Mercury-arc Lamp

Abstract

A mercury-arc lamp formed of a generally cylindrical-shaped tube with electrodes mounted at either end thereof and comprising a line source of light when illuminated from the central portion of the tube between the electrodes and with the tube enlarged adjacent the electrodes so as to reduce devitrification of the tube and provide a long-life mercury-arc lamp.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a mercury-arc lamp and particularly to a mercury-arc lamp which provides a source of fine-line radiation with a high degree of brightness.

2. Description of the Prior Art

In television color picture tubes of the aperture grill or Chromatron (registered trademark) type which are formed of many parallel grid elements constructed of metal wires or strips which extend across the face of the tube at predetermined intervals and are mounted adjacent the color phosphor screen, an electron beam is directed to the screen through the grid elements to excite the phosphors to obtain particular colors.

The phosphor screen of the color picture tube of these types are made up of a plurality of phosphor strips that emit red, green and blue emissions. The strips are sequentially arranged in a repeating cyclic order to obtain the desired color combinations.

The present preferred method for making such phosphor screens comprises placing a grid device on a face plate which has been coated over the entire interior surface with a phosphor slurry of a particular color emissive phosphor and a photosensitive material and the coated interior surface is then exposed to radiation by light from a line source. The exposed photosensitive material hardens and the unexposed photosensitive material is removed so as to leave the phosphor slurry only at those areas which have been exposed. It has been difficult to reduce the tube diameter of the mercury-arc lamp so as to obtain high resolution and the light emitted from such prior art lamps has been fairly wide and the mercury-arc lamps have not been satisfactory as a line source of light. Attempts have been made to use optical lens for reducing the prior art lamps to a line source but this is difficult and a substantial portion of the light is lost due to the lens.

Although the luminous portion in mercury-arc lamps may be rendered extremely fine by reducing the diameter of plasma produced between the electrodes, which can be achieved by reducing the tube diameter of the lamp, this results in a disadvantage in that the reduced tube diameter causes devitrification of the tube envelope due to heat by radiation which shortens the life of the lamp.

SUMMARY OF THE INVENTION

The present invention comprises a mercury-arc lamp which has a tube diameter which is reduced in the central portion to provide a line source of illumination and which has enlarged spherical cavities formed in the tube at either end of the active portion adjacent the electrodes of the lamp so as to avoid devitrification of the tube envelope and to provide fine and linear radiation of high intensity that may be used to make phosphor screens of color picture tubes without using a lens system.

Accordingly, one object of this invention is to provide a mercury-arc lamp of high luminance which provides fine and linear radiation.

Another object of this invention is to provide a mercury-arc lamp which is not subject to devitrification of the tube envelope and is long lasting.

Still another object of the invention is to provide a mercury-arc lamp which is suitable for use as a line source of light in making phosphor screens of color picture tubes without the need of a lens system.

Other objects, features and advantages of the invention will be readily apparent from the following description of certain preferred embodiments thereof taken in conjunction with the accompanying drawings, although variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure, and in which:

FIG. 1 is a schematic view illustrating the optical printing method for making a color phosphor screen;

FIG. 2 is a cross-sectional view of a prior art mercury-arc lamp;

FIG. 3 is an enlarged cross-sectional view illustrating one example of a mercury-arc lamp according to this invention; and

FIGS. 4, 5 and 6 are graphs illustrating the degree of devitrification of the tube envelope and are utilized for explaining the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates apparatus for exposing a phosphor screen for constructing a color television tube, for example. A phosphor slurry 2 is coated on the entire inside surface of a panel 1 of the tube envelope of a color picture tube upon which it is desired to form a color phosphor screen. The phosphor 2 might be formed, for example, of a red color emissive phosphor and a photosensitive binder. An optical mask 3 which has an optical pattern corresponding to the pattern the desired red phosphor strips of the color phosphor screen which will be ultimately formed is placed between the panel 1 and a light source 4 so as to expose the red phosphor strips at the desired locations. The phosphor slurry 2 is placed on the inside surface of the panel 1 and is exposed through the optical mask 3 to irradiation from the light source 4 to form in the slurry a latent image of the optical pattern of the mask 3. Then the interior surface of the panel 1 is subjected to a developing process to obtain red color phosphor strips of a predetermined pattern. The unexposed phosphor between the exposed red color phosphor strips is removed and the process is repeated to place green and blue color emissive phosphors on the panel 1.

