  Miniature atomic beams can provide new functionalities for atom based sensing
instruments such as atomic clocks and interferometers. We recently demonstrated
a planar silicon device for generating well-collimated thermal atomic beams
[Nat Commun 10, 1831 (2019)]. Here, we present a near-source fluorescence
spectroscopy (NSFS) technique that can fully characterize such miniature beams
even when measured only a few millimeters from the nozzle exit. We also present
a recipe for predicting the fluorescence spectrum, and therefore, the source
angular distribution, even under conditions of strong laser saturation of the
probing transition. Monte Carlo simulations together with multi-level master
equation calculations fully account for the influence of optical pumping and
spatial extension of the Gaussian laser beam. A notable consequence of this
work is the agreement between theory and experimental data that has allowed
fine details of the angular distribution of the collimator to be resolved over
3 decades of dynamic range of atomic beam output flux.
