Zirconium Lamps

The pages of our journal have already discussed the question of the unique properties of a new point source of light—the lamp with a concentrated arc (see the abstract by G. N. Rokhlin1). The principal working part of this lamp is a specially treated zirconium cathode. It consists of a tantalum cup, in which a small rod of pressed zirconium oxide is placed. A few tenths of a millimeter from it there is a molybdenum plate that plays the role of an anode. At its center, coaxially with the cathode, a round aperture is cut, serving as a window for the exit of the beam of light emitted by the lamp. When a voltage of 1000–2000 volts is applied to the lamp electrodes, a discharge is ignited between the anode and the metallic cup of the cathode. After the zirconium oxide has been sufficiently heated and becomes electrically conducting, the arc transfers from the walls of the cup to the oxide surface facing the anode.

The zirconium oxide melts and, under the action of the high temperature and ionic bombardment, is partially reduced. A luminous film of metallic zirconium forms on the surface of the cathode.

Through the aperture in the anode there passes a narrow beam of light resting on an extremely small circular cathode spot, whose diameter in some lamps reaches 0.085 mm. At the same time the lamp has a brightness of from 4000 to 10,000 stilbs, which is several times greater than the brightness of powerful incandescent lamps and is only slightly inferior to the brightness of carbon arcs with simple carbons.

Recently a number of articles have appeared in print reporting certain features of zirconium lamps that were not reflected in the first work of Becking and Deibert2. Among them, first of all, one should mention the article by Hawksford and Platt3. This article draws attention to the following circumstance. The zirconium lamp, as has already been indicated, possesses great brightness. Thus, for example, the brightness of a 100-watt lamp is approximately 6000 stilbs. Such brightness corresponds to the radiation of an absolutely black body at a temperature of 3200°K. Assuming that the radiation of the lamp is thermal radiation, we come to the conclusion that the zirconium film must have a temperature of not less than 3200°K.

However, from the position of the maximum of the radiation curve of a 100-watt lamp, the authors conclude that the temperature is equal to 2800°K. In this connection it is necessary to state that, along with thermal radiation, radiation of some other kind occurs in the lamp, and it is this that provides such great brightness.

A second circumstance to which Hawksford and Platt drew attention is that the zirconium lamp is well modulated in amplitude, phase, and frequency. This likewise confirms that it is not thermal radiation alone that is characteristic of it.

A detailed investigation of this question with the aid of modulation characteristics makes it possible to judge also the fraction constituted by thermal radiation in the total radiation of the lamp. Thus, examining the constant decays of modulated light, Hawksford and Platt came to the conclusion that in the infrared region 10% of the luminous flux decays instantaneously ($\tau < 10^{-5}$ sec), 60%—considerably more slowly ($\tau = 5 \cdot 10^{-4}$), and 20%—over a larger interval of time. This last part is sensitive only to long-term changes of the mean current. It is thermal radiation in origin and corresponds to a temperature of 2800°K, i.e., it has a considerably longer-wavelength maximum than the other two parts.

The first part, with a decay constant of less than $10^{-5}$ sec, is produced by the radiation of gases and vapors in the arc. Finally, the middle part, in the opinion of Hawksford and Platt, is characteristic neither of the radiation of gases and vapors in the arc nor of thermal radiation, and the question of its origin remains open.

Both the first and the second part, having a short-wave maximum, correspond to a temperature of 3200°K.

This circumstance—that the zirconium lamp is well modulated—can be used for its application in a light-beam communication system, as is described in detail in the work of Buckingham, Deibert, and Morgenstern.^4

An extremely favorable factor for such an application is that the intensity of the modulated radiation of the zirconium lamp proved to be maximal in the infrared region. Long waves have a number of advantages in organizing communication by means of a light beam, since, first, a number of photoelectric cells possess high sensitivity in this range, which is necessary for good operation of the receiving device; second, this is the invisible part of the spectrum, which is convenient for achieving secrecy of transmission; third, in the case of fog, the attenuation of long-wave radiation is considerably less than the attenuation of short-wave radiation.

Theoretically, the greatest beam intensity is obtained at maximum brightness, i.e., in the case of low-power lamps. However, the extremely small diameter of the cathode spot of a two-watt lamp possessing maximum power requires the use of lenses of a high class of accuracy. In view of the difficulty of obtaining such lenses, it is more expedient to use medium-power lamps, which, with optics of average quality, can give a more intense beam than low-power lamps with the same optics. Even high-power lamps give a very narrow beam. Thus, the beam of a 100-watt lamp at a distance of 1.5 km has a diameter equal to 8 m. Less powerful lamps give still narrower beams.

Under this condition, communication over long distances requires accurate focusing and firm mounting of the source and receiver. In portable installations only high-power lamps can be used. The modulation characteristics of low-power lamps surpass the modulation characteristics of more powerful lamps. Thus, for example, the modulation coefficient of a 100-watt lamp is twice as small as the modulation coefficient of a two-watt lamp.

A concrete example of the use of zirconium lamps for light-beam communication is a communication line that has been operating in Manhattan since April 1943.

The transmitter in this line consisted of a 10-watt lamp focused by a parabolic mirror 15 cm in diameter with a focal length equal to 1200 mm. It gave a beam which, at a distance of 1200 m, had a diameter of 3 m. The receiver consisted of a compound Fresnel lens 45 cm in diameter, focusing the received light flux onto a cesium photoelectric cell. This system operated at a speed of 65 words per minute in one direction. The narrow beam made it unnecessary to filter out infrared waves in order to ensure transmission secrecy—it was secure without that. Under ideal conditions—absolutely clear weather—the system can operate at a distance of 50 km.

Neither sun nor clouds have any noticeable effect on the intensity of the beam. However, rain and fog worsen transmission by a factor of two, while dense fog and snowfall stop communication altogether.

In conclusion, it should be noted that the installation, after operating for 3½ years, had only about 3% of operating time lost because of the lighting-engineering part of the equipment.

V. V. Fedorov

CITED LITERATURE

1. Rokhlin G. N., UFN, 31, 1 (1947).
2. Buckingham and Deibert, JOSA (May, 1946).
3. Huxford and Platt, JOSA, 37, 1 (1947).
4. Buckingham, Deibert and Morgenstern, El. Eng. 66, 10 (1947).