This paper describes the measurement methods and instrumentation used in the realization and transfer of the NBS scale of spectral radiance. The application of the basic measurement equation to both black-body and tungsten-strip-lamp sources is discussed. The polarization properties, response linearity, spectral responsivity function and "size-of-source effect" of the spectroradiometer are described. The analysis of sources of error and estimates of uncertainty are presented. The assigned uncertainities in spectral radiance range from about 2.1% at 225 nm to 0.3% at 2400 nm.