1 Classification of electric light sources

Light source can be divided into natural light source and artificial light source. In artificial light sources, the light sources that emit light in the form of electricity are collectively referred to as electric light sources. According to the different forms of converting electric energy into light energy, electric light sources can be divided into the following categories: gas discharge light sources, thermal radiation light sources, solid light sources and laser light sources. The first three light sources belong to incoherent light sources, and the laser light source belongs to a new type of coherent light source. The research of light source is a special technical discipline, which involves many disciplines such as optics, atomic physics, electric vacuum and colorimetry.

1.1 Gas discharge light source

The gas discharge light source is a light source placed between two electrodes in the gas to discharge light. Gas discharge light source is widely used because of its high luminous efficiency. There are also many types of gas discharge light sources. If the pressure of the gas filled in the lamp tube is used, it can be divided into low voltage (3 × Below 104Pa) gas discharge light source, high voltage (3 × l04～3 × 105 Pa) gas discharge light source and ultra-high voltage (3 × Above l05Pa) gas discharge light source.

1.2 Thermal radiation light source

Thermal radiation light source is a light source made of thermal radiation of objects. The luminous efficiency of the thermal radiation light source is low. Under good design and process conditions, the vacuum bulb is only 60% - 70%, the ordinary gas filled bulb is only 8% - 10%, and the tungsten halogen lamp is about 16% energy converted into light. However, because of its simple structure, mature technology, good color rendering, low price and convenient use, it is still a common light source used by * * *.

1.3 Solid state light source

Solid state light source, gas discharge light source and thermal radiation light source belong to incoherent light source. For example, electroluminescent screen and light-emitting diode are solid light sources.

The basic principle of electroluminescence is that some phosphors are excited and emit light under the action of electric field. Although the luminous efficiency of the electroluminescent screen is very low, it has the advantages of low power consumption, long service life, simple structure, convenient and reliable use, and is widely used for indicating lighting in the lightless areas such as basements and tunnels; At the same time, it can also be used in image intensifier, image invertor, flat panel TV, etc.

LED, also known as semiconductor lamp, is a diode composed of a P-type semiconductor and a N-type semiconductor. Although the luminous efficiency of LED is low, it is small in size, simple in power supply, low in power consumption and long in life, so it is widely used in signal lamps, display light sources, optical communications and other fields.

1.4 Laser light source

As a new type of light source, the laser makes use of the stimulated emission principle and the filtering and amplification effect of the laser cavity to make its light waves have small beam divergence angle (its divergence angle is about 0.1 milliarc), good directivity or accuracy; Good monochromaticity; High beam brightness.

2 Common gas discharge light sources

There are many kinds of common gas discharge light sources, including low-voltage gas discharge light sources and high-pressure gas discharge light sources:

2.1 Basic principle of gas discharge light source

2.1.1 Gas discharge and electron emission

Gas discharge refers to the discharge phenomenon when the current passes through the gas medium. There are many kinds of gas discharge. Glow discharge and arc discharge are two kinds of light sources.

1. Positive ion bombardment emission: Positive ion bombardment emission is the main emission form of glow discharge cathode. For this kind of emission, the cathode does not need to be heated at all, so it is also called cold cathode emission.

2. Hot electron emission: When the cathode is heated by electricity, the average speed of electrons in the cathode increases. With the increase of cathode temperature, more and more electrons get enough speed to escape from the cathode, so that the emission of electrons increases. Hot electron emission is a main emission form of arc discharge. High pressure mercury lamp, xenon lamp, sodium lamp and metal halide lamp will be introduced later, and their cathode emission adopts this form.

