1. Field of the Invention
The present invention relates to a lamp and a manufacturing method of that lamp that has a special structure of electrode sealing and whose internal pressure become one atmosphere or more when operated to light it.
2. Description of the Prior Art
Conventionally, high intensity discharge lamps have been widely used for ordinary illumination in homes, facilities and stores. In recent years, these lamps are being used as light sources for overhead projectors, projection televisions and motion picture projectors. The reason for this is because high intensity discharge lamps emit an extremely bright light.
In particular, in recent years, research has been active on ways to bring lamps closer to a point light source by shortening the length of the discharge arc. However, reductions in lamp voltage occur following efforts to shorten the length of the discharge arc. Therefore, when an attempt is made to operate a lamp using an identical voltage, increases in lamp current generate. These increases in lamp current are linked to large increases in electrode loss, actively vaporize electrode materials worsening the early stages of electrodes. Namely, the increases in lamp current cause the lifecycles of the lamps to shorten. From this type of reason, when shortening the arc length, it is normal to increase the mercury vapor pressure when operating the lamp is made to protect against reductions in lamp voltage (increases in lamp current).
When the mercury vapor pressure and other similar parameters are made to increase when operating the lamp, the lamp must be constructed such that it will not be cracked due to that high operating pressure.
FIG. 11 shows the structure of a conventional discharge lamp. In the figure, 100 is a light emission portion wherein exists a discharge arc and 101 is a side tube portion that extends from light emission portion 100. Light emission portion 100 and side tube portion 101 are both comprised by quartz glass.
A gas that becomes a high pressure when the lamp is operated is sealed in light emission portion 100. Further, 102 is an electrode that functions to supply electrical current into light emission portion 100. The electrode material is normally tungsten. In comparison to the thermal expansion coefficient of tungsten of 5.2.times.10.sup.-6, the thermal expansion coefficient of quartz glass is 5.5.times.10.sup.-7 that is almost one decimal place different. Technology for sealing methods of two types which differ greatly in this way is difficult.
For a sealing method for this case a foil sealing structure is known wherein a metal foil 104 connects between electrode 102 and an external electrical current supply line 103 and glass being sealed airtight in this metal foil. By carrying out plastic deformation on an extremely thin metal foil, the difference in the thermal expansion coefficient between the glass and the metal is absorbed making it possible to obtain a seal.
Conventionally, pinch sealing is a manufacturing method of this foil sealing structure lamp. In the following, conventional pinch sealing will be described referring to FIG. 12. Glass tube 110 is formed by a separate process in which a quartz glass tube is heated and allowed to expand forming light emission portion 100 in a specified shape. A quartz glass tube that is not deformed is connected to both end portions of light emission portion 100 as side tube portion 101. Glass tube 110 is retained by a chuck 113. The end portion of electrode 102 is disposed on light emission portion 100 to maintain a discharge arc. And also, electrode 102, metal foil 104 (connected to the other end portion of electrode 102) and electrical current supply line 103 are disposed on side tube portion 101.
Further, in order to prevent electrode oxidation during the sealing process, side tube portion 101 maintains in a rare gas environment. The glass of this side tube portion 101 is thermally fused by a burner 111 and then pressure formed by a forming die 112 from two directions perpendicular to the surface of metal foil 104.
Problems to be Solved
The following two problems exist when using this type of sealed lamp.
Electrode 102 and the glass of side tube portion 101 have different thermal expansion coefficients and there is no airtight seal. Thereupon, a gap can be opened between electrode 102 and the glass of side tube portion 101.
FIG. 13 shows the cross sectional shape of the side tube portion along line 105 shown in FIG. 11. In the figure, 120 is a side tube portion glass. Further, 121 is a gap between electrode 102 and side tube portion glass 120. The shape of gap 121 has a sharp notch 122 due to squeezing from two directions of the glass. There was a problem of a concentration of stress acting on sharp notch 122 and the lamp being damaged due to a pressure lower than the pressure strength actually possessed by the glass.
The second problem is a crack 106 shown in FIG. 11. This crack 106 occurs in the side tube portion glass at the position of electrode 102. The percentage of cracks which occur during sealing is larger than cracks which occur due to differences in the thermal expansion coefficients of the electrode and the glass. However, this crack has an action that is said to lessen the stress occurring between the electrode and the glass when lighting and extinguishing the lamp. Because of this, cracks which occur due to differences in the thermal expansion coefficients do not interfere with the lamp.
Cracks which occur because of differences in the thermal expansion coefficients however, occur due to another mechanism. The electrode does not cause plastic deformation as with a metal foil. Because of this, if the electrode is struck by the side tube portion glass with a strong force, the glass will crack due to that impact. A concentration of stress will generate at the tip of this crack which will further lower the pressure strength of the lamp. In other words, there is a problem of cracks occurring due to factors other than differences in the thermal expansion coefficients of the glass and the electrode.
