Hefner Lamp

HEFNER LAMP, LAMP, or more precisely, the Hefner-Alteneck lamp, is currently accepted in Germany, the USSR, and many Western European countries as the unit for measuring the intensity of light sources. In the specialized German literature, the Hefner lamp is usually denoted by the letters H.K. (Hefners Kerze). Such designations can often be encountered on many lighting devices, on which there are markings indicating their light intensity, for example, on electric bulbs, kerosene burners, etc. The H. lamp is a small metal lamp (see illustration), with a simple burner without glass. In the reservoir A of the lamp, amyl acetate is poured, which burns with a yellow, very even, non-smoky flame. The wick in the lamp is asbestos, 8 mm in diameter, passes through a nickel-silver tube B (burner), the wall thickness of which is 0.15 mm. The wick is cut completely flat at the edge of the burner. The height of the flame is regulated by a special screw g and must be exactly 40 mm. For precise adjustment of the flame length, there is a special device in the form of a small tube-ocular, mounted on a leg C, equipped on one side with a biconvex lens P, and on the other side with a ground glass with a transverse line d. When the lamp is burning, the lens gives an image of the upper end of the flame on the ground glass. By adjusting with the screw, the tip of the flame is set at the level of the line, which corresponds to a flame length of exactly 40 mm. The H. lamp well satisfies one of the most basic requirements imposed on any unit used for measuring light intensity, namely, it has great constancy of light emission, i.e., its light intensity varies very little during the time interval necessary for making measurements. The fluctuations in light intensity of the H. lamp do not exceed ±1%, while the English standard candle from spermaceti, the German standard candle from paraffin (V.K.=Vereinskerze), the French Carcel lamp with colza oil (huile de Colza), and the old French standard candle (de l'Etoile) give fluctuations up to 10%. Only the so-called standard decimal candle, equal to V₂₀ units of the Violle, surpasses the Hefner lamp in constancy of light emission; however, the practical implementation of this candle presents enormous difficulties, which have not yet been overcome. The Violle unit is the intensity of light emitted by 1 square centimeter of molten platinum during its solidification. All the standard candles listed above, also accepted as a unit for measuring the intensity of light sources, have far from identical intensity and differ significantly from the intensity of the H. lamp. For converting one value to another, the following data are used: 1 H.K.=0.817 V.K.=0.87 de l'Etoile = 0.092 Carcel = 0.885 decimal candle = 0.044 Violle. In very precise studies of any light source, it should be borne in mind that the intensity of light of the H. lamp, although weakly, fluctuates depending on barometric pressure and absolute air humidity, as well as on its carbon dioxide content. The normal intensity of light for the H. lamp is considered to be at 760 mm barometric pressure, 6 mm absolute air humidity, and 0.04% CO₂ in the air. A change in barometric pressure of 40 mm causes a change in the intensity of light of the H. lamp by 0.4%. An increase in carbon dioxide by 0.10% decreases the intensity of light by 0.72%. An increase in water vapor by 1 liter in 1 cubic meter of air decreases the intensity of light by 0.55% (Khvolson). According to the research of Tumlirz, the total amount (E) of energy emitted by the H. lamp is equal to 0.1483 small calories, with the mechanical equivalent of light (e) being only 0.00361 small calories, or 151,500 ergs. K. Ångström, when measuring with a compensation heliometer, found other, apparently most accurate values, namely: E=0.215 small calories, e = 0.00194 small calories, or 81,000 ergs. At a distance of 1 meter from the lamp, 8.1 ergs fall per 1 square centimeter of area per second.