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 perceived by people - had the greater 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 measurements, 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.
It is generally believed that the most accurate means of measuring the CIE tristimulus values X, Y, Z or chromaticity coordinates x,y from a display is by using a spectroradiometer. Nevertheless, tristimulus colorimeters employing three or four colored filters find wide use because of their simplicity and lower cost. These devices cannot be calibrated to give accurate results in all situations because the spectral responsivities of their filtered detectors are not exactly the CIE color-matching functions. However, for a display that produces a linear superposition of three primary colored lights of fixed spectra, a tristimulus colorimeter can be correctly calibrated to measure all colors on that display. Signals from all of the filtered detectors are used to compute each of the X, Y, z values. The calibration matrix is computed by data fitting to a reference colorimeter. An improvement to the previously published method is reported, an a numerical example is shown. This technique is more tractable with today's digital instrumentation than it was when it was discovered, yet it remains underused. The American Society for Testing and Materials, through its Committee on Color and Appearance, is revising its standard on display measurements using tristimulus colorimeters to encourage the adoption of the technique.
Until recently, the source-based photometric scale of NIST had been derived from the gold- point black-body using a long derivation chain resulting in a scale uncertainty of about 1%. In order to improve the accuracy of the photometric scale, a new detector-based illuminance scale has been realized with an uncertainty about twice as small as that of the previous source- based scale. The realization of the new illuminance scale was made by photometers calibrated for absolute spectral response, traceable to NIST primary standard detectors using a short derivation chain. The luminous intensity scale is derived from the illuminance scale. With the introduction of the new photometric scale, the photometric calibration services of NIST will include luminous intensity and luminance calibration of different kinds of light sources, measured with standard photometers. Luminous intensity measurements of LEDs are discussed as an example to illustrate the new calibration services. In addition, the new photometric scale will allow calibration of illuminance and luminance meters with standard photometers, using different kinds of light sources.