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Water-cooled lamp (MD-water-cooled high-pressure mercury lamp)
When the water-cooled high-pressure mercury lamp is ignited, the surface temperature of the discharge tube is about 1000℃, and the outer surface in contact with water is about 50℃, so the tube wall thickness must be designed to withstand thermal stress and mercury vapor pressure of 50 to 200 atmospheres. The typical inner diameter of the discharge tube is 2 mm, and the tube wall thickness is 1 to 2 mm. Although the lamp has a forced cooling device, the operating temperature of the quartz glass has exceeded the devitrification temperature, so the lamp life is short.
A transition sealing method should be adopted between the tungsten lead and the quartz arc tube. The electrodes are made of tungsten rods, and the distance between the two electrodes must be fixed during sealing. The arc tube is exhausted in the usual way, and filled with excess mercury and about 40torr of argon, and then sealed. Mercury must be stored in the pit behind the electrode, and the electrode must be about 0.5 mm above the mercury surface. If the voltage of the manufactured lamp is low, the sealing head can be shortened and more mercury can be forced to be filled. This lamp will have liquid mercury when it is ignited, which is different from ordinary high-pressure mercury lamps.
Another method is to use a process of sealing molybdenum foil and quartz glass, and connect the molybdenum foil to an electrode of a certain shape without back gap. The dosage of mercury should be controlled so that all mercury can evaporate, so the lamp voltage can be determined in advance and hardly changes with the load of the lamp. The air pressure inside the lamp is high, and a strong structure is required.
The water-cooling sleeve is usually made of two coaxial glass tubes. Water flows between the discharge tube and the inner sleeve, and then returns between the inner and outer sleeves. A high water flow rate (60 ml per second) is required to prevent boiling and bringing bubbles that can affect the optical system.
If the lamp uses AC power, a high-resistance transformer is generally required. If DC power is used, in addition to the same other devices, a rectifier circuit must be connected.
The lamp must generally be lit horizontally. The load of a water-cooled lamp is high, so the heating time can be as short as 5 seconds. Because of water cooling, the restart time is also similar. When the lamp is fully heated, the lamp filled with excess mercury works under saturated vapor pressure, which may produce instantaneous changes in voltage and light output. After the lamp goes out, the mercury must be restored to a uniform distribution before it can be restarted.
Light output The initial luminous efficiency of a water-cooled high-pressure mercury lamp is about 60 to 65 lumens/watt. The brightness increases with the increase of load and the decrease of the discharge tube diameter and tube wall thickness. However, these parameters also affect the rated life of the lamp, so compromise measures must also be taken. Like short-arc high-pressure mercury lamps, the spectral output of water-cooled high-pressure mercury lamps is composed of mercury spectrum lines and continuous spectrum. Among them, the mercury spectrum lines are widened due to the high density of mercury atoms, and the continuous spectrum also accounts for a considerable proportion of the light output due to the high pressure of the lamp. Although the color rendering of water-cooled high-pressure mercury lamps is improved compared with ordinary high-pressure mercury lamps, the red light rate is only 6%.
In order to obtain better light color and light efficiency under high load conditions, the current pulse can be made only a few milliseconds wide, so that the peak current reaches 5 to 10 times the effective stable current. However, this method can only be used in lamps with controlled charging doses and will greatly shorten the life of the lamp.
The light output of the lamp also decays more rapidly due to the accelerated evaporation of tungsten, but halogen can be added to the discharge tube to control the blackening and slow the light decay.
As the lamp is used for a long time, its brightness will decrease due to devitrification and tungsten evaporation. The tube voltage also increases due to corrosion of the cathode and deformation of the discharge tube. The control device directly affects the change of power dissipation and light intensity with life. This aspect will be discussed in Chapter 18.
Life The end of life is usually caused by devitrification, which normally starts around the electrode and then spreads along the tube. Quartz glass devitrifies rapidly after repeated changes in temperature, so the length of life is greatly affected by the switching frequency. If it needs to be switched on and off several times per hour during the use cycle, the life of the lamp is only 30 to 100 hours. Damage is often caused by the explosion of the tube, and the water in the sleeve is a buffer that will not cause external damage. Because damage cannot be predicted in advance, multiple sets of lamps are often installed so that when one fails, another can be used to replace it.
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