A system to determine the density and temperature of ground‐state hydrogen atoms in a hydrogen plasma by measurement of the absorption of Lyman‐beta or Lyman‐gamma radiation is described. The Lyman‐series radiation is generated by resonant four‐wave sum‐frequency mixing in mercury vapor. A wide range of hydrogen atom densities can be measured by employing these two transitions. A sample measurement on a H−ion‐source discharge is presented. Extensions to Lyman‐alpha and other vacuum‐ultraviolet wavelengths are discussed.