2.1.2 Gas radiation

Before the electrons emitted by the cathode reach the anode, they will have complex interactions with the gas atoms, causing ionization and excitation. Compared with thermal radiation light source, gas discharge light source has the following significant advantages:

1. It is not subject to * * * of filament melting point as thermal radiation light source, so it can obtain very high temperature;

2. The radiation spectrum can be selected as long as appropriate luminescent materials are selected;

3. The luminous efficiency is much higher than that of thermal radiation light source;

4. Long service life, good stability of light output during the service life.

2.2 Common gas discharge light sources and their applications

Generally speaking, gas discharge light sources can be divided into low pressure discharge light sources and high pressure gas discharge light sources. The gas pressure in the gas discharge lamp has a great influence on the characteristics of the light source. It not only affects the light color of the light source, but also affects the luminous efficiency of the light source.

2.2.1 Low pressure gas discharge light source

Low pressure gas discharge light sources mainly include low pressure sodium lamp and low pressure mercury lamp, while carbon arc is a special gas discharge light source.

1. Low pressure mercury lamp: There are two types of low pressure mercury lamp: one is cold cathode glow discharge type, mainly used as ultraviolet light source, used for sterilization in the pharmaceutical industry and food manufacturing industry, or used in scientific research such as fluorescence analysis and photochemical reaction. The other is the hot cathode arc discharge type. The commonly used fluorescent lamp is the hot cathode arc discharge low-pressure mercury lamp.

2. Low pressure sodium lamp: low pressure sodium lamp has high luminous efficiency. Low power low pressure sodium lamp (GP20Na) is mainly used in spectral instruments, and can be used as a monochromatic light source in polarizers, polarimeters, refractometers and other optical instruments. Sodium lamps above 45W are mainly used as lighting lamps or signal lamps.

3. Other low pressure gas discharge light sources: commonly used hydrogen lamps, krypton lamps, hydrogen arc lamps, atomic spectrum lamps and mercury flush lamps are low pressure gas discharge megasources, which can be used as light sources for many optical instruments (such as interferometers, refractometers, etc.).

4. Carbon arc: The carbon arc light source can be divided into ordinary carbon arc, flame carbon arc and high-strength carbon arc. High intensity carbon arc and ordinary carbon arc are generally DC powered, while flame carbon arc can work in DC and AC. Carbon arc light source is a typical open arc.

2.2.2 High pressure gas discharge light source

Common high pressure gas discharge light sources include high-pressure mercury lamp, ultrahigh pressure mercury lamp, metal halide lamp, high-pressure sodium lamp and pulsed xenon lamp. In modern optoelectronic technology, pulsed xenon lamp can be used as the "optical pump" of solid-state lasers.

1. High pressure mercury lamp: the mercury vapor pressure of the high pressure mercury lamp is about 1-5atm (note: 1 atmosphere~101.33 kPa). It has high luminous efficiency and can emit strong ultraviolet light; It can not only be used for lighting, but also be used in areas such as blueprint, health care sunbath treatment, aging test of rubber and plastics, fluorescence analysis and ultraviolet flaw detection.

2. Ultrahigh pressure mercury lamp: The mercury vapor pressure of the ultrahigh pressure mercury lamp is greater than 10-20 atmospheres, and its spectral distribution is similar to that of the high-pressure mercury lamp. At the same time, the ultrahigh pressure mercury lamp has the characteristics of continuous radiation spectral lines.

3. High pressure sodium lamp: The light efficiency of the high pressure sodium lamp is close to that of the low pressure sodium lamp, but the light color is better than that of the low pressure sodium lamp. At the same time, it is small in size, high in power density, high in brightness, and low in ultraviolet radiation. It is very suitable for use in places where high brightness and high ultraviolet radiation are required, such as urban street lighting.

4. Metal halide lamp: metal halide lamp is used in high-pressure mercury lamp

It is made by adding halides of certain metals (mainly iodine other substances of metals - thallium iodide, dysprosium iodide, mercury iodide, sodium iodide, indium iodide, etc.). Thallium lamp, dysprosium lamp and indium thallium lamp are metal halide lamps. Because of its high luminous efficiency and good light color, this kind of lamp is widely used in square, station, wharf, street and underwater lighting and other light sources.

