Metal vapor discharge lamp

Abstract: Disclosure is given for a technique of preventing arc displacement from occurring in an iron-filled metal vapor discharge lamp which additionally contains magnesium so as to prevent the iron from depositing on the internal wall surface of the bulb (1) of said lamp and forming a thin iron film. Namely, in the disclosed metal vapor discharge lamp arc displacement can securely be prevented by incorporating thallium in an amount of 3.2 x 10⁻⁹ to 2.0 x 10⁻⁷ mol per cc of content volume of the bulb.

Description: This invention relates to a metal vapor discharge lamp to be employed for photochemical reactions or curing of paints and inks. Ultraviolet rays are often used for inducing photochemical reactions or curing paints and inks, and those having a wavelength in the range of about 280 to 400 nm are useful for such purposes. Metal vapor discharge lamps are often used as sources of such ultraviolet rays. Such a metal vapor discharge lamp has a pair of electrodes disposed at each end of a tubular bulb to oppose each other, and the tubular bulb contains a metal halide together with sufficient amounts of mercury and a rare gas so as to maintain an arc discharge, in order to to increase the quantity of light emission in the effective wavelength range. Particularly those metal vapor discharge lamps having incorporated therein iron in the form of a metal halide are conveniently used for the purpose of curing paints and inks, since they can continuously emit ultraviolet rays in the wavelength range of 350 to 400 nm. In other words, since the iron-filled metal vapor discharge lamp can emit a large quantity of ultraviolet rays, the ultraviolet rays can penetrate the internal portion of the paint or ink layer quickly, to allow curing of the material in a short time. However, if such an iron-filled metal vapor discharge lamp is lit for a long time, the iron deposits on the internal wall surface of the bulb to form a thin film, so that the amount of the iron which can contribute to light emission will be reduced, and the thus-formed thin iron film prevents transmission of ultraviolet rays therethrough so reducing the output of the ultraviolet rays. In order to solve the above problem, the present inventors have previously developed a metal vapor discharge lamp in which magnesium is additionally incorporated so as to maintain the output of the ultraviolet rays for a long time (Japanese Patent Application No. 220325/1985; USP 4,769,576). Such a metal vapor discharge lamp is occasionally used at a lower power demand than the rated power depending on the application. Once the metal vapor discharge lamp is turned off, it requires several minutes until it can be lit again. Accordingly, when materials are irradiated with ultraviolet rays intermittently, on/off operation of the metal vapor discharge lamp cannot be repeated in short cycles, so that the lamp must be shielded with a shutter when the radiation should be screened. Thus, while the shutter is closed, the metal vapor discharge lamp is lit on at a power of, for example, 1/2 the rated power. Therefore, in the case of a magnesium-filled metal vapor discharge lamp and when it is lit at a power which is lower than the rated power, the following problems arise. As shown in Fig. 1, the arc 4 to be generated between the electrodes 2,3 disposed to oppose each other at both internal ends of the tubular bulb 1 displaces in the vicinity of the electrodes 2,3 as indicated with X. Such an arc displacement phenomenon occurs at either or both of the electrodes 2,3, and the degree and state thereof vary widely. When such an arc displacement occurs, the portion of the bulb immediately above the displaced portion X is overheated by the arc approaching thereto. Accordingly, the quartz glass becomes opaque and has a reduced life. In the case where the metal vapor discharge lamp is used in combination with a long converging reflector having an ellipsoidal cross section or the like, the position of the arc in the metal vapor discharge lamp coincides with the focal point of the reflector. However, the light at the displaced portion X deviates from the focal point and cannot be utilized, a low light utilization efficiency resulting. It is a first object of this invention to provide a metal vapor discharge lamp having mercury, iron and magnesium filled therein, in which no arc displacement occurs when it is lit at a power which is lower than the rated power, so that the bulb does not become opaque. It is a second object of this invention to provide a metal vapor discharge lamp having mercury, iron and magnesium filled therein, in which the light utilization efficiency does not drop when it is used in combination with a converging reflector. These objects are achieved by a metal vapour discharge lamp according to the claim. The present inventors found that thallium (Tℓ) is effective for achieving the above objects, and after they made various experiments they accomplished this invention. To describe in detail, displacement of the arc can be prevented by incorporating thallium in an amount of 3.2 x 10⁻⁹ to 2.0 x 10⁻⁷ mol per cc of the content volume of the bulb even when the metal vapor discharge lamp is lit at a power which is considerably lower than the rated power. Therefore, the bulb is prevented from becoming opaque by overheating and