Radiometric Reference and Calibration Methods
In this section, we discuss both calibration and characterization of solar radiometers. According to he United States National Institute of Standards and Technology (NIST) Engineering Statistics Handbook (available at http://www. nist. gov/div898/ handbook):
Calibration is a measurement process that assigns value to the property of an artifact or to the response of an instrument relative to reference standards or a designated measurement process. The purpose of calibration is to eliminate or reduce bias in the user’s measurement system relative to the reference base.
Note that characterization is distinct from calibration, as defined in the same NIST handbook:
The purpose of characterization is to develop an understanding of the sources of error in the measurement process and how they affect specific measurement results.
Once calibration has been accomplished, characterization can further address specific sources of error to produce results with improved accuracy. Also important is the concept of traceability of measurements. In the International Standards Organization Vocabulary of International Metrology (VIM), (ISO 1996), definition 6.10, “traceability” is defined:
The property of the result of a measurement or 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.
For a measurement program to claim traceability the provider of a measurement must document the measurement process or system used to establish the claim and provide a description of the chain of comparisons that establish a connection to a particular stated reference.