What Is a Mercury Vapor Lamp?

Mercury vapor lamp, referred to as mercury lamp, is the oldest HID lamp. It is mainly used for street lighting. Its luminous efficiency is 26 ~ 60lm / W, and the light is cold blue / green and white. Mercury vapor lamps are used less often in performance and stadium lighting, and metal halide lamps are used more because the latter has better color rendering (color rendering index of 70, mercury vapor lamp of 50) and better luminous efficiency (70 ~ 115lm / W). However, the life of mercury vapor lamps (16000 ~ 24000h) is longer than that of metal halide lamps ^[1] .

Mercury vapor lamps are light bulbs that use hot mercury vapor in the lamp tube after being energized. Mercury vapor lamps emit a discontinuous spectrum of light. In its spectrum, the red light component is lacking, and the green, blue, and ultraviolet regions have strong line spectra, so it is not suitable for light sources for color photography, and it is mostly used for urban street lighting ^[2]

The working principle of gas discharge lamps is based on the collision of electrons, atoms and ions in the discharge tube. The generation of the lamp current depends on the ionization of the atoms caused by the electron collision, and the excitation of the atoms or molecules is a necessary process of the radiation of the characteristic line spectrum or the band. The radiation of the continuous spectrum is mainly the result of the recombination of electrons and ions. The main radiation spectral lines of mercury are in the ultraviolet, blue, green, and yellow light regions.

The luminous efficiency of the mercury vapor discharge changes with the mercury vapor pressure, see Figure 1. Light effect with mercury vapor in AB section

Mercury vapor lamps are lamps made using the principle of mercury vapor discharge. According to the difference of mercury vapor pressure and use, it can be divided into low-pressure mercury vapor lamps, low-pressure mercury fluorescent lamps, high-pressure mercury vapor lamps, ultra-high-pressure mercury vapor lamps, and so on.

Taking the fluorescent high-pressure mercury vapor lamp as an example, the main structure of the mercury vapor lamp is briefly introduced. The fluorescent high-pressure mercury vapor lamp is composed of main parts such as a discharge tube, an electrode, an outer glass shell and a lamp cap.

Mercury vapor lamp discharge tube

The discharge tube of the high-pressure mercury vapor lamp is made of a high-temperature-resistant transparent quartz tube. The tube is filled with a small amount of argon. At startup, a discharge between the main electrode and the auxiliary electrode is established, and then an arc discharge between the main electrode is established. The discharge tube The temperature of the tube wall can reach 1000K during operation, and the internal mercury vapor pressure can reach 0.2 ~ 0.4MPa. It is required that the discharge tube can transmit ultraviolet rays to excite fluorescent substances on the inner wall of the outer glass shell and emit light ^[4] .

Mercury vapor lamp electrode

A discharge electrode is provided at each end of the discharge tube, and the structure is that a double-layer tungsten wire spiral is sheathed on the outer surface of the sharpened tungsten rod, and the spiral is coated with barium, strontium, and calcium oxide electron-emitting materials. The inner layer is wound with a spiral gap, and as many electron-emitting materials as possible can be filled between the spirals. The outer layer is tightly wound to prevent the electron-emitting materials from falling off, while reducing ion bombardment ^[4] .

Mercury vapor lamp outer glass case

A fluorescent high-pressure mercury vapor lamp is a double-walled glass bulb, and an oval-shaped outer glass shell is placed outside the quartz discharge tube. The outer glass shell is filled with hundreds of Torr (1 Torr = 133.32Pa) of nitrogen. The nitrogen prevents the oxidation of molybdenum foil and metal parts and keeps the temperature of the discharge tube stable. The outer glass shell can also cut off the ultraviolet rays of the quartz discharge tube to ensure lighting safety. The fluorescent powder on the inner wall of the outer glass shell of the fluorescent high-pressure mercury vapor lamp converts the ultraviolet rays to red light, improving color rendering and luminous efficiency. The outer glass shell is mostly made of hard glass with excellent thermal stability, to avoid bursting due to rain and snow splashing during outdoor use ^[4] .

Mercury vapor lamp holder

The fluorescent high-pressure mercury vapor lamp adopts the standard E40 screw cap to facilitate installation and use. Because the working temperature of the bulb is very high and the life is long, the mechanical clamping lamp base without welding mud is mostly used ^[4] .