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100 Years of Photometry and Radiometry Harnessing Light



Jonathan E. Hardis


Measurement of light is an old subject, though the past 100 years have seen significant advances. 100 years ago, photometry the art and science of measuring light as it is preceived by people had the greatest technological importance. Even today, SI (the metric system) retains a base unit for photometry, the candela. However, early work at NBS included pivotal projects in the field of radiometry the measurement of the physical characteristics of light. These included the validation of Planck's newly-minted theory of blackbody radiation, determining the radiation constants with good accuracy, and the definitive analysis of the spectral responsivity of human vision, so as to relate photometry to radiometry. This latter work has only increased in importance over the past 75 years as the definition of the candela has changed and improved. Today, NIST makes radiometric, and hence photometric measurments with unprecedented precision. Cryogenic radiometers based on the principle of electrical substitution measure optical flux with uncertainties of 0.02%. Additional facilities enable measurement of spectral responsivity, spectral radiance, and spectral irradiance. Novel detectors, such as light-traps, allow the best accuracy to be transferred from the primary standards to routinely-used instruments and to calibration customers. Filtered detectors are used to realize photometric scales, radiation temperature scales, and other, specialized measurements. Indeed, the story of the metrology of light is the story of continuous improvement, both driven by and enabled by advances in technology. We touch upon some of these as a prelude to the other talks in this conference.
Proceedings Title
Harnessing light: Optical Science and Metrology at NIST, Conference | | Harnessing Light: Optical Science and Metrology at NIST | SPIE
Conference Dates
July 29-August 3, 2001
Conference Title
Proceedings of SPIE--the International Society for Optical Engineering


blackbody radiation, candela, NIST history, photometry, radiometry


Hardis, J. (2001), 100 Years of Photometry and Radiometry Harnessing Light, Harnessing light: Optical Science and Metrology at NIST, Conference | | Harnessing Light: Optical Science and Metrology at NIST | SPIE, [online], (Accessed March 2, 2024)
Created June 1, 2001, Updated March 7, 2017