The calculation method for light emission efficiency splits external quantum efficiency (EQE) into internal quantum efficiency (IQE) and light extraction efficiency (LEE) independently. Consequently, the IQE connected to Purcell factor and the LEE are calculated separately. This traditional method ignores the interplays between the Purcell factor and transmittance coefficient in spectral domain, which all strongly depend on emitting directions. In this work, we propose a new figure of merit to describe the light emission process accurately by using the direction-dependent Purcell factor and transmittance coefficient simultaneously. We use a specific LED structure as a numerical example to illustrate the calculation method and optimization procedure.