Arc discharge lamp having large anode radiating surface

#The invention relates to an electric arc discharge lamp comprising a cathode, an anode, means for applying an electrical potential between the cathode and anode to cause an electrical arc discharge therebetween, and a refractory metal diaphragm located between the cathode and anode, the diaphragm having an aperture to restrict the area over which the discharge occurs.

#UK Pat. No. 1486514 shows a discharge lamp comprising a lamp bulb which has a window which is highly permeable to the relevant range of wavelengths emitted by the arc discharge. The interior of the bulb is filled with a mixture of deuterium and krypton. The discharge is effected between a heated cathode and an annular anode. The arc discharge is concentrated by the aperture so that a particularly intensive radiation is delivered through the window.

#In order to produce an intense localized discharge it is necessary for the aperture of the annular anode to have a small diameter. As a consequence the anode is made relatively small and thus has a small surface area and has little heat radiating capacity. Consequently the heat generated by the arc will cause the temperature of the anode to increase to a relatively high temperature which makes the use of refractory metals for the anode necessary if a reasonable life for the lamp is to be achieved.

#An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which:

#FIG. 1 shows an exploded elevation of an electric arc discharge lamp according to the invention,

#FIG. 2 shows in cross-section the assembled cathode diaphragm and anode of FIG. 1,

#FIG. 1 is an exploded elevation of an electric arc discharge lamp which comprises a base 1, a cathode assembly 2, a substantially tubular member 3, a diaphragm 4, a substantially tubular member 5, a cup shaped anode 6, an anode extension 7, and an envelope 8.

#The base 1 carries three leads 11 which are surrounded over most of their length by ceramic tubes 12, three further leads 13, two of which are shown, surrounded by ceramic tubes 14, and three leads 15, one of which is shown.

#The cathode assembly 2 is shown on a larger scale and in more detail in FIG. 2. An impregnated cathode pellet 21 is mounted on the end of a metal can 22 which encloses a heater coil 23. The can 22 is mounted within an electrically conducting generally tubular member 24 by means of three equally spaced wires 20, of which only one is shown, which extend radially between the members 22 and 24 to minimize the heat conduction between the members whilst establishing an electrical connection between them. The member 24 is mounted in a ceramic disc 25 which also carried two terminal pins 26 and 27. A further terminal pin 28 is welded to the tubular member 24 and serves to form an electrical connection to the cathode pellet 21 via the wires 20 and metal can 22. The ceramic disc 25 is surrounded by a metal skirt 29. The cathode heater 23 is connected to the terminal pins 26 and 27 via metal plates 261 and 271 and helically coiled leads 231 and 232, this construction being used to minimize heat conduction and still provide rigid support for the heater. The metal plates 261 and 271 are welded to the pins 26 and 27 respectively and are provided with tails 262 and 272 for connection to two of the leads 15 while terminal pin 28 is connected to the third lead 15.