Nowadays, projection-type image display devices are rapidly becoming commonplace. Such a display device is often equipped with a high pressure discharge lamps as its light source. Among various high pressure discharge lamps, short-arc type high pressure discharge lamps that function virtually as point sources of light and are easy to arrange and control, are widely used as the light sources of liquid crystal projectors, which are becoming particularly popular these days. Typical examples of said short-arc type high pressure discharge lamps include xenon lamps and metal halide lamps.
A liquid crystal projector is an apparatus adapted to project images onto a screen by using a liquid crystal display panel and is capable of clearly projecting not only static images but also moving pictures. As liquid crystal projectors are capable of projecting various images based on signals representing the images regardless of whether the input signals are analog or digital, they are used as a tool for making presentations, a screen in a make-shift theater, or as the screen of a large-screen television, and their demands are expected to keep increasing in the future.
Compared with such inexpensive image display devices as CRT displays, liquid crystal projectors are very expensive. Therefore, efforts are being exerted to provide a less expensive liquid crystal projector by making it compact or otherwise reducing its production costs. Furthermore, as there is still room for further improvement in brightness, progress is being made to employ a special optical system.
As it is mentioned above, a high pressure discharge lamp is popularly used as a light source device for a liquid crystal projector, and what are now required of a high pressure discharge lamp are a reduced distance between the electrodes, a higher efficiency and a longer life span. In addition to these demands, ensured reliability and reduction of production costs, which is the present objective of the liquid crystal projector, are required of high pressure discharge lamps.
In most cases, a high pressure discharge lamp used in such an image display apparatus as mentioned above is installed in the apparatus in such a manner that it will be lit in the horizontal position. When a lamp is lit in the horizontal position, a convection current generated in the arc tube makes the temperature of the upper part of the inside of the arc tube higher than the other part, thereby making the temperature distribution in the entire arc tube nonuniform. In order to solve the problem of nonuniformity of temperature distribution, an air cooling device, such as a fan, is installed.
One of the important processes of producing a high pressure discharge lamp is the sealing process to form the inside of the arc tube into an airtight space. The sealing process is conducted by the pinch sealing method, which enables the quick and efficient processing. A portion sealed by the pinch sealing method can be maintained in a sufficiently airtight state having the ability to withstand the high internal pressure and ensures reliable functioning of the lamp in desired performance characteristics for a long period of time.
The arc tube has to be provided with two sealed portions: one each at the cathode side and the anode side. A metal foil conductor which is made of molybdenum and connected to the cathode is attached to one of the sealed portions, while another molybdenum metal foil conductor, which is connected to the anode, is attached to the other sealed portion. A lead wire made of molybdenum extends from each sealed portion to the outside of arc tube. Inside each sealed portion, the metal foil conductor and the lead wire are connected by welding. As the outside atmosphere enters the portion where they are welded, the welded portion is in a condition similar to that if it were exposed to fire. As a result, when it is in a high temperature environment for a long time, it is prone to oxidation and possible breakage. There is a demand for an effective means to solve this problem by maintaining the welded portions at a lower temperature.
This may be achieved by somehow reducing the heat conducted from the heat source or cooling the welded portion to limit increases of temperature. However, a cooling method is almost never employed, because noises produced by an air blower, which may be an air cooling fan or the like, make it difficult to attain the objective of reducing noises. Therefore, lamps produced these days usually employ a method which calls for increasing the length of either one of or each sealed portion in order to reduce the heat conducted from the source of heat generation. FIG. 7 is a characteristic graph showing the relationship between distances from various points to the surface of the discharge space of the arc tube, which is the source of heat generation, and temperatures at said points. When the distance from the surface of the discharge space of the arc tube exceeds 30 mm, the temperature is in the permissible range to ensure that the sealed portion achieves the expected life expectancy.
However, in case the lamp is a small high pressure discharge lamp described above with the length of the portion to be sealed exceeding 30 mm, it is difficult to seal the target portion by the pinch sealing method. In other words, in case of a small high pressure discharge lamp, it is impossible to provide a sufficiently long distance from the center of the arc, which serves as the source of heat generation.
Accordingly, the first object of the present invention is to ensure that the sealed portion achieves the expected life expectancy and to provide a high pressure discharge lamp, a high pressure discharge lamp device and a light source device using said high pressure discharge lamp, wherein the lamp has sealed portions formed by the pinch sealing method, and wherein a sealed portion is maintained at a desired distance from the heat source.
