We use the modifications of spatial mode distributions in planar microcavities to address the issue of light outcoupling in devices such as LEDs. Whereas detailed calculations are needed to fully model and optimize the structures, we show that a simple picture can be obtained that clarifies the main physical effects of the microcavity. In particular, it is shown that the cavity order mc is the chief parameter in determining the extraction efficiency according to η=1/mc under some reasonable, simplifying assumptions. When the source in the cavity has a linewidth, light redistribution among both angles and wavelengths occurs and can either be a limiting factor for the overall extraction efficiency or a positive one in case of spectral narrowing requirements. In extraction-optimized systems, one gains a factor of up to ten on the single-face extraction as well as brightness (radiance).