Calibrations of Brewer spectrophotometers for total ozone observations can be performed either by intercomparisons towards a reference instrument (the most frequent method) or by absolute calibration using Langley plot method (feasible only at specific locations). The world Brewer calibration scale is defined by the Triad of Brewer instruments maintained by MSC (Meteorological Service of Canada) in Toronto. Each Brewer of the Triad is calibrated absolutely at the Mauna Loa Observatory, Hawaii (MLO). Instruments in the world network are routinely calibrated by comparisons with travelling references, outside of Canada mostly by the Brewer 017 operated by the International Ozone Service - a Canadian commercial company. Brewer 017 is regularly checked towards the reference Triad.
In November 2003 the WMO/GAW Regional Calibration Center for RA-VI region (RBCC-E) has been establish at the Observatory Izaña of AEMET, Canary Islands (IZO). RBCC-E owns a full calibration and reference-maintenace equipment composed by three Brewer spectroradiometers (The IZO Triad): a Regional Primary Reference (Brewer 157), a Regional Secondary Reference (Brewer 185) and a Regional Travelling Reference (Brewer 183) which can be transported for calibration campaigns outside IZO. Besides, an Optics Laboratory is available for indoors calibrations as well as equipment tests.