This high-definition thermal test unit for the ultraviolet spectrometer represents the object that flew on the Apollo-Soyuz mission in July 1975. The spectrometer was part of an experiment designed to measure the local concentration of atomic oxygen and nitrogen in the atmosphere using optical absorption spectroscopy. Extremely low concentrations of gases occur even at the altitudes at which manned spacecraft orbit. Those low concentrations mean that a very long light path is required to measure the prevalence of those gases by means of optical spectroscopy. To perform the experiment the two spacecraft assumed positions at precisely 150, 500 and then 1000 meters apart. Apollo astronauts sent a laser beam to a reflector built into the side of Soyuz. The reflected beam was then passed through the spectrometer. Only the 500 and 1000 meter passes gave useful data. Concentrations of gases were computed from the difference in intensity between the outgoing and reflected beams.

The artifact was transferred to NASM by NASA's Johnson Space Center in November 1975.