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Photoinjectors are the main high brightness electron sources for x-ray free electron lasers (XFELs). Photoinjector emittance reduction is one of the key knobs for improving XFEL lasing, so precise emittance measurement is critical. It is well known that rms emittance is very sensitive to low intensity tails of particle distributions in the phase space, whose measurement depends on the signal to noise ratio and image processing procedures. Such sensitivities make the interpretations of beam transverse brightness challenging, leading to different emittance definitions to reduce the impact of tail particles. In this paper, transverse phase space is analyzed in action and phase coordinates for both analytical models and experiments, which gives a more intuitive way to calculate the beam core brightness.