Cesium Lamp

In the tenth issue of J. O. S. A. for 1946*) a cesium lamp recently developed in the USA is described, with a high output of resonance radiation and a service life of several hundred hours.

To eliminate the interaction of cesium ions with the glass, which leads to destruction of the glass and disappearance of cesium from the discharge, in the lamp described the inner surface of the discharge tube is coated with a thin layer of a special protective glass, the composition of which, however, is not given. To maintain the required cesium-vapor pressure, the discharge tube is placed in a vacuum jacket. The electrodes are small tungsten spirals coated with barium and strontium oxide. The lamp is filled with argon at a pressure of 200–220 mm Hg, which considerably increases the output of resonance radiation. As the experiments showed, the indicated pressure is the best. At lower argon pressures the output of resonance radiation falls, while at higher pressures the fraction of visible light increases and the stability of the discharge is disturbed. Filling the lamps with krypton increased the output of resonance radiation by a further 10–15% compared with argon.

Lamps of 50, 100, and 500 W power were developed.

The table below gives data on the intensity distribution in the spectrum of the lamp described, with a power of about 100 W (arc length 8 cm, inner diameter of the tube 35 mm, current 5–5.5 A, voltage drop across the lamp 17.5 V), operated in an oven to maintain the thermal regime. The temperature of the discharge tube was 300°C, which corresponds to a cesium-vapor pressure of 2 mm Hg.

Although no special measurements of the shapes of the resonance lines were carried out, the author states that they are not self-reversed.

) N. C. Beese, Cesium Vapor Lamps, J. O. S. A. 36*, No. 10, 555, 1946.

CESIUM LAMP

The output of infrared radiation is 21.4 W, i.e. 22% of the consumed power. For a 500 W lamp (arc length about 40 cm, tube diameter 50 mm, current 8–10 A, voltage drop across the lamp 50 V) it reaches up to 33%, owing to a reduction in the share of the cathode-anode drop (7 V) in the total voltage drop across the lamp.

Investigations have shown that the slightest impurities of oxygen, nitrogen, and carbon dioxide cause considerable quenching of the resonance radiation. Conversely, small additions of hydrogen to pure argon prove to be even useful: thus an addition of 0.006% \(H_2\) gives an increase in the output of infrared radiation by 30–50%. To absorb harmful gases, a getter was introduced into the lamp in the form of metallic zirconium or tantalum, which also served as a component of the electrodes.

Table 1

Certain difficulties are presented by ignition of the lamp. For this purpose it is necessary to heat the spirals for 1 min., then switch the lamp on to an increased alternating voltage (for example, a 100 W lamp to 300 V at a current of 1 A), and only after this switch it to the normal operating regime. In this case thermal equilibrium is established after about 10–15 minutes of burning.

The infrared radiation of such lamps is excellently modulated by audio frequencies up to 10,000 oscillations per sec. The depth of modulation of the radiation intensity (at 100% modulation of the current) at 200 osc./sec. is 80% and decreases somewhat with increasing frequency; however, even at 5000 osc./sec. it is still equal to 45–25%. In this respect a cesium lamp is already, at a modulation frequency of 1000 hertz, 1000 times more effective than a 60 W incandescent lamp, which is likewise a source of infrared radiation.

The service life of cesium lamps operating in the continuous-modulation regime is somewhat reduced, but nevertheless remains equal to 100–200 hours.

G. N. Rokhlin