As described above, an arc tube to be lit in the horizontal position may be provided with an air cooling device, such as an air blower, in order to make the temperature distribution uniform throughout the inside of the arc tube. However, installing an air cooling device not only presents the problem of noises but also presents another problem in that it may become impossible to obtain necessary luminescence characteristics if the arc tube is cooled excessively. In other words, control of the air cooling device is difficult. Although this problem may be overcome by lighting the arc tube in the vertical position, vertical lighting causes substantial increase of the temperature of the upper sealed portion and, therefore, tends to enhance oxidation of the metal foil sealed therein and makes the metal foil even easier to break.
Accordingly, the second object of the present invention is to ensure that the sealed portion achieves the expected life expectancy when the lamp is used in the vertical position and to provide a high pressure discharge lamp, a high pressure discharge lamp device and a light source device using said high pressure discharge lamp, wherein the possibility of breakage of a metal foil is reduced even if the lamp is used in the vertical position.
As it is mentioned above, floodlight illumination and image display devices are rapidly becoming commonplace over recent years, and high pressure discharge lamps are used as their light sources in most cases. Among various high pressure discharge lamps, short-arc type high pressure discharge lamps that function virtually as point sources of light and are easy to arrange and control, such as metal halide discharge lamps, are widely used as the light sources of liquid crystal projectors, which are becoming particularly popular these days in addition, using a metal halide discharge lamp as the light source of a vehicle headlight is becoming more commonplace, while small, short-arc type high pressure discharge lamps are widely used as spot lights for store illumination and other purposes.
Next, examples of conventional high-pressure discharge lamps for liquid crystal projectors are explained hereunder, referring to drawings.
FIG. 19 is a front view of a first example of conventional high-pressure discharge lamps for liquid crystal projectors.
Referring to FIG. 19, numeral 101 denotes a discharge casing, numeral 102 an anode, 103 a cathode, 104, 104 sealed molybdenum foils, 105 an external lead wire, and 106 a base.
The discharge casing 101 consists of a discharge space portion 101a and a pair of sealed portions 101b, 101c respectively extending from the two opposing ends of the discharge space portion 101a. A sealed molybdenum foil 104 is airtightly buried in each sealed portion 101b, 101c. An appropriate quantity of discharge medium comprised of mercury, a rare gas and a halide of luminous metal is sealed in the discharge space portion 101a.
The anode 102 has an electrode shaft 102a, which is inserted in the sealed portion 101b and welded to an end of the sealed molybdenum foil 104. The external lead wire 105 is welded to the other end of the sealed molybdenum foil 104.
In the same manner as the anode, the electrode shaft 103a of the cathode 103 is inserted in the other sealed portion, i.e. the sealed portion 101c, and welded to an end of the sealed molybdenum foil 104 buried therein. Another external lead wire (not shown in the drawing) is welded to the other end of the sealed molybdenum foil 104 and connected to a cathode terminal 106a of the base 106.
The base 106 has a cylindrical body 106b, which is made of stainless steel and bonded to the cathode-side sealed portion 101c.
Next, a liquid crystal projector is explained.
A liquid crystal projector is a device adapted to project an image formed on a liquid crystal screen, regardless of whether the image is static or moving picture. As it is capable of projecting various images based on signals which may be video signals or computer-generated images input into the projector, the projector is used in various fields; for example, it may be used as a tool for making presentations, a screen in a make-shift theater, or as the screen of a large-screen television.
The rear-projection TV has become more commonplace among large-screen televisions over recent years. When a liquid crystal projector is used as a television, performance characteristics that are equivalent to those of a television of the cathode-ray tube type are required of the projector. For example, the projector may be required to have a life span of more than 10,000 hours and be ensured to operate to project sufficiently visible images immediately after being switched on, and be capable of immediate restart even if it is switched off by mistake.
Among various factors required of the liquid crystal projector, superior brightness and lower costs are given the priority. Therefore, metal halide-type ultra high pressure discharge lamps are most widely used nowadays. In order to obtain more intense luminance, a ultra high pressure discharge lamp is usually designed such that the internal pressure in the discharge casing amounts to more than 30 atmosphere (30 atm) during the time that the lamp is lit. Although safety is sought for by increasing the thickness of the wall of the discharge casing, which is made of quartz glass, or devising a safer shape, there is still the possibility for the discharge casing to burst before the end of its life span. Breakage of the discharge casing may result in damage to other optical parts.
