The principle and spectrum of illumination of various lamps

1, incandescent lamp

An incandescent lamp is heated by a tungsten wire and emits light by means of thermal radiation. It can be seen from the spectrum of the incandescent lamp that it gradually decreases from the near infrared of 780 nm until the purple wavelength is about 380 nm.

A common incandescent lamp is a simple construction in which a filament is placed in a glass envelope and a lamp cap is mounted. There are two kinds of single spiral filaments and double spiral filaments made of fine tungsten wire. The twisting of the tungsten wire is mainly to reduce the evaporation of the tungsten wire and prolong the life. The gas charged in the glass envelope is a mixed gas of argon gas and nitrogen gas which is almost the same as the gas pressure when the gas is not lit, in order to reduce the evaporation of tungsten in the filament. The long-term failure of incandescent lamps is mainly caused by the evaporation of tungsten (the glass shell is black), and in order to overcome this problem, halogen lamps are produced.

2, halogen lamp

The halogen lamp emits light in the same way as an incandescent lamp, except that a small amount of halogen is added to the filling gas, which is a small and highly efficient light source. In order to overcome the high temperature, the glass material is also changed to quartz glass or hard high silica glass or aluminate glass. Halogen lamps generally have small bulbs, high power, and high luminous efficacy compared with ordinary incandescent lamps, and have a long life. There is a cyclic chemical reaction in the halogen lamp called "tungsten halogen cycle".

Figure:

After the halogen is added, the evaporated tungsten atoms react with the halogen at the wall attachment to form a tungsten halide molecule. This tungsten halide molecule is highly susceptible to evaporation. As long as the temperature of the glass tube wall rises to a certain extent, it will not adhere to the tube wall and will be decomposed into tungsten and halogen vapor by convection or diffusion to a filament attachment with a high temperature. The decomposed tungsten atoms are deposited at a lower temperature of the filament, and the halogen vapor is repeatedly reacted with other tungsten atoms. In order for this reaction to function steadily, the temperature of the glass tube needs to be above 250°, which is why quartz glass is required. The tungsten halogen cycle avoids blackening of the tube wall due to evaporation of tungsten.

Why are incandescent lamps low in light efficiency? Mainly due to the loss between the input power and the output light, refer to the following figure:

Now I finally know why the luminous efficacy of incandescent lamps is only 10~20lm/W.

3, fluorescent lights

The fluorescent lamp is a cathode low-pressure mercury vapor discharge lamp. A light source that is converted into visible light by a fluorescent powder by using ultraviolet rays generated by discharge. In 1938, it was put into practical use by Inman et al. of GE Corporation of the United States.

The filament is a tungsten double helix or a triple helix, and the surface of the filament is coated with an electron-emitting substance (emitter). The glass tube is filled with a system of rare gases such as argon, or a mixture of helium and neon, and an amalgam. The inner wall of the glass tube is coated with a trichromatic phosphor. The principle of illumination is that the electronic ballast provides a pulse voltage to preheat the filament and activate the electron-emitting material on the filament (cathode) to generate electrons. The electron collides with the mercury atom inside the tube to generate ultraviolet light, and the ultraviolet light reflects the visible light through the phosphor powder coated on the wall of the tube.

Fluorescent lamps also have a lot of losses in various capacity conversion processes. Below is the fluorescent lamp energy distribution:

Look at the picture to see why fluorescent lamps are more effective than incandescent lights.

Due to the use of three primary color phosphors and the presence of ultraviolet light, the spectrum of fluorescent lamps has many small spikes.

Due to the wide variety of fluorescent lamps, a wide variety of phosphors are used, and there are many types of spectra, and the above are only common types.