A laboratory apparatus and experimental techniques have been developed to investigate the properties of a 5–30 keV electron beam fluorescence probe system in air, nitrogen, and helium test gases over a pressure range from 10−3to 10 Torr at 302 K. The design and performance characteristics of the electron beam generating system and optical detector system are described. The electron beam system used in this work was especially designed for wind tunnel remote measurements of local gas density in low density high velocity flow fields. A 1 mm diam collimated beam of 5–30 keV electrons was used to irradiate each of the test gases and excite fluorescence radiation, specifically the 391.44 nm (0,0) First negative band ofN2+in air and nitrogen, and the 501.57 nm (21S–31P) He line in helium. A precision optical detector system has been designed and used to observe the spatial distribution and pressure dependence of this fluorescence radiation excited by the electron beam. Calibration and test procedures have been implemented to reduce significantly experimental errors and provide relative fluorescence intensity measurements with a precision better than ±2%. Typical results are presented and discussed which illustrate data obtained using the developed apparatus and test procedures.