From Current Literature

A New Point Source of Light*

A new type of arc lamp, developed by Buckingham and Deibert, makes it possible to obtain a stationary light spot of unusually small dimensions with high and uniform brightness. This light source, called a “concentrated-arc lamp” or “pin-point light,” gives a brightness several times greater than the brightness of tungsten lamps. The size of the light spot in these arcs, depending on the power, may have a diameter from 0.07 mm for a two-watt lamp to 8–9 mm for a 1500-watt lamp.

This type of lamp consists of a tube of tungsten, molybdenum, or tantalum filled with zirconium oxide. Its open end is located at a distance of several tenths of a millimeter behind the hole in the middle of a flat, sufficiently large anode made of the same metals (see figure). The electrodes are mounted in a glass bulb, which is filled with argon to a pressure of 1 atm.

The light source in this arc is the incandescent spot formed on the end face of the cathode. This spot, whose spectrum is very close to the radiation of a gray body with a maximum near 10,000 Å, provides the overwhelming part of the lamp’s visible radiation. A small fraction of the radiation—predominantly the line spectrum of argon and of singly ionized zirconium, situated at a distance of several hundredths of a millimeter from the cathode surface—gives a cathode glow.

Authors believe that, as a result of activation of the cathode, a thin layer of metallic zirconium is formed on the surface of the oxide, which at the place where the spot forms is in a molten state.

This film is so thin that surface-tension forces firmly hold it on the oxide, thanks to which the lamp can operate stably in any position.

* W. D. Buckingham and C. R. Deibert, The Concentrated-Arc Lamp, J. O. S. A., vol. 36, No. 5, 245, 1946.

The service life of these lamps is determined by the lifetime of the cathode and, despite the forced operating conditions, is fairly long, reaching several hundred hours. (For two-watt lamps the average is 170 hours, and for lamps of higher power: 10, 25, 100 W—700–1000 hours.) This is explained by the fact that zirconium particles evaporating from the surface of the cathode are ionized in the arc and return, to a considerable extent, to the cathode, thereby continuously restoring the metallic zirconium film.

As for the luminous characteristics of this type of arc, the brightness of a series of standard lamps (2, 10, 25, 100 W) under normal operation varies from 10 kilostilbs for the two-watt lamp to 4–5 kilostilbs for lamps of higher power. Fluctuations in brightness during lamp operation do not exceed 10%. The brightness and the diameter of the spot can be varied within known limits by changing the current, since both the brightness and the spot diameter increase with the current. The luminous intensity over a wide interval is directly proportional to the current. At the same time, the spatial distribution is very close to Lambertian.

Ordinary lamps of this type operate on direct current with a ballast resistance connected in series to limit the current to its normal value. Igniting the lamps requires a high voltage on the order of 1000–2000 volts. However, the operating voltage of the lamps is small: for a two-watt lamp it is 130 volts, and for larger lamps 24 volts.

These lamps may find broad application in all cases requiring narrow, intense beams of light, such as, for example, in lensless projection, in photographic enlargements, in the study of optical systems, and in many other devices requiring the use of a bright point source of light.

G. N. Rokhlin