Thereupon, a shrink seal method is used to solve the above-mentioned two problems. An example of a shrink seal method is shown in FIG. 14. Glass tube 110 is retained by chuck 126. The end portion of electrode 102 is disposed on light emission portion 100 to maintain a discharge arc. And also, electrode 102, metal foil 104 (connected to the other end portion of electrode 102) and electrical current supply line 103 are disposed on side tube portion 101. A reduced pressure state is maintained inside glass tube 110. While this glass tube 110 is rotated in the circumferential direction of the tube (approximately indicated by arrow 128), side tube portion 101 is thermally fused uniformly by burner 127. Side tube portion 101 glass undergoes diameter reduction by means of a pressure difference between the inside and outside of glass tube 110 and then metal foil 104 and side tube portion 101 glass positioned where the metal foil is located are sealed airtight.
According to this method, because the glass undergoes diameter reduction towards the electrode, the shape of the gap between the glass and the electrode becomes almost circular eliminating the notch portion that generates a concentration of stress. Further, because the sealing pressure does not exceed the atmospheric pressure, the glass does not receive any impact when sealed.
However, because the sealing pressure of the metal foil portion does not exceed one atmosphere in this shrink seal method, there are still remaining problems of an insufficient amount of plastic deformation of the metal foil and a weak seal between the metal foil and the glass tube.
Thereupon, a method has been attempted that uses a die to evenly squeeze the glass (for example, a polygon shaped die or a circular die) in order that the shape of the gap between the electrode and the glass does not have a notch portion and that additionally removes cracks occurring in the side tube portion glass positioned where the electrode is located from the rear.
For example, Japanese Patent Laid-open Publication (Kokai) HEI 5-159743 discloses a method which attempts to eliminate cracks by reheating and gradually cooling the side tube portion after pinch sealing.
However, in order to eliminate cracks, the glass temperature must be increased up to the softening point. The softening point of quartz glass is 1683.degree. C. FIG. 11 shows the state of crack 106. In particular, a location 122 (FIG. 13) where a crack occurs is adjacent to light emission portion 100. Because an electrode is embedded in side tube portion 101, light emission portion 100 is also greatly affected by temperature. Light emission portion 100 is formed in an approximate spherical shape and the tip of the light emission portion adjacent to the side tube portion has a thin skin of glass making it especially vulnerable to deformation due to temperature increases. Deformation of the light emission tube changes the temperature of the coolest point inside the light emission tube when the lamp is operated (lowest point in the direction of the gravitational force of light emission portion 100 when the axial direction of the side tube portion is set in the horizontal direction and used in that manner). The vapor pressure of the light emitting material inside the lamp is determined by the coolest temperature point inside the light emission tube when the lamp is operated. In other words, deformation of the light emission tube causes the vapor pressure of the light emitting material inside the lamp to change thereby changing the spectral distribution characteristics. Because of these factors, eliminating cracks after the sealing process is passed through is difficult.
Further, an explanation was provided for the above-mentioned discharge lamp although this is not a specific problem for a discharge lamp and when hermetically sealing an electrical current supply line inside a glass tube, the same problems occur. In other words, the same problems exist in an incandescent lamp of a halogen light bulb.
The object of the present invention is to take these factors into consideration and provide a lamp with the following improvements. Eliminates concentration of stress occurring in the gap between the glass and electrode. Controls to a minimum the occurrence of cracks which occur due to factors other than differences in the thermal expansion coefficients of the glass and the electrode. And in addition has a high pressure strength structure with improved adhesiveness between the metal foil and the glass.
Means for Solving the Problem
In order for the present invention to achieve the above-mentioned objects, a method is used to produce a discharge lamp wherein an electrode assembly is hermetically sealed that comprises at least an electrical current supply line and a metal foil connected to the electrical current supply line to produce a lamp in the following process. A side tube portion housing the metal foil portion is compressed at the metal foil portion by a pressure higher than the pressure that compresses the side tube portion at the electrical current supply line. As a result, the process hermetically seals the metal foil portion in a state in which the electrode assembly (positioned such that one portion of the electrode is inside the light emission portion) is inserted inside a glass bulb that comprises at least a light emission portion and a side tube portion extending into the light emission portion.
Further, the lamp of the present invention is characterized by having a light emission portion comprised by glass, a side tube portion that extends from the light emission portion and is comprised by glass as well as an electrical current supply line with one portion arranged inside the light emission portion, one end portion connected to a metal foil, and that is hermetically sealed in the side tube portion. The lamp is further characterized by the lateral cross sectional shape in the perpendicular direction of the axis of the electrical current supply line of the gap between the electrical current supply line and the side tube portion having a shape similar to the cross section of the electrical current supply line as well as the side tube portion glass located at the position of the metal foil portion being compression formed by a die.
The lamp of the present invention is further characterized by having an electrode with one portion arranged inside the light emission portion together with one end portion connected to a metal foil. The end portion connected to the metal foil is hermetically sealed in a side tube portion extending from the light emission portion. Furthermore, the lateral cross sectional shape in the perpendicular direction of the axis of the electrical current supply line of the gap between the electrical current supply line and the side tube portion has a smooth shape without notches which cause a concentration of stress, and the side tube portion positioned where the metal foil is located is compression formed by a forming die.