Detailed explanation of the principle of metal halide lamp

1. The light-emitting process of metal halide lamps

Metal halide lamps are filled with a small amount of metal halide and gas. It takes more than a minute from triggering to normal lighting, which can be roughly divided into three stages.

1. Triggering stage. Metal halide lamps have no filaments, only two electrodes. Directly applying the working voltage will not ignite them. High voltage must be applied first to ionize the gas in the lamp. The high voltage is generated by a special trigger.

2. Ignition stage. After the bulb is triggered, the discharge voltage of the electrode further heats the electrode, forming a glow discharge and creating conditions for arc discharge.

3. Normal light emission stage. Under the action of glow discharge, the electrode temperature becomes higher and higher, the number of electrons emitted increases, and the transition to arc discharge is rapid. As the temperature rises further, the light emission of the lamp becomes stronger and stronger until it reaches normal. The whole process takes more than one minute. If the starting current is large and the power supply has good starting performance, this process can be shorter.

2. The luminous mechanism of metal halide lamps

Metal halide lamps mainly rely on metal halides as luminescent materials, and metal halides exist in the lamp in solid form. Therefore, a small amount of ignition gas hydrogen or xenon must be filled in the lamp to ignite the bulb. After the lamp is ignited, it first works in a low-pressure arc discharge state. At this time, the voltage between the two poles of the lamp is very low, about 18~20V, and the light output is also very small. At this time, heat energy is mainly generated to heat the entire lamp body. The metal halide introduced into the lamp evaporates continuously as the temperature rises, becoming metal halide vapor. Under the action of thermal convection, it continues to flow to the center of the arc. A part of the metal halide is decomposed by the arc at 5500~6000K high temperature to become metal atoms and halogen atoms. Under the action of the electric field, the metal atoms are excited to emit light; the other part of the metal halide is not decomposed by the high temperature of the arc. Under the dual action of high temperature and electric field, it is directly excited to form molecular light emission.

Since various metal halides have different evaporation temperatures, these particles evaporate one after another and participate in luminescence, so different atomic spectra appear one after another. As the temperature gradually rises, the density of metal atoms in the arc gradually increases, resulting in resonant absorption, and the atomic characteristic spectrum gradually weakens until it disappears and expands to the long-wave band. As the lamp temperature further increases and thermal equilibrium is established, all metal halides evaporate, and molecular spectra appear. The light color and brightness also tend to be stable. The gas pressure in the lamp can reach dozens of atmospheres, and the arc in the lamp changes from low-pressure arc discharge to high-pressure arc discharge. The voltage across the lamp rises from 18~20V and gradually stabilizes to about 100V, entering a normal luminous state.

The luminous efficiency of a lamp is related to its overall size, process structure and the type of metal it contains.

3. Advantages and disadvantages of metal halide lamps

The biggest advantage of metal halide lamps is that they have a very high luminous efficiency, up to 80~90Lm/W, and generate less heat during normal luminescence, so they are a cold light source. Since the spectrum of metal halide lamps is a dense linear spectrum superimposed on a continuous spectrum, the color rendering index is very high, that is, the color reproduction is very good, up to 90%. In addition, the color temperature of metal halide lamps is high, up to 5000~6000K, and special projector lamps can reach 7000~8000K. Under the same brightness conditions, the higher the color temperature, the brighter the human eye feels.

Metal halide lamps have a relatively short lifespan due to their high brightness and small size. Due to material and process limitations, the lifespan of domestically produced metal halide lamps is still less than 1,000 hours, while the lifespan of imported metal halide lamps can reach several thousand hours.

Another disadvantage of metal halide lamps is that they are difficult to start and require a special trigger. After starting, the brightness gradually increases. If the starting energy is too large or the starting speed is too fast, it will affect the life of the bulb, which should be fully considered when designing the circuit.