5. Pulse xenon lamp: The spectral characteristics and distribution of the pulse xenon lamp are similar to sunlight, and it has a flash duration of the order of nanoseconds (10-9~10-2S). Pulse xenon lamps are widely used in high-speed photography, aerial photography, stroboscopic observation instruments, optical instruments, laser weapons and other fields.

3 Common gas of electric light source

3.1 Gas for electric light source and its role

The main gases used for the electric light source are argon, helium, neon, krypton, xenon, nitrogen, lamp argon, iodomethane, dibromomethane, bromomethane, nitrogen trifluoride, etc. They are often used to control various physical and chemical processes. In some cases, they also use the special properties of the gas itself to generate light.

In the gas discharge lamp, argon, helium, neon and xenon are mainly used to help start the discharge and play the role of buffer gas in the main discharge zone. Such as reducing the ion loss caused by bipolar diffusion moving towards the pipe wall; Control the mobility and conductivity of electrons; The capacitor is easy to be broken down under low trigger voltage; The sputtering and evaporation of the electrode are reduced, thereby prolonging the service life of the electrode; In the fluorescent lamp, the fluorescent powder is protected from the bombardment of mercury ions.

Argon has been used in electric light source for more than 80 years. As the molecular weight of argon is larger than that of nitrogen, it is helpful to reduce the sublimation rate of luminescent tungsten wire, so the service life of tungsten wire can be prolonged. At the same time, the low thermal conductivity of argon reduces the heat loss, thereby improving the electrical efficiency. Argon can be used to fill some phototubes in addition to various lamps.

Helium group gases (helium, neon, argon, krypton, xenon) have been widely used in modern light sources and laser technology.

Nitrogen molecules have the ability to prevent destructive arcs between components with different potentials in the lamp; In addition, in order to avoid oxidation and corrosion of lamp structural materials, the working environment in the lamp must be composed of inert gas, so the filling gas of the bulb is generally nitrogen or a mixture of nitrogen argon and krypton nitrogen.

In the halogen tungsten lamp, the active gases hydrogen iodide, hydrogen bromide, methyl bromide, nitrogen trifluoride, etc. are used to generate the transport cycle of tungsten, which prevents tungsten from accumulating on the inner surface of the blister shell, and promotes the vaporized metal molecules to return to the tungsten wire.

3.2 Impact of harmful gas on electric light source

Because the working temperature of the lamp is very high, the chemical activity of many lamp materials will be greatly enhanced under high temperature. In order to prevent the serious damage of lamp materials due to oxidation and corrosion, oxidation and corrosion must be avoided. Therefore, the purity of the gas used for electric light source and the harmful impurities therein need to be strictly controlled. Such harmful impurity gases generally include oxygen, hydrogen, carbon monoxide, carbon dioxide, hydrocarbons and water.

These harmful gases are quite common contamination agents, and their harm can be reduced as much as possible by means of vacuum treatment; The residual harmful gas in the lamp can also be eliminated through the absorption, adsorption and chemical reaction of degassing agent (such as phosphorus, barium, magnesium, molybdenum, titanium, niobium, zirconium, etc.) after the lamp is sealed. In the filling gas of most lamps, the content of such harmful gas is only allowed to be controlled within a few parts per million (10-6), for example, the content of oxygen and water should be less than 2 × 10-6。

4 Common varieties, specifications and typical composition of electric light source mixture

The electric light source gas mixture is mainly used as the filling gas for incandescent lamps, special light source lamps (such as infrared lamps, strong overflow lamps, fluorescent lamps, luminous signals, sun lamps, ozone lamps, photochemical lamps, sterilization lamps, ultraviolet lamps, glow lamps, zirconium arc lamps, halogen gas lamps, etc There are four kinds of radioactive gas mixtures and bulb argon mixtures. In order to prolong the service life of the lamp, the purity of the unit pure gas used to prepare the mixed gas for the electric light source should generally be greater than 99.99%, and the content of oxidized impurities should be strictly controlled. Generally, the content of water and oxygen should be less than 2 × 10-6。 Helium gas is used in almost all electric light source mixtures.