An apparatus is described which consists of a double grating–mirror dispersion system, and which can be used both as a spectrophotometer and as a tristimulus calorimeter of high accuracy. Three masks, cut to the exact requirements of the CIE distribution coefficients for equal energy stimuli, can be placed alternately in the plane of the spectrum. Chromaticities were calculated for some fluorescent lamps from their spectral irradiance data and also directly measured by utilizing the three masks. These results are compared and discussed. Results are also given of measurements made on a set of fluorescent lamps both by a mask photometer at NPL, Teddington, and by the NPRL mask photometer.
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