Free-piston-driven expansion tubes are capable of generating flow conditions over a wide range of enthalpies ranging from orbital up to superorbital velocities. Initial optical measurements aimed at investing the flow in such a facility are presented. Emission studies were used to identify impurities in the flow and to investigate spectral regions that are accessible by optical techniques. At moderate enthalpies, it was found that significant radiation resulted from metallic contaminants. At high enthalpies, the spectrum consisted of a number of atomic lines together with a broadband background component indicative of the presence of electrons. The presence of this radiation may limit the applicability of optical techniques that require spectral regions free from the influence of atomic transitions or background radiation. Emission spectroscopy (through Stark broadened hydrogen lines) and two-wavelength holographic interferometry were used to measure the electron number density behind a bow shock on a blunt body at conditions where significant ionization was observed. They yielded average concentrations of (3 ± 1) × 1017cm-3 from the emmission measurements and (3.8 ± 0.6) × 1017 cm-3 from the interferometry.