A simple method of evaluating absolute number densities in atom reservoirs is described. It consists of measuring the difference in the integrated absorption signals obtained from a reference cell and an analytical cell with a frequency-modulated diode laser as primary source. The evaluation of the number density of atoms in the analytical cell is straightforward and, in optically thin conditions both in the vapor cell and in th analytical cell, does not even require the knowledge of the absorption oscillator strength of the transition. Conversely, for a given, known value of the atomic density, the number density in the analytical cell can be obtained.
