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Working principle of mercury lamp

The operation of the discharge lamp is based on the collision process between electrons, atoms and ions in the discharge tube.

Due to the collision of electrons, the ionization of atoms occurs, thus generating an electric current. The excitation of atoms or molecules is a necessary process for the emission of characteristic spectral lines or spectral bands. At the same time, a continuous spectrum is emitted due to the recombination of ions and electrons. The main resonant wavelength of mercury is in the ultraviolet region, and the other main spectral lines are in the blue, green and yellow regions. The effect of vapor pressure can be seen in the mercury lamp from the moment it is connected to the power supply. The voltage passing through the lamp tube is very low at first, only about 20 volts. The discharge fills the discharge tube and appears blue. The discharge at this time is similar to that of a fluorescent lamp, and the radiation emitted is mostly in the ultraviolet region. Later, the temperature of the discharge tube gradually increases with time, and the mercury vapor pressure also increases accordingly. The discharge shrinks along the axis into a narrow band and gradually brightens. When the pressure is further increased, the radiation energy gradually concentrates in the spectral line region with longer wavelengths, producing a small amount of continuous radiation, thereby turning the discharge light into white. When the high-pressure mercury lamp discharge tube is fully discharged, the vapor pressure is about 2~10 atmospheres (1 atmosphere = 1.013x10 Newtons/square meters) depending on the power of the lamp. When the vapor pressure of the water-cooled high-pressure mercury lamp (MD) and the short-arc high-pressure mercury lamp (ME) is as high as 10~100 atmospheres, the spectrum can be further broadened and the continuous radiation is relatively increased. As a result, radiation is generated in a wider spectral region, thereby improving the light color.

The familiar mercury lamp is equipped with an ellipsoidal outer glass shell and the inner wall is coated with phosphor. It includes an arc tube with electrodes, internal wiring and general components. This lamp is usually called a fluorescent high-pressure mercury lamp (MBF/U). The discharge tube of this lamp emits the green-white light of typical mercury discharge and some ultraviolet light. The ultraviolet part is absorbed by the phosphor on the inner wall of the outer glass shell and converted into visible light. The phosphor usually emits red light between 600~750 nanometers because the discharge itself rarely emits light in this range. The light thus combined has a fairly good color rendering and is suitable for street lighting, high-span industrial building lighting, and indoor lighting in some shopping malls. The light efficiency of the lamp depends on the phosphor used and the power, which is generally about 50 lumens/watt.

The MA type is one of the earliest mercury lamps. Its discharge tube is made of aluminosilicate glass and can only work at 1 atmosphere. This lamp has now been eliminated and replaced by ordinary high-pressure mercury lamps (MB/U) with good light color, high light efficiency, and wider application. These improvements are mainly due to the use of high-temperature resistant quartz glass discharge tubes. The outer dimensions of ordinary high-pressure mercury lamps are the same as those of the MA type, with two types of tubular and transparent elliptical outer glass shells.

The reflective fluorescent high-pressure mercury lamp (MBFR/U) evolved from the fluorescent high-pressure mercury lamp. The shape of its glass shell enables it to produce a unique polar coordinate distribution of light intensity. This lamp provides a simple light source for high-span buildings and industrial lighting.

The self-ballasted or composite lamp (MBT) works by connecting the mercury discharge tube and tungsten filament in series. The filament not only controls the current in the discharge tube, so there is no need for an external ballast device, but also emits red light that is lacking in mercury discharge as a supplement. The new lamp also coats the inner wall of the outer glass shell with fluorescent powder to further improve the spectral power distribution.

Small low-pressure mercury lamps (M1 and M2 types) are developed to provide small, cheap ultraviolet light sources and certain visible radiation. They are mainly used in biological and entomological research, as well as to excite fluorescent materials. The M1 type uses a resistor ballast and a 24-volt DC power supply, while the M2 type uses a series inductor ballast and an AC power supply.

Mercury discharge can produce a large amount of ultraviolet and blue radiation, which is widely used in optoelectronic printing and other photochemical reactions. It can also be used for health care and sterilization. Different uses require lamps of different sizes and different rated powers. There are now lamps ranging from a few watts to several kilowatts. The wavelength of radiation is very important. For example, for sterilization, the minimum wavelength is about 260 nanometers, and for general health care, it is about 297 nanometers. In photoelectric printing, there are many ranges of spectral sensitivity available, mainly in the near ultraviolet, as well as in the blue and green ranges, and although mercury lamps still have some specialized uses in this area, metal halide lamps are now finding many new uses in photoelectric printing.

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