Modern Pulsed Light Sources

Recently, articles devoted to pulsed light sources and their practical application have been appearing in the literature with increasing frequency. Pulsed light sources are characterized by high values of energy and current densities during the flash, which arises as a result of the discharge of a capacitor through a gas-filled tube. Depending on the electrical parameters of the circuit and on the design of the pulsed lamp, the duration of the flash lies in the range from \(10^{-6}\) to \(10^{-3}\) sec. Increasing the capacitance of the discharge capacitor increases the duration of the radiation and its total energy. To obtain the shortest possible flashes at a selected capacitance value, the self-inductance of the discharge circuit must be minimal. However, from the point of view of the service life of the lamp, in a number of cases it is useful to introduce into the discharge circuit a self-inductance with minimal ohmic resistance.

In the works reviewed, pulsed lamps consisted of a glass or quartz tube with an internal diameter from 4 to 24 mm, with tungsten or aluminum electrodes sealed into the ends and with an interelectrode gap from 3 to 78 cm. In most cases, lamps longer than 30 cm were made in the form of a spiral. The lamps were filled with inert gas, predominantly krypton or xenon, at a pressure of several tens of millimeters of mercury. The instant of the lamp flash is determined by applying a high-voltage pulse to the ignition electrode. Most often the ignition electrode consists of several turns of wire wound around the lamp. In the works of Laporte[^1],[^2], discharge control was carried out by means of a thyratron connected in series with the lamp. The lamp had a length of 20 cm, an internal diameter of 8 mm, and was filled with xenon to a pressure of 35 mm Hg. The author indicates that for a tube of this size this pressure is optimal. With a discharge capacitor capacitance of 10 μF and a voltage of 6000 V, Laporte obtained average luminous fluxes of 70 million lumens, which, for a duration of 100 μsec (the flash duration was measured by the method of mirror scanning), gives a luminous efficacy of 40 lumens/watt. The radiant energy of the lamp was proportional to the energy stored in the discharge capacitor. The author indicates that when the tube is filled with krypton, its luminous efficacy decreases by a factor of 1.5.

Warmoltz and Gellmer[^3], using, for illumination of a Wilson chamber, a xenon-filled tube 20 cm long and with an internal diameter of 4 mm, obtained approximately the same luminous efficacy value—42.5 lumens/watt—and proportionality between the radiant energy and the energy stored in the capacitor. In the same work it is indicated that the radiation of a xenon-filled pulsed lamp is close to the radiation of a black body at \(6000^\circ\text{K}\).

Various designs of tubular pulsed lamps intended for energies from 62 to 16,000 joules per flash are described in paper 4. Owing to the instantaneous heating and expansion of the inner surface of the tube, at such high energies rupture of the lamp occurs. The authors point to the necessity of using quartz as the material for pulsed lamps at elevated discharge energy. The article gives curves of current, voltage, instantaneous power, and luminous flux versus time (see figure) for a lamp filled with xenon to a pressure of

1 — current, 2 — voltage, 3 — power, 4 — luminous flux.

60 mm Hg, with a capacitor capacitance of 25 μF and a voltage of 2000 V. From these curves it is evident that the maximum of the luminous flux noticeably lags behind the maximum of the power.

In all the papers reviewed, numerous photographs are presented of tracks of nuclear particles obtained in a Wilson chamber when illuminated by pulsed lamps. The possibility is indicated of using pulsed lamps as a source of powerful short-duration pulses of infrared and ultraviolet radiation. Thus, according to approximate measurements by Laporte², the ratio of the energy of infrared radiation to the energy of the visible region of the spectrum is 25%. According to Aldington and Meadowcroft⁴, the region of infrared radiation of xenon-filled pulsed lamps extends at least to 11,000 Å. The same authors note the presence of rich ultraviolet radiation (to 22,000 Å), which can be used to alter the color of a pulsed discharge. The possibility is also indicated of using pulsed lamps for night aerial photography.

L. Ch.

REFERENCES CITED

1. M. Laporte et Teillac. J. Phys. et Rad. 9, 253 (1948).
2. M. Laporte, R. Gros et J. Roux. Comptes Rendus 226, 1235 (1948).
3. N. Warmoltz and A. M. C. Helmer. Philips Techn. Rev. 10, 178 (1948).
4. J. N. Aldington and A. J. Meadowcroft. J. Inst. Electr. Eng. 95, 671 (1948).