Optimization of light emitting devices when light is trapped within the dielectric structures opens the question of the management of light propagation and of control of the directionality of extracted light. These phenomena are closely connected to the so-called extraction length of light propagating within the device, a parameter that is poorly discussed when considering the design and architecture of devices. In this work, a straightforward method for the measurement of the extraction length in the case of luminescent sol–gel films patterned with a 2D photonic bandgap structure is presented. Quantitative information usually obtained by spectro-angular measurements can be retrieved from simpler standard spectrometry, allowing discussion of the influence of pattern depth, which determines the extraction by the photonic structure as compared to other uncontrolled pathways such as diffusion from defects in the film coating. These assessments provide a quick and safe method for the quantitative evaluation of light extraction strategies toward a rational design of light emitting devices with optimized properties.