Therefore, a structure that calls for integrating a reflecting mirror with the high pressure discharge lamp is employed in order to prevent or minimize scattering of the glass in case of breakage of the discharge lamp, or in order to facilitate maintenance of the lamp, such as adjusting the position of the optical parts. Other examples of widely employed structures include one that calls for covering the aperture of the reflecting mirror with a glass protector plate.
Next, a metal halide discharge lamp used in a headlight of a vehicle is explained hereunder.
A metal halide high pressure discharge lamp for a vehicle headlight is primarily required to be capable of performing instantaneous start-up and restart more quickly than does a metal halide high pressure discharge lamp for a liquid crystal projector. For this reason, a metal halide high pressure discharge lamp for a vehicle headlight is typically filled with xenon gas at approximately 5 atm at normal temperature as well as mercury that will serve as a buffer medium during the time when the lamp is in the lit state, with the pressure of mercury vapor then amounting to more than 20 atm.
Other than metal halide discharge lamps described above, mercury lamps are included in the examples of a high pressure discharge lamp whose discharge medium includes mercury serving as a luminous body or a buffer medium. Whereas metal halide discharge lamps use mercury as a buffer medium, mercury lamps use mercury as a luminous medium.
In case of any one of the high pressure discharge lamps mentioned above, the mercury contained in the discharge casing condenses on the surface of the electrodes when the lamp is turned off. This is because the electrodes cool down more quickly than does the inner surface of the discharge casing.
In case of a high pressure discharge lamp of a DC-powered type, whose cathode is smaller than the anode and therefore cools faster, the mercury tends to become attached to the cathode.
Mercury thus adhering to the surface of an electrode reduces electron emission from the cathode and consequently impairs such performance of the lamp as start-up and restart.
In cases where a high pressure discharge lamp is enclosed by a reflecting mirror and a glass protector plate or disposed in an outer tube, the high pressure discharge lamp does not easily cool down after it is switched off. Restarting such a high pressure discharge lamp when the vapor pressure of the discharge is still high requires start-up voltage having substantial pulse energy to be applied between the electrodes.
However, increasing the pulse energy of the start-up voltage not only makes insulation of the lighting components difficult and expensive but also increases radiated noises. In case of a device incorporating a microcomputer, increase in noises may cause malfunction of the microcomputer,
A structure aimed at improving start-up performance is disclosed in Japanese Provisional Publication No. 61957/1990.
FIG. 20 is a front view of a second example of conventional high-pressure discharge lamps, which has the same structure as the one disclosed Japanese Provisional Publication No. 61957/1990. The elements corresponding to those in FIG. 19 are identified with the same reference numerals, explanation of which is omitted herein.
The second example of conventional high pressure discharge lamps shown in FIG. 20 is characterized in that a starting conductor 107 having the same electrical potential as that of the cathode 103 extends along the cathode-side sealed portion 101c and, after being wound once around the borderline between the discharge space portion 101a and the sealed portion 101c, further extends along the discharge space portion 101a and that the end portion of the starting conductor 107 is then wound once around the borderline between the discharge space portion 101a and the anode-side sealed portion 101b, which is located opposite the sealed portion 101c.
FIG. 21 is a front view of a third example of conventional high-pressure discharge lamps. The elements corresponding to those in FIG. 19 are identified with the same reference numerals, explanation of which is omitted herein.
The third example of conventional high pressure discharge lamps shown in FIG. 21 has a structure such that a starting conductor 108 having the same electrical potential as that of the cathode. 103 extends along the cathode-side sealed portion 101c and that the end portion of the starting conductor 107 is wound once around the borderline between the discharge space portion 101a and the sealed portion 101c.
Compared with the first example of conventional high pressure discharge lamps, both the second and third examples described above are capable of reducing the start-up voltage. However, neither example is capable of reducing the start-up voltage to a desired level. Furthermore, there is no significant difference in effectiveness between a high pressure discharge lamp according to the second example and a high pressure discharge lamp according to the third example.
In other words, according to any one of the examples of conventional art described above, restarting the lamp within several dozens seconds of turning off the lamp requires application of a pulse voltage having substantial energy. Thereafter, the pulse energy gradually decreases. In addition, from 3 to 5 minutes after turning off the lamp, there is a period when great pulse energy is needed again.
Accordingly, the third object of the present invention is to provide a high pressure discharge lamp having superior performance characteristics in start-up and restart-up while having a simple structure, and also provide a high pressure discharge lamp device and a light source device using said high pressure discharge lamp.