can enjoy a long life. When the metal vapor discharge lamp is used in combination with a converging reflector, the arc coincides with the focal point of the reflector, so that no reduction in the light utilization efficiency will occur. Incidentally, if the amount of thallium is less than 3.2 x 10⁻⁹ mol per cc of the content volume of the bulb, the effect of preventing displacement of the arc may not sufficiently be exhibited; whereas if it is more than 2.0 x 10⁻⁷ mol per cc, the output of the luminescence spectrum of mercury, iron and magnesium will be lowered, whereby the intensity of the ultraviolet rays in the effective wavelength range of 280 to 400 nm will consequently be lowered, disadvantageously. Fig. 1 is an illustration of a metal vapor discharge lamp. The metal vapor discharge lamp according to this invention has a basic structure as shown in Fig. 1; wherein a pair of electrodes 2,3 are disposed in a tubular bulb 1 made of quartz having an inner diameter of 22 mm at each end to oppose each other with a distance of 750 mm therebetween. This metal vapor discharge lamp is operated at a rated power of 9 KW and has a content volume of 285 cc. The bulb 1 contains 285 mg of a metal mercury (Hg), 30 mg of mercury iodide (Hg₂I₂), 5.0 mg of iron (Fe), 0.7 mg of magnesium, 1.9 mg of thallium iodide and 4000 Pa (30 mmHg) of xenon gas. The amount of thallium to be incorporated is 2.0 x 10⁻⁸ mol per cc of the content volume of the bulb 1. When such a metal vapor discharge lamp was lit at a rated power of 9 KW, the lamp current was 10.3 A with no displacement of the arc 4. When the above metal vapor discharge lamp was used in combination with a converging reflector, the arc entirely coincided with the focal point of the reflector, so that no reduction in the light utilization efficiency occurred. Next, such lamps containing thallium in various amounts were prepared (20 pieces per amount) to investigate the arc displacement and the variation in the ultraviolet output by operating them at a power of 4.5 KW which is 1/2 the rated power, and the test results are as shown in Table 1. Incidentally, in Table 1, the mark X in the column of Arc displacement means that an arc displacement occurred at both end portions of the lamp in 18 lamps out of 20, and at either end portion of the lamp in the remaining 2 lamps; and the mark ○ means that no arc displacement occurred in any of 20 lamps. The numerical values in the column of the Ultraviolet output are relative values in the wavelength range of 280 to 400 nm. As apparent from Table 1, when the amount of thallium incorporated is 2.5 x 10⁻⁹ mol/cc, an arc displacement was observed. However, in those cases where said amount is 3.0 x 10⁻⁹ mol/cc or more, no arc displacement was observed. On the other hand, it can be seen that the ultraviolet output tends to be lowered as the amount of thallium is increased. In other words, when the amount of thallium added is 2.0 x 10⁻⁷ mol/cc, the loss in the ultraviolet output is only 10 %, such a loss being not at all significant and allowing practical application of these metal vapor discharge lamps. However, when the amount of thallium incorporated is 2.2 x 10⁻⁷ mol/cc, the loss in the ultraviolet output will greatly be increased to 17 %. Consequently, 3.2 x 10⁻⁹ to 2.0 x 10⁻⁷ mol/cc of thallium to be added is practical since the ultraviolet loss in the effective wavelength range of 280 to 400 nm is small and practical with substantially no arc displacement. In the metal vapor discharge lamp according to this invention, the iron will not be deposited on the internal surface of the bulb 1 even after extended lighting to form a thin iron film, so that the output of the ultraviolet rays can be maintained. On the other hand, the effect of the thallium additionally incorporated to the metal vapor discharge lamp on the spectrum output of mercury and those of iron and magnesium was investigated. It was found that in the metal vapor discharge lamp of this embodiment, the loss in the spectrum output of mercury was 4 %, that of iron was 4 %, and that of magnesium was 1 %, which are insignificant values compared with the metal vapor discharge lamp containing no thallium. Accordingly, the above data proves that if thallium is added in an amount of 2.0 x 10⁻⁸ mol/cc of the content volume of the bulb 1, the spectrum outputs of mercury, iron and magnesium will not be affected by the thallium. Further, the addition of thallium has no substantial effect on the electrical characteristics such as starting current and reignition voltage. While a thallium halide was used in this embodiment, it may be replaced by the metal thallium. The same effect can be obtained by using a mixture of such materials.

Claims: A metal vapor discharge lamp comprising a bulb and a pair of electrodes disposed therein, containing suitable amounts of iron, halogen and magnesium together with sufficient amounts of mercury and a rare gas so as to maintain an arc discharge, characterized in that thallium is additionally incorporated in an amount of 3.2 x 10⁻⁹ to 2.0 x 10⁻⁷ mol per cc of the content volume of the bulb.

Inventors: FUSE TETSUO, OHYAMA MASACHIKA, YAMAGUCHI AKIYASU, FUSE, TETSUO, OHYAMA, MASACHIKA, YAMAGUCHI, AKIYASU
Applicants: USHIO ELECTRIC INC, USHIO DENKI KABUSHIKI KAISHA
Application year: 1991
Main classification: H01J61
IPC codes: H01J61