A dual beam acousto-optic tunable spectrometer utilizes an optics system to isolate the two radiation beams tuned by an acousto-optic filter, one of which is used to analyze a sample, the other of which is used as a reference. The tuning of the filter and the analysis of the signals is conducted by a microprocessor, which corrects for the many possible sources of noise. A new fluid sample cell utilizing glass balls to collimate radiation incident on the sample and focus radiation transmitted from the sample, is also disclosed.