern depth, the stronger the coupling between the guided light and the photonic crystal. RCWA simulations allow evaluating quantitatively the extraction length, characteristic of the efficiency of light extraction, from the transmission spectra, in good agreement with direct measurements.