[en] A system to determine the density and temperature of ground-state hydrogen atoms in a plasma by vacuum ultraviolet laser-absorption spectroscopy is described. The continuous tunability of the spectrometer allows for analysis at any of the Lyman transitions. The narrow bandwidth of the laser source allows for the accurate determination of the hydrogen absorption line shape and, hence, the translational temperature. The utility of the system is exemplified by data obtained on an ion source plasma. The measurements show the quality of the data as well as illustrating the behavior of these sources under varying discharge conditions