The calibration set 666.000 consists of three neutral density glass filters for checking the absorbance accuracy, and a holmium oxide glass filter for checking the wavelength accuracy. The glass filters are mounted in precision aluminium frames. They are designed for use with the standard 10 mm cell holder provided with spectrophotometers. The set, along with an empty mount, is supplied in a sturdy wooden case.

For identification purposes the set number is engraved on each filter mount.
The absorbance values and/or peak position wavelengths of every filter are quoted in the accompanying calibration certificate. A copy of the values, for laboratory use, can be found on the inside face of the case lid.