Sodium lamps - lighting devices that use metal pairs as a working substance. Unlike the two other classes of bit devices. For example, mercury lamps use the discharge in gases, emit a family of lighting fixtures where metal compounds become the working substance.
Key features of discharge sodium lamps
It is believed that sodium bulbs have the highest light output, which implies an impressive efficiency. Products are characterized, among other things, by a long service life. During operation, the light output decreases slightly. The operating parameters( high-pressure lamps) depend little on the ambient temperature( overheating is excluded by a properly implemented design).Sodium light bulbs are in demand for street lighting. There are serious drawbacks:
- Not too reliable color rendition( coefficient values - 25).This has long been considered the main limitation for the use of discharge lamps in everyday life. Extremely bad looking with a similar lighting human skin.
- Discharge in sodium vapor is inherent in deep pulsation, which leads to rapid visual fatigue. The flicker effect is harmful to the nervous system and several aspects of human health. The mentioned phenomenon is explained by the complete inertia of the arc in sodium vapor — the luminescence follows the law of the applied voltage( in a network, usually a 50 Hz sinusoid).
- As the use of the life resource is consumed, the power consumption of the sodium lamp gradually grows and increases by 40% relative to the original.
- The sodium starter control gear is bulky( takes up a lot of space) and is characterized by large losses( up to 60% of the total consumed energy).
- The presence of a starting choke predetermines a low power transmission coefficient( up to 0.35).That requires a solid block of compensating capacitors to eliminate the reactive part.
The above explains the use of sodium light bulbs mainly for night lighting, in particular, non-residential objects: shops, warehouses, railway stations. In addition - for storages, highways, architectural structures. The yellow light of a low-pressure sodium lamp allows a person to distinguish parts with a relatively low intensity of radiation, excellently passes through the fog in bad weather conditions. The specified specificity makes it possible to create on the basis of the described devices a variety of signal installations.
Some of the above disadvantages can be eliminated by using electronic inverter-type ballasts. This reduces power consumption, due to the absence of a starting choke, the power factor reaches 0.95.Of course, the mass of the electronic ballast is small. This is known to a person who knows the benefits of LED and discharge lamps with Edison E27 thread. All electronics here fit in the basement.
The service life of high pressure sodium lamps varies between 12 and 28 thousand hours. This is a competitive value, in terms of workdays is 4 - 9.5 years. Gradually, the voltage drop across the lamps increases at a rate of 1–5 V per year. What becomes a cause provoking failure.
Low-pressure bulb flask is usually cylindrical. For high-pressure products, it is sometimes mushroom with an internal reflector or ellipsoidal. In the latter case, the emission spectra are graded in power: for its average values, the pressure in the flask is maximum, explaining the above division. Spectral characteristics are affected by the mains voltage( if electronic ballast is not used).The service life and amplitude are critical: an increase or decrease in the voltage of only 5% leads to a sharp aging of the product.
For ordinary consumers, lamps with improved color rendering are of interest. The corresponding product ratio reaches 83, which is recognized as an excellent indicator. For example, for LED bulbs, typical values are 70 or more. The latter are massively used in everyday life, few will be found who wish to complain about such parameters. And given the efficiency of sodium light bulbs, we believe that the devices will become a worthy competitor for other families of lighting devices.
The principle of operation of sodium lamps
In a sealed flask, conditions are created for the evaporation of sodium. To obtain light, D-lines at 589 and 589.6 nm are used. Sodium lamps come in high and low pressure. According to the generally accepted classification, it is, respectively, from 30,000 to 1 million Pa and from 0.1 to 10,000 Pa. Such a situation has arisen on the basis of long research into the specifics of the discharge.
It has been established that the maximum light output is observed at pressures of 0.2 and 10,000 Pa. The first sodium lamps, created in 1931 by Marcello Pirani, function on the first extremum of the function within the specified interval at a current density of 0.1 - 0.5 A per square centimeter. The most favorable conditions for the emission of light are achieved at temperatures of the liquid phase in the range of 270 - 300 degrees Celsius( the temperature of the base is at least twice as low).Lamps operating at a pressure of 0.2 Pa, more efficient.
The second extremum is reached by further heating the vapor. At temperatures of 650 - 750 degrees Celsius. High-pressure sodium lamps could not be created for a long time. The difficulty was the lack of a suitable material for the flask. Only aluminum ceramics managed to withstand the onslaught of an aggressive environment at temperatures above 1000 degrees( 1300 - 1400 degrees Celsius).Artificial materials gave humanity a lot, which was indirectly mentioned in the review on Electrical Circuits.
Low Pressure Sodium Lamps
Low pressure lamps are extremely effective. The above wavelengths become dominant, but far from the only ones in the emission spectrum. In low-pressure lamps, most of the lines lie in the region of the eye's sensitivity. This means the light is as bright as possible. In other words, low pressure lamps have an attractive efficiency.
In laboratory models, the efficiency reaches 50-60%.As a result, the light output rises to 400 lm / W( the theoretical limit for the current level of technology is 500 lm / W).
For comparison.9W EKF LED bulb( analog of a 75 W filament) gives 830 lm flux. The figure is considered a good indicator of energy saving. Although the light efficiency, it is not hard to guess, is “only” 92 lm / W.It becomes clear how effective low-pressure sodium lamps, invented long ago, in 1931.
In practice, one has to make sacrifices( Philips bulbs are still good and achieve luminous efficacy of 133-178 lm / W).The temperature of the flask rises to the required 270-300 degrees Celsius due to special measures for thermal insulation( if the radius of the flask exceeds the maximum effective) and some increase in the operating current to the optimum. As a result, the efficiency of real products released for mass sale does not reach the above limits. But it remains elevated, so that sodium bulbs are called energy efficient.
Thermal insulation is sometimes supplemented with other measures. Reflective jacket of semiconductor materials transmits the useful yellow radiation, but reflects infrared inside. The temperature inside rises further. But the sodium lamp design is more complicated.
Arc firing is facilitated by the addition of some neon and argon. This greatly reduces the voltage developed by the driver. Due to the presence of impurities, the glass of the flask does not absorb argon. The radius of the lamp is taken slightly more than the optimum and is 15-25 mm. The oxide cathode is usually bifilar or labeled( sintered from powder).Tungsten activated by alkaline( alkaline earