hefner unit, or hefner candle

A unit of luminous intensity, late 19th – early 20th centuries, equal to the horizontal intensity of the light from a lamp developed by the German engineer Friedrich Franz von Hefner-Alteneck (1845-1904) in 1884. Abbreviation, HK. Mainly used in Germany before 1942. One hefner unit is approximately 0.903 candela.

The lamp, known as a Hefner lamp, burned amyl acetate (C₅H₁₁C₂H₃O₂). The wick was solid, completely filling a tube with an inner diameter of 8 millimeters. Incorporated in the lamps was an optical device for measuring precisely the height of the flame, which was to be 40 millimeters. The chief source of lack of reproducibility was humidity, the lamp being about 10% brighter in dry air than at high humidities, which was corrected for by standard tables. In Germany the hefner unit superseded the vereinkerze for scientific work, and was replaced by the candela.

Friedrich Franz von Hefner-Alteneck.
Vorschlag zur Beschaffung einer konstanten Lichteinheit (“Recommendation for provision of a constant light standard”).
Electrotechnische Zeitschrifte, volume 5, pages 20-24 (1884).

Photographs of two 1904 Hefner Lamps “approved by the Physikalish-Technichen Reichanstalt as the German Photometric Standard Source of Light, burning amyl-acetate”, can be seen at http://physics.kenyon.edu/EarlyApparatus/Optics/Lamp/Lamp.html, courtesy of Professor Greenslade.