Car headlights are generally composed of three parts: bulb, reflector, and light distribution mirror (diffuser).
- Bulb
The bulbs used in car headlights include incandescent bulbs, halogen tungsten bulbs, and new high-brightness arc lamps.
(1) Incandescent bulb: Its filament is made of tungsten wire (tungsten has a high melting point and strong luminescence). During manufacturing, in order to increase the service life of the bulb, an inert gas (nitrogen and its mixed inert gas) is filled into the bulb. This can reduce the evaporation of the tungsten filament, increase the temperature of the filament, and enhance the luminous efficiency. The light emitted by an incandescent bulb has a light yellow color.
(2) Halogen tungsten bulb: Halogen tungsten bulbs are filled with inert gas and infiltrated with a certain halogen element (such as iodine, chlorine, fluorine, bromine, etc.). The principle of halogen tungsten regeneration cycle reaction is used, that is, the gaseous tungsten evaporated from the filament reacts with the halogen to generate a volatile tungsten halide, which diffuses to the high temperature area near the filament and decomposes under heat, causing the tungsten to return to the filament. The released halogen continues to diffuse and participate in the next cycle reaction, and the cycle repeats over and over again, thereby preventing the evaporation of tungsten and the blackening of the bulb. Halogen tungsten bulbs are small in size, and the bulb shell is made of high-temperature resistant and mechanically strong quartz glass. At the same power, the brightness of halogen tungsten lamps is 1.5 times that of incandescent lamps, and the life is 2 to 3 times longer.
(3) New high-brightness arc lamps: This type of lamp does not have a traditional filament in the bulb. Instead, it is replaced by two electrodes installed in a quartz tube. The tube is filled with xenon and trace metals (or metal halides). When there is enough arc-starting voltage on the electrode (5000-12000V), the gas begins to ionize and conduct electricity. The gas atoms are in an excited state and begin to emit light due to the energy level transition of the electrons. After 0.1s, a small amount of mercury vapor evaporates between the electrodes, and the power supply immediately switches to mercury vapor arc discharge, and then switches to halide arc lamp operation after the temperature rises. After the ignition reaches the normal working temperature of the bulb, the power to maintain the arc discharge is very low (about 35w), so 40% of the electricity can be saved. - Reflector
The function of the reflector is to maximize the aggregation of the light emitted by the bulb into a strong beam to increase the irradiation distance.
The surface shape of the reflector is a rotating parabola, which is generally stamped from a thin steel plate of 0.6-0.8mm or made of glass or plastic. Its inner surface is silver-plated, aluminum-plated or chrome-plated, and then polished; the filament is located at the focus of the reflector, and most of its light is reflected and becomes a parallel beam to the distance. The brightness of a bulb without a reflector can only illuminate the surrounding distance of about 6m, while the parallel light beam reflected by the reflector can illuminate the distance of more than 100m. After passing through the reflector, there is still a small amount of scattered light, of which the light upward is completely useless, while the light to the side and below helps to illuminate the road surface and curbs 5 to 10m away. - Light mirror
The light distribution mirror, also known as astigmatism glass, is made of light-transmitting glass. It is a combination of multiple special prisms and lenses, and its shape is generally round and rectangular. The function of the light distribution mirror is to refract the parallel light beam reflected by the reflector, so that the road in front of the car has good and even lighting.