The light source 4 is usually a mercury-arc lamp. It is desired that the light source 4 be a linear light source which extends in the longitudinal direction of the phosphor strips that will be ultimately formed so as to insure uniform exposure of the phosphor strips to light throughout their entire lengths. However, prior mercury-arc lamps have not been bright enough to satisfactorily serve as linear light sources and an optical system has been required. It is difficult to produce a linear light beam using a lens system and also the use of a lens system causes a substantial loss of the light energy.

FIG. 2 illustrates a prior art mercury-arc lamp. A pair of electrodes 7A and 7B project into a tube 6 made of quartz glass from either end in the axial direction. Mercury holes 8A and 8B which are filled with mercury 9 are formed at both ends of the tube 6 and an inert gas such as argon, xenon or the like is sealed in the tube 6. It is to be particularly noted that in prior art mercury-arc lamps the inner diameter of the tube 6 is small at the inner ends of the mercury holes 8A and 8B as indicated by the numerals 10A and 10B so as to prevent the mercury 9 from flowing out through the holes 8A and 8B. The inner diameter of the tube 6 is substantially uniform between the ends of the electrodes 7A and 7B.

So as to render the luminous portion of the mercury-arc lamp linear, the diameter of plasma produced between the electrodes 7A and 7B is made small by reducing the diameter of the tube 6. However, this causes devitrification of the tube 6 which results in a loss in the amount of light produced and shortens the service life of the mercury-arc lamp.

The devitrification of the tube 6 is caused by crystallization of the quartz glass or thermal decomposition of silicon dioxide into silicon and oxygen. The crystallization of the quartz glass occurs when it is heated or cooled to about a temperature in the vicinity of the transition point of the quartz glass and once the devitrification of the tube 6 has started it progresses rapidly and spreads. In some cases, this introduces heat distortion in the glass tube and decreases the pressure which the glass can withstand resulting in explosion on the tube 6.

When the inner diameter of the tube 6 is small, the inside surface of the tube 6 is very close to the electrodes 7A and 7B. This allows negative ions emanating from the electrodes to impinge upon the inner walls of the tube with great energy and the electrodes are heated above 1000.degree. or more. Such intense heat causes the closely spaced glass tube to devitrify as described above resulting in short service life of the mercury-arc lamp. When the quartz glass is heated to a high temperature it becomes conductive and creeping discharges occur resulting in a loss of energy. The creeping discharge is directed to the positive electrode but when the mercury-arc lamp is ignited by an AC current, the current reverses in polarity and the creeping discharge is directed to the center between the electrodes of the tube 6. In the case of lighting with AC current, the creeping discharge is intensely produced and loss in energy is as much as several hundred times that which occurs during lighting by DC current. Also, once the creeping discharge has started, it absorbs heat generated by the plasma and devitrification of the tube is increased. This also causes a local increase in the vapor pressure of the tube, thus lowering the luminous efficiency of the lamp and increasing the likelihood of explosion of the tube.

The present invention is illustrated in Figure 3. A substantially cylindrical tube 11 is constructed of quartz glass and rodlike electrodes 12A and 12B partially extend into the tube from both ends thereof in the axial direction.

The ends 12a and 12b of the electrodes 12A and 12B are formed in the form of truncated cones with the base of the cones having a diameter of about 0.5 mm. and the point of the electrodes are tapered to a diameter of about 0.3 mm., respectively. The distance between the electrodes 12A and 12B may be approximately 15 mm.

The tube of this invention is formed such that the inner diameter of the tube 11 is within the range from 0.5 to 1.4 mm. which is much smaller than that of conventional mercury-arc lamps. Also, the inner walls of the tubes adjacent the end portions 12a and 12b of the electrodes 12A and 12B are enlarged to form spherical cavities 14A and 14B. The diameters of the cavities 14A and 14B may be selected such that the distance between the inner wall of the cavity and the end of the electrode is greater than the diameter of the tube in the central portion. When the diameters of the inner diameter of spherical cavities 14A and 14B has been in the range of 2 to 2.5 mm., very satisfactory results have been obtained. Such diameters provide a substantial spacing between the active ends of electrodes 12A and 12B and the inner walls of spherical cavities 14A and 14B and substantially inhibit devitrification.

Narrow portions of the tube adjacent the cavities 14A and 14B are filled with mercury 15. The portions of the tube adjacent the ends are formed of quartz glass, gradiant seal glass 16 as shown and tungsten glass 17 through which the electrodes 12A and 12B pass.

