Book excerpt: A detailed review of the theoretical methods of optical pyrometry and the application of these methods at the National Bureau of Standards in realizing, maintaining and distributing the International Practical Temperature Scale above 1063 C degrees is presented.In the theoretical presentation, the concepts of effective and mean effective wavelengths are introduced, and various equations relating these parameters to each other and other physical quantities are derived.The important features of precision visual optical pyrometers are discussed and a number of blackbody sources and tungsten strip lamps described. Detailed experimental procedures and results of primary and secondary calibrations of optical pyrometers at NBS are given.Finally, recommendations for achieving high precision and accuracy and the fundamental limitations in visual optical pyrometry are presented.(Author).