National Standards Laboratories (NIST, NPL, etc.) work together to agree a common definition for measurement units. These then make up the International System of units, SI. e.g. kilogram, second, metre, ampere, candela, and the Standards Laboratories will then "realise" units from internationally agreed SI definitions to establish primary national measurement scales.

Traceability is defined in the "International Vocabulary of Basic and General Terms in Metrology (ISO, 1993)" as the."property of the result of a measurement of the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties."

Where appropriate, calibration certificates are issued where the certification process has established traceable links to a USA National Institute for Standards and Technology (NIST) Standard Reference Material (SRM) to determine the appropriate filter parameters.

Using procedures consistent with the operation within an ISO 17025 environment, all certification measurements are bracketed by use of an appropriate NIST primary SRM. In addition the fundamental characteristic of the reference spectrophotometer are periodically established using physical references. For example, wavelength calibration is verified using line spectra from a mercury emission source.