Electric lamp and discharge devices – With optical device or special ray transmissive envelope – Reflector
Reexamination Certificate
2001-07-26
2003-02-04
Wong, Don (Department: 2821)
Electric lamp and discharge devices
With optical device or special ray transmissive envelope
Reflector
C315S20000A, C313S111000
Reexamination Certificate
active
06515406
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a short-arc high-pressure mercury vapor discharge lamp including a pair of discharge electrodes opposed to each other in an arc tube and enclosing mercury and a rare gas in the arc tube. The invention further relates to a lamp unit provided with such a high-pressure mercury vapor discharge lamp.
BACKGROUND ART
High-pressure mercury vapor discharge lamps have the advantage of high luminance and therefore, combined with reflectors (for example, parabolic mirrors), are used as the light source for liquid crystal projectors, and so forth. With a trend toward larger screen sizes and higher resolution images, particularly in recent liquid crystal projectors, there has been a demand for lamps that achieve a higher illuminance on the projection screen. In order to obtain such a lamp, it is required to shorten the arc length (distance between the electrodes) and to increase luminous flux by increasing lamp power (rated power and input power).
The above-described shortening of the arc length is required so that light emitted by the lamp reaches the target (the projection screen) with minimal loss. In other words, the closer the light emitting portion (arc) of the lamp is to a point light source, the more loss of converging light caused by the optical system, such as a reflector, can be reduced (i.e., the optical efficiency is improved). More specifically, the luminous flux per unit arc length &PHgr;/d, where &PHgr; (lm) is the luminous flux and d (mm) is the arc length, is equivalent to the arc luminance L (cd/m
2
), and this arc luminance L determines the screen illuminance on the projector during projection.
An example of a so-called short-arc lamp, in which the above-described shortening of the arc length is attempted, is disclosed in, for example, Japanese Unexamined Patent Publication No. 2-148561. This lamp is a high-pressure mercury vapor discharge lamp in which the lamp power is set at 30 to 50 W and the arc length is set at 1.0 to 1.2 mm. The above-described arc length is very short as compared to, for example, a 40 W high-pressure mercury vapor discharge lamp for general-purpose illumination (HF40 available from Matsushita Electric Industrial Co., Ltd.) which has an arc length of 12 mm. That is to say, this kind of lamp is distinguished from lamps for general purpose illumination and the like in that it normally has an arc length of about 2 mm or less or at the longest about 3 mm or less. Therefore, in the present invention, an arc having an arc length of 3 mm or less is referred to as “short arc.”
The above-described increase in lamp power may be achieved by increasing lamp current or by increasing lamp voltage. However, for a drive circuit that drives a lamp, it is generally easier to increase output voltage than it is to increase current capacity. In addition, when the lamp current is increased, a rise in the temperature of the electrodes is effected by increased Joule loss to the electrodes, and as a result, blackening, caused by vaporization of the electrodes and deposition of this vapor on the inner wall of the arc tube, is more likely to occur. For these reasons, in conventional high-pressure mercury vapor discharge lamps, various methods have been proposed for achieving an increased lamp power by increasing lamp voltage. For example, in a lamp disclosed in the above-described Japanese Unexamined Patent Publication No. 2-148561, by increasing the amount of mercury enclosed and/or by increasing the tube wall loading (lamp power/internal surface area of the arc tube (W/mm
2
)), the operating pressure of the lamp can be set as high as 200 to 300 atm., to achieve a lamp voltage of 76 to 92 V. In this case, a lamp power of 30 to 50 W is achieved with a lamp current of about 0.33 to 0.66 A. (It should be noted that the lamp power can be easily increased by lengthening the arc length; however, this leads to a reduction in optical efficiency as described above, and therefore it is not possible to achieve a screen illuminance that improves according to the degree of increase in lamp power.)
However, in conventional high-pressure mercury vapor discharge lamps in which the lamp power is increased by increasing the operating pressure and the like as described above, it is difficult to substantially increase lamp power due to a limitation on the strength of the arc tube to withstand pressure.
DISCLOSURE OF THE INVENTION
In view of the foregoing problems, it is an object of the present invention to provide a high-pressure mercury vapor discharge lamp which has a short arc length and achieves a high luminous flux by substantially increasing lamp power.
It is another object of the invention to provide a lamp unit utilizing such a high-pressure mercury vapor discharge lamp.
In order to achieve the above-described substantial increase in lamp power, first, the present inventors tried increasing lamp voltage. However, although the operating pressure of a lamp varies by the size, shape, or the like of the arc tube, the highest possible operating pressure is about 400 atm. In addition, while the operating pressure of a lamp is proportional to the amount of mercury enclosed, the lamp voltage is proportional to only about ½ power of the amount of mercury enclosed (Elenbaas, “The High Pressure Mercury Vapour Discharge,” North-Holland Publishing Company, 1951, p30). For this reason, it was difficult to increase lamp voltage to about 90 V or higher, and thus impossible to substantially increase lamp power to, for example, about 125 W or higher. As a result, the highest achievable light output was about 60 (lm/W). (It should be noted that a limitation on the strength of the arc tube to withstand pressure such as described above is due to the limitations of the sealing technique. However, improvement in sealing technique, by which a substantial increase in strength to withstand pressure can be obtained, is not easily achieved, as there are still many technical problems to overcome.)
Further, since it was difficult to increase lamp voltage above the above-described level, the present inventors thought of another technique wherein after having increased lamp voltage as much as possible, lamp current is increased. In doing so, however, an increase in electrode diameter is required in order to reduce Joule loss to the electrodes such as was described above and prevent blackening of the arc tube. However, an increase in electrode diameter increases the area of contact between the sealing portions and the electrodes in the arc tube, causing very small cracks or gaps to be more likely to occur. In other words, because the strength of the sealing portions is reduced, the probability of damage to the arc tube is increased. Hence, in this case also, because there was a limitation on the strength of the arc tube to withstand pressure, it was not possible to substantially increase the lamp current (specifically, above about 1 A, for example), and therefore it was difficult to substantially increase lamp power.
At this point, in order to achieve a substantial increase in lamp power, further and various studies were carried out. From these studies, it was found that the limitations of the drive circuit in terms of an increase in the lamp current such as described above have technically nothing to with the drive circuit itself. In addition, the problem of the strength of the arc tube to withstand pressure, which accompanies an increase in electrode diameter intended to prevent blackening of the arc tube, in fact is due to the approach adopted as described above in which the operating pressure is increased so as to increase the lamp voltage. Thus, the present inventors came up with an approach in which the lamp power may be increased by increasing the lamp current while allowing for a reduction in the lamp voltage.
In other words, power is basically the product of current and voltage, and thus, electrically speaking, an increase in power is equivalent to an increase in voltage and an increase in current. However, in actual high-pressure mercury vapor discha
Kai Makoto
Kitahara Yoshiki
Takeda Mamoru
Matsushita Electric - Industrial Co., Ltd.
Parkhurst & Wendel L.L.P.
Tran Chuc
Wong Don
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