The laser induced stimulated emission (LISE) of the Balmer &agr; line is used to measure the ground state hydrogen atom density in an efficient source of hydrogen atoms generated by a low‐pressure (1 Torr), pulsed microwave discharge at 2.45 GHz. The H atom kinetics in pure hydrogen discharges and in hydrogen–air mixture discharges are compared. It is shown that, for high atomic density and long plasma length crossing, the LISE Balmer &agr; line is partially absorbed by the plasma. This provides an estimation of the H(n=2) atom density. ©1996 American Institute of Physics.