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.

© 1971 Optical Society of America

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Figures (2)

Fig. 1
Fig. 1

Schematic elevation of apparatus. A, diffuser; B, entrance slit; C and F, spherical mirrors; D and G, gratings; E, focal plane of spectrum and central slit; H, exit slit; I, detector.

Fig. 2
Fig. 2

Perspective drawing with cut-away portions to show some detail. A, entrance slit; B, first mirror in housing with adjusting screws; C, first grating; D, plane of spectrum and central slit; E, position of detector; F, machined aluminum framework; G aluminum disk on which masks are mounted; H, precision bearings on which disk runs; I, tapered pin for accurate disk positioning; J millimeter scale with vernier and magnets connected to central slit mechanism; K, reed switch assembly with micrometer adjustment screw for accurate setting; L, housing for second mirror; M, base plate.

Tables (2)

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Table I Values of Scattered Light Expressed as Fractions of Radiation Intensity at 632.8 nm

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Table II Comparison of Chromaticities of Lamps as Measured by Different Instruments and Methods

Equations (3)

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D = 2 f tan i / λ ,
δ = [ D ( 550 ) - D ( 380 ) ] / D ( 380 ) = 0.007 ,
δ = [ D ( 760 ) - D ( 380 ) ] / D ( 380 ) = 0.022 ,