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Most radiometric measurements require that calibration by the use of standards be performed prior to the measurements of unknowns. The requirements for standards fall naturally into two different types: flux measurements and material measurements. The flux measurements include power, incidance, and radiance, while the material standards include those for reflectance, transmittance, emissivity, absorptance, detector responsivity, and refractive index.
10.1 Types of Standards
Standards may be divided into primary and secondary standards as well as flux and material standards. Another type, the need for which has been created more by lawyers and contract managers, is the traceable standard. This is one that can be traced to an appropriate official agency for purposes of satisfying a contract.
A primary standard is based on the measurements of quantities other than that of the standard. For instance, one primary standard is a cavity radiator at the National Institute of Science and Technology (NIST) formerly the National Bureau of Standards (NBS). This cavity radiator has its emissivity calculated by one of the methods described above to be at least 0.9999, and its temperature is then measured. By measurement or calculation of two non-flux quantities (temperature and emissivity), the flux from the cavity is determined. Such is a primary standard, and only NIST, or other national standards labs, have such standards. NIST also has secondary (or working) standards. One example is a set of tungsten bulbs that are compared to the primary cavity radiator just described, and are used in other laboratories as calibration sources. It is often good practice in individual labs to follow this procedure of using a primary and several working standards. In this case, the âprimaryâ standard is actually a secondary standard obtained from NIST, and the working standards are compared to it as often as is necessary.
10.2 Photometric Standards
NIST has been required by Congressional mandate to maintain the standard of luminous intensity, a long, cylindrical body with a conical end. The tube, shown in Figure 10-1, is made of platinum and is surrounded by an insulator made of unfused thoria and then fused thoria. The temperature is monitored in several places with thermocouples (and sometimes other temperature sensors), and the emissivity was calculated by the methods of Gouffé and DeVos.
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