Electric light replaced candles and kerosene lamps almost a century ago. Today, three types of light sources are the most widely used. One was invented at the end of the century before last, one in the middle of the last century and one was invented about thirty years ago.
The first source: Incandescent lamps
In the familiar light bulb, light is emitted by a tungsten filament 40-50 micrometers thick, coiled into a spiral. The melting temperature of tungsten is 3400ºC, and the operating temperature of the filament does not exceed 2900ºC, which is much lower than the temperature of the Sun (5770ºC). Therefore, artificial light gives a yellowish light, not white. We do not notice this, but pictures taken without flash and without “white balance” turn out yellow.
The glass bulb of a lamp is filled with an inert gas, often with an admixture of bromine or iodine vapor (halogen lamps). The tungsten atoms that evaporate from the filament react with the halogens and are released at high temperature, settling back on the filament. This raises the filament temperature to 3000ºC, which is achieved in small high-current and more durable 12 volt bulbs.
The average lifetime of an incandescent lamp is 1000 hours (halogen lamps up to 4000). Lamps usually burn out at the moment they are switched on. The specific resistance of the cold tungsten filament is small (only three times that of copper), so at the first moment a current pulse passes through the lamp, the strength of which is about 10 times greater than the nominal. It burns out the defective filament.
Incandescent lamps emit a continuous spectrum, but mostly in the infrared (thermal) region, and only 15% of the energy, and even then only in the best models, accounts for visible light. They are not economical and safe: a very hot bulb can burn a paper or fabric lamp-shade and cause a fire. So there is an urgent need to gradually replace them with more promising light sources.
The second source: Fluorescent lamps
A fluorescent lamp is a glass tube filled with low-pressure mercury vapor. When the lamp is plugged in, the mercury atoms are ionized by a stream of electrons escaping from the hot cathode, and a so-called glow discharge occurs in the tube. The ions receive energy (are excited) and immediately “discharge” it in the form of radiation. The spectrum of mercury radiation is linear, it contains several bright lines in the blue, violet and ultraviolet regions, unpleasant and even harmful to the eyes. Therefore, the walls of the lamps are covered with a layer of phosphor, a substance that emits a bluish or warmer light reminiscent of natural light (they used to be called daylight lamps). And quartz lamps without phosphor are used for disinfecting rooms and for tanning.
A fluorescent lamp has a negative resistance: the greater the current flowing through it, the lower the resistance. That is why it must be turned on with a ballast, a device that limits the amount of current flowing through it. In lamps used to illuminate various workplaces, the ballast is a choke or inductance coil. Automatic ignition of the lamp is provided by a starter – a neon bulb with one moving electrode. At the moment of switching on, the electrodes of the starter are closed and supply current to the filament, which heats up and emits electrons. The next moment the electrodes heat up and open, breaking the circuit. In the choke, due to self-induction, a powerful voltage peak occurs, which lights the lamp. This scheme has a number of disadvantages: the device is quite bulky, the choke hums during operation, and the lamp flickers at twice the frequency of the mains voltage (100 Hz).
More convenient and reliable is a compact electronic ballast that converts the line voltage of 50 hertz to a high-frequency 20-60 kilohertz. It is mainly used in the new generation of fluorescent lamps, called energy-saving lamps.
In these lamps, the thin tube of mercury vapor is usually coiled and connected to a standard socket to be screwed into the socket. An electronic ballast is mounted in the base, which heats the cathode of the lamp and applies a high-frequency voltage to it that triggers the discharge. The lamps are silent and flicker-free. They emit either a “cool”, bluish light or a “warm” light that resembles incandescent bulbs. It is believed that with the same light output such lamps consume five times less electricity: 12-watt gives the same amount of light as a 60-watt bulb, and their life is up to 5-6 thousand hours, but these figures are very approximate. And the high price calls into question their economic benefit.
Energy-saving lamps do not tolerate overheating and frequent on/off switching. Therefore, they should not be placed in enclosed fixtures, in bathrooms and toilets, where you have to turn on the light many times a day.
All fluorescent lamps have a common and very significant disadvantage: Each contains up to 70 milligrams of mercury. Because mercury vapor is poisonous, used lamps must be disposed of. But today they are simply thrown away, contaminating the soil and air.
The third source: LED lights
A semiconductor diode is a two-layer structure of carriers of electric charges of different types. In one, the main carrier is free electrons carrying negative charges. It is an n-type semiconductor (from negative). In the other, the role of carriers is played by holes, quantum states in a solid that are not occupied by electrons. They are equivalent to positive charges in a p-type semiconductor (positive). Between these layers there is a narrow zone of p-n junction. When an electric current passes through this zone, the recombination of electrons and holes occurs, that is, the filling of empty quantum states with electrons. The recombination is accompanied by emission of light due to the transition of an electron from one energy level to another, lower one. A semiconductor device operating in this mode is called an LED. The presence in the structure of several zones of p-n junctions gives simultaneous emission of different frequencies. By changing the composition of semiconductors, it is possible to create LEDs that emit light from ultraviolet to the infrared part of the spectrum. LEDs are very cost-effective: their efficiency reaches 50% and higher. Service life is at least 100 thousand hours.