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Short arc high pressure mercury lamp

The design and production of short-arc high-pressure mercury lamps are basically the same as high-pressure mercury lamps, but the current is higher and the mercury vapor pressure is 20 to 40 atmospheres.

The glass shell must be made of high-quality quartz glass to prevent the possibility of dark spots in the projected beam. The glass shell is basically spherical, but can also be made into a pear or ellipsoid shape to make the surface temperature more uniform. The load per unit surface area (in watts) must be limited to prevent possible devitrification and shortened life caused by overheating. If the glass shell is small, the lumen retention performance of the lamp is poor because the material sputtered by the electrode will be deposited in a smaller area. If the glass shell is large, the heating time of the lamp will be extended, resulting in faster electrode evaporation and inconvenience in use. The glass shell around the seal is the coldest part of the lamp and can be coated with a heat-reflecting coating or a metal Reflector to retain heat and help warm up. Glass shells with rated power of 100 watts to 5000 watts have a diameter of 10 to 120 mm and a wall thickness of 1.5 to 5 mm.

If the lamp breaks when burning or cooling, it will explode with great energy, so it can only be used in appropriate lamps to ensure safety.

Sealing The sealing is made of molybdenum foil. High-power lamps should be sealed with multi-layer foil or cup-shaped sealing. The discharge tube of some lamps must be sealed in an outer glass shell filled with nitrogen at about half an atmospheric pressure to prevent oxidation of the sealed part, which can extend the life of the lamp.

Electrode Due to the large current used, the electrode of the short-arc high-pressure mercury lamp should be larger than that of the high-pressure mercury lamp, but the design principle is the same. The electrode is generally made of machined tungsten rods with tungsten wire windings, and may contain a small amount of thorium oxide to reduce the work function and lower the starting voltage. In order to make the temperature of the seal and the entire glass shell uniform, the gap between the two electrodes should be below the center point of the glass shell. If the lamp is used in alternating current, the two electrodes should be the same; if it is direct current, the anode must generally be enlarged, because the voltage drop of the anode is higher, and the power consumption is also greater.

Short arc high pressure mercury lamps generally do not use electron emission materials, because such materials will evaporate quickly from the electrode and cause the glass shell to blacken early. After the lamp is used for a long time, the shape of the electrode will change due to local melting and the growth of the tip, thus affecting the discharge characteristics and stability. This is mainly affected by the high temperature of the electrode, which can be improved by selecting raw materials and changing the process method.

Some lamps also install a tungsten auxiliary electrode through the third sealing point to start at the usual voltage. The structure of this lamp is more complicated, but if a lamp without an auxiliary electrode is used, the required control device is more complicated.

Inflating usually uses argon-neon-pan mixed gas with a pressure of about 60 Torr. Some lamps (such as short arc xenon high pressure mercury lamps (MEX) are also filled with 0.5 to 2 atmospheres of xenon. Although this lamp requires a dedicated starting circuit, the initial discharge of xenon can provide quite good light color when it is in the process of forming all mercury vapor. At the same time, xenon filling can also speed up the heating rate, improve lumen maintenance performance and extend life.

Starting and heating up The starting process of short arc high pressure mercury lamps is similar to that of high pressure mercury lamps. The time required for starting varies with the lamp current and filling gas, and may also be extended due to excessive cooling of the glass shell. If the lamp is turned off and restarted immediately, a high voltage of 15,000 to 50,000 volts is required. If it is started only with the supply voltage, it generally takes 5 to 10 minutes of cooling time. The design of some circuits adopts a compromise between the above two voltages.

Working characteristics of lamps AC is generally cheaper, but DC is also used in situations where stable discharge is required. This is mainly because the short-arc high-pressure mercury lamp has a lower tube voltage and a larger current than ordinary high-pressure mercury lamps, so the control device is also more clumsy and more expensive. The change in lamp characteristics caused by voltage changes depends on the rated power and operating conditions of the lamp, but in general, this trend is similar to that of ordinary high-pressure mercury lamps.

Short-arc high-pressure mercury lamps generally require vertical burning. Because if a horizontal burning point is used, the tube wall above the discharge area will be overheated due to the convection of hot ion vapor, resulting in devitrification and shortened life.

The light output of the short-arc high-pressure mercury lamp is about 40 to 50 lumens/watt. Because this lamp is often used for projection, it is more important to understand the discharge brightness and light intensity. The brightness distribution along the length of the discharge tube is almost equal except for a section near the electrode, but the lateral brightness at the discharge center is close to Gaussian distribution, such as 10 The peak value of a 00-watt lamp is 4x10 candlepower/square centimeter, and the brightness at 1.25 mm from the axis is 2x10 candlepower/square centimeter.

Since the mercury vapor pressure of a short-arc high-pressure mercury lamp is higher than that of a general high-pressure mercury lamp, its spectral line broadening and the increase in continuous emission are directly related to its linear spectral emission, especially at the shorter wavelength end. Figure 14-11 is a typical spectral power distribution diagram of a short-arc high-pressure mercury lamp, and the red light rate of the lamp is about 4%.

Metals such as cadmium can be added to the mercury discharge to increase the red light rate. This method is not very effective in old-fashioned mercury lamps and high-pressure mercury lamps, and will cause the relative spectral power of the light efficiency to decrease significantly. However, in high-power short-arc high-pressure mercury lamps, adding cadmium can double the high red light rate without reducing the light efficiency. A better way to improve the light color and light efficiency of short-arc mercury lamps is to add metal halides to the discharge, which will be described in Chapter 15.

Lamp life After the lamp is used for a long time, the emission performance of the thoriated tungsten electrode will deteriorate, and the glass bulb will gradually turn black due to tungsten evaporation, resulting in either too little light or failure to start. Both of these situations will be accelerated by frequent switching. Depending on the usage and load conditions, the life of the lamp is generally between 500 and 5000 hours.

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