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.

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