Because of the variation in emissivity of tungsten within the visible spectrum, the relative spectral energy distribution of a tungsten ribbon filament lamp calibrated as CIE source A will deviate from the Planckian distribution adopted and tabulated by the CIE for this source. Likewise, the energy distributions of standard sources B and C, when used with a calibrated tungsten ribbon filament lamp, will deviate from those generally given. It is shown that this deviation can introduce a change in chromaticity coordinates which is bigger than the smallest chromaticity differences distinguishable in a large field of view under good observing conditions. Reasons are given for believing that the relative energy distribution values of a source A lamp with coiled filament will be nearer to the tabulated data. The actual relative energy distribution of a properly calibrated source A lamp may therefore vary somewhat according to the type of filament used in the lamp. Two ways of defining the standard source A lamp and its energy distribution so as to reduce this uncertainty are suggested.
B. HISDAL, "Relative Spectral Energy Distribution of Standard Light Sources for Colorimetry," J. Opt. Soc. Am. 48, 608-612 (1958)