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B. Standard Artificial Sunlight.—There are various methods for producing artificially an approximate reproduction of average noon sunlight, as above specified. The most accurate is that of Priest which consists in passing the radiation from a gas-filled tungsten lamp, operated at a color temperature of 2848°K, (about 15.6 l.p.w., for concentrated filament lamp), through a pair of crossed nicol prisms between which is inserted

Fig. 7

a crystalline quartz plate 0.500 mm. thick with surfaces perpendicular to the optic axis of the crystal (78). The resulting energy distribution is shown, in comparison with that of actual sunlight, by the broken line in Fig 6. The proper energy distribution for the radiation from the lamp, before passing through the nicol prisms and the quartz plate, is given in Fig 7. Lamps yielding