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Determination of the Constants of Total Radiation From a Black Body

This paper [1] presents a critical review of the state of knowledge about the constant of nature that relates the quantity of radiant heat and light to the temperature of the radiant object. Studies of this issue at the beginning of the 20th century ultimately led to the discovery of quantum mechanics, the keystone of modern physics. The continuity of this work at NIST for the remainder of the century also led to fundamental changes in the way that optical metrology is now carried out. In 1905 Samuel W. Stratton, the Director of the National Bureau of Standards, persuaded a young research associate at the Washington Carnegie Institute to come to the NBS laboratories to undertake research programs in infrared spectroscopy. Thus William Weber Coblentz began a nearly 40-year career at NBS that resulted in over 400 publications in diverse areas of optical research. Stratton’s original intent was to use Coblentz as an assistant in his own laboratory, but these demands were modest and Coblentz was able to pursue his own interests in infrared spectroscopy and radiometry, begun as a graduate student at Cornell. One of the suggestions offered by Stratton led to the work cited here and to the establishment of the science of radiometry at NBS. The first decade of the 20th century was a very important time in the development of concepts in modern physics. One of the of the great mysteries of the era was the underlying principle behind blackbody radiation, the heat and light emitted from hot objects. (Black objects are studied in order to eliminate the effect of reflection.) Max Planck in Germany had developed a theory that appeared to explain the phenomenon in proper detail, but it required the introduction of a revolutionary new idea. In Planck’s theory, the energy of light and other electromagnetic radiation was quantized in discrete increments. The proportionality between the energy increment and the frequency of the radiation became known as Planck’s constant. The spectral distribution of blackbody radiation, that is, its radiance at different wavelengths, is determined by its temperature. Early on, the quantitative details were described by presuming two constants of nature, which became known as the first and second radiation constants. Additionally, the total energy of the radiation emitted was shown to be proportional to the fourth power of the temperature. This proportionality constant was named the Stefan-Boltzmann constant after the theoretical physicists who first calculated the relationship. The Stefan-Boltzmann constant can be expressed in terms of the other radiation constants or the fundamental constants now known as Planck’s constant, the Boltzmann constant, and the speed of light. This process of unifying basic phenomena and verifying the ideas with accurate experiments was central to the development of physics at the turn of the century. On a visit to Europe, Stratton observed that a number of the national laboratories in Europe were expending considerable effort in measuring the radiation constants for describing the spectral distribution and total radiation from blackbody sources. Stratton suggested to Coblentz that it would be good if NBS could contribute to this effort and recommended that Coblentz direct his efforts in this regard.