FIGS. 4, 5 and 6 are graphs illustrating the degree of devitrification of the tube 11. In these curves the particular tube of the invention tested had an outer diameter of 4 mm. and an inner diameter of 1 mm. The distance between the electrodes 12A and 12B was 15 mm. The diameters of the electrodes 12A and 12B were 0.5 mm. at the base and 0.3 mm. at the small ends, respectively.

The degree of devitrification is plotted on the basis that "10" indicates that the tube is opaque to a degree such that an electrode in each cavity cannot be seen from the outside of the tube.

FIG. 4 illustrates the degree of devitrification of the tube at the cavities 14A and 14B relative to the lighted time of the mercury-arc lamp when the pressure P of cooling air fed to the lamp was 1.5 kg./cm..sup.2, the diameter d of the cavities 14A and 14B being used as a parameter. For example, it is to be noted that the top curve in FIG. 4 which is labeled d=1.0 mm. has a devitrification degree much higher than diameters of d=1.5 mm., d=2.0 mm. or d=2.5 mm.

FIGS. 5 and 6 show, respectively, the degree of devitrification of the cavities and luminous portion of the tube as a function of the diameters d of the cavities 14A and 14B of the tube 11 with the pressure P of the cooling air being used as a parameter.

The graphs show that the devitrification degree decreases with an increase in the distance between each electrode to the inner wall of the tube but when d=2.5 mm. or more, the improvement of the devitrification degree does not substantially increase with increase of the distance d. An increase in the diameter of the cavities causes an increase in the pressure within the cavities resulting in less mechanical strength of the tube. When d is equal to or less than 2 mm. the devitrification degree is large. Therefore, it is desired that the diameter of the cavities 14A and 14B be in the range of about 2 to 2.5 mm.

The diameter of the main portion of the tube 11 between the electrodes with the exception of the cavities 14A and 14B is selected to be in the range between 0.5 to 1.4 mm. because tubes having diameters of less than 0.5 mm. are low in working efficiency whereas tubes with diameters exceeding 1.4 mm. are not preferred because the diameter of the plasma is increased to such an extent that the lamp is inadequate as a line source of light and also the surface tension of the mercury 15 is exceeded which allows the mercury to flow from the mercury holes when the lamp is mounted in a vertical direction. The tube according to this invention is selected so that it is small and linear irradiation can be produced. Therefore, the use of the mercury-arc lamp of this invention as the light source for optical printing of the color phosphor screen does not need the optical system 5 illustrated in FIG. 1. As a matter of fact, it has been discovered that the mercury-arc lamp of this invention increases the brightness five to 20 times over that of conventional mercury-arc lamps such as illustrated in FIG. 2 with conventional optical systems.

Also the mercury-arc tube of the invention comprises a tube with a small diameter which has small overall area which decreases the overall pressure on the tube and allows it to withstand increased pressure. This increases the mercury vapor pressure and enhances the luminous efficiency of the lamp.

Also, since the electrodes are spaced a substantial distance from the inner wall of the tube due to the cavities 14A and 14B surrounding them, the probability of impingement of negative ions from the electrodes 12A and 12B upon the inner wall of the tube can be decreased and the energy of any ions impinging upon the wall of the tube will also be reduced.

In addition, due to the spacing of the electrode from the inner wall, the so-called thermal layer is provided in the space which reduces the transmission of heat from the electrode to the wall of the tube so that devitrification of the tube can be effectively prevented and the service life of the lamp will be substantially increased.

The tapering of the ends of the electrodes as illustrated at 12a and 12b, reduces the transmission of heat from the heated electrodes and heat radiation is low. Therefore, the electrodes can be maintained at a high temperature and an electric charge concentrated on the electrodes to insure efficient discharge between the electrodes.

It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of the present invention.

Claims

We claim as our invention:

1. A mercury-arc lamp comprising:

a tube made of glass;

two electrodes sealed in the tube at opposite ends thereof; mercury contained in the tube at both ends thereof; an inert gas sealed in said lamp;

enlarged spherical cavities formed in the tube about the ends of said two electrodes;

the inner diameter of the main portion of the tube being in the range of 0.5 to 1.4 mm.;

the inner diameters of said spherical cavities being greater than 1.5 mm.; and

said two electrodes being circular in cross section and having diameters of about 0.5 mm.

2. A mercury-arc lamp according to claim 1 wherein said two electrodes are formed with truncated tapered ends.

3. A mercury-arc lamp according to claim 2 wherein the inner ends of said two electrodes are about 0.3 mm.