GB1152719A - Gas Discharge Lamps. - Google Patents

Abstract

1,152,719. Metal-vapour lamps. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 25 Aug., 1966 [28 Aug., 1965], No. 38114/66. Heading H1D. A thermionic-cathode discharge lamp in which radiation emission from the positive column occurs, contains a rare gas at a pressure between 0À1 and 10 Torr and an alloy of at least two vaporizable metals, at least one of which is an alkali-metal, the operating wall temperature of the lamp being chosen such that, in the positive column, at least 20% of the total number of atoms of the metal having the lower ionization potential is ionized whilst the density of the vapour of the metal having the higher ionization potential is such that optimum radiation generation is obtained. Thus, one of the metals provides for current conduction and the other for radiation. Due to the low pressure of the first constituent a certain degree of saturation of the ionization in the positive column is reached. A positive voltage/current characteristic is obtained so long as the potential gradient is insufficient to cause ionization of the second constituent or of the rare gas. Hence the usual ballast resistor may be omitted. The graphs of Fig. 2 (not shown) show that the maximum of the voltage/current characteristic is reached at a current value which increases with wall temperature. Since the pressure of the light-producing constituent also depends upon the wall temperature, this may be chosen as a compromise between efficiency and light output. It may be regulated by means of a heating wire in the outer tube 6. The pressure p‹ of the current-conducting constituent may be determined from the desired current density using the formula 1À8 x 10<SP>-3</SP> j/V d < pD < 9 x 10<SP>-3</SP> j/V d where p<SP>D</SP> is given in Torrs and V d is the drift velocity of electrons in the positive column. The rare gas may be argon at a pressure of 3 mm. (krypton and xenon may also be used). The alloy may consist of 45 atom per cent of Hg and 55 atom per cent of Cs and the lamp may be operated at a wall temperature between 353‹ and 403‹ K. In a further example, the alloy consists of 98 atom per cent of Na and 2 atom per cent of Hg, the temperature being about 483‹ K. When Na is chosen as the lightemitting constituent, the alloy may contain less than 1% of Cs at a temperature between 483‹ and 543‹ K. The tube may be operated on A.C. or D.C. and comprises two cathodes 2, 3 surrounded by nickel rings 4, 5.