A method for measuring the population of atoms in a vapor cell comprises collecting a sample of atoms, applying radio frequency (RF) spectroscopy to the sample such that a first portion of the atoms are in an upper ground state and a second portion of the atoms are in a lower ground state, and applying light to the sample to produce a first fluorescence such that all atoms are left in the lower ground state. The method further comprises measuring a population of the atoms in the upper ground state based on the first fluorescence, applying an RF pulse to the sample to transfer the atoms in the lower ground state to the upper ground state, and applying light to the sample after the RF pulse is applied to produce a second fluorescence. A population of all the atoms in the sample is then measured based on the second fluorescence.