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10 Candle Standard Pentane Lamp

Three horizontal pictures that show parts of a carbon-filament lamp created in 1887.

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When work in photometry was initiated at the National Bureau of Standards (NBS) around 1903, a group of carbon-filament lamps calibrated against Hefner flame standards were set up as the first National Reference Standard of luminous intensity. Comparisons of incandescent lamps calibrated against this reference standard group and also against flame standards both at NBS and in the national laboratories of origin of the flame standards (Hefner at Physikalisch-Technische Reichsanstalt in Germany, Carcel at the International Electrical Commission in France, and Vernon-Harcourt at the National Physical Laboratory in England) resulted in establishment of the relative magnitudes of the unit of luminous intensity in these four countries and culminated in the adoption in 1909 of the "international candle" as the unit in England, France, and the United States, with the Hefner candle of Germany considered as 0.9 as large as the international candle.

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10 Candle Standard Pentane Lamp

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