Test-tube cuvettes were popular for making measurements in the Thermo Scientific™ SPECTRONIC™ 20 Spectrophotometers popular in teaching and basic analytical laboratories in the last century.  Many well established methods still call for the use of test-tube cuvettes.  

Modern methods (and instruments) generally rely on square cuvettes with a highly reproducible pathlength, optical surfaces that are perfectly perpendicular to the instrument light beam, and have a high degree of reproducibility from cuvette to cuvette, even among low cost disposable cuvettes sold by the hundred.

Spectrophotometers designed for use with cuvettes often have restrictions on the maximum height of the cuvettes that they can accommodate.  This requires that special short test-tubes be purchased if a customer wishes to work with test-tubes in a modern instrument.  

Thermo Fisher Scientific continues to supply specialized test-tube cuvettes to support our customers’ ongoing analytical needs.  However, we strongly recommend that customers establish new methods to use square or rectangular cuvettes, and that older methods requiring test tubes be reviewed and, if possible, updated to use square cuvettes as a more modern, more accurate, and more available alternative.