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A National Measurement System for Radiometry, Photometry, and Pyrometry Based upon Absolute Detectors

V. Conclusions and Suggestions

The advent of cryogenic electrical substitution radiometers with unprecedented accuracy has provided the opportunity to recast the procedures and techniques utilized to maintain radiometric and photometric units. In the case of photometry the impetus for change is further assisted by the development of stable, reliable, and inexpensive silicon photodetectors for use in photometric measurements. The availability of high quality solid state current amplifiers allows silicon detector systems to provide measurement over many decades of optical or photometric power. These detector systems can be calibrated at NIST or other calibration laboratories and, with the measurement strategies outline above, photometric and radiometric units can be defined and maintained for the calibration of many types of optical sensing equipment. As pointed out in the photometry section of this Technical Note, the maintenance of a single photometer by an organization interested in photometric measurement could suffice to define all the calibration parameters ordinarily required and thereby obviate the need for the care and maintenance of a large number of lamp standards.

Obtaining the national optical measurement units from an accurate cryogenic radiometer allows for their simultaneous improvement as the techniques for better utilization of the cryogenic radiometer are developed. While improvement in the accuracy of the cryogenic radiometer itself would not necessarily be of immediate benefit, the ability to perform improved transfer measurements to other devices can produce corresponding improvements to the measurement chain. Improvements in transfer standard detectors in all wavelength regions will assist the efforts to propagate the benefits of the high accuracy of the HACR to radiometric enterprises in general. NIST has programs in these areas to develop better mechanisms for calibrations in the infrared and the ultraviolet. An additional effort is underway to improve the accuracy of the NIST synchrotron ultraviolet radiation facility, SURF II, by relating its spectral output to the HACR through a FR system. The SURF II facility provides a source of radiation from about 1 nm to the far infrared for use in a variety of scientific endeavors including far UV radiometry. This will assist the effort to unify the NIST radiometric units maintained in differing ways by various technologies.

NIST customers of radiometric and photometric services are encouraged to renew their thinking on calibration strategies for the future to take advantage of the savings and increased accuracy suggested by the developments outlined in this note. Using a well characterized detector system offers the possibility of a significant reduction in the number of calibration artifacts necessary to maintain a direct traceability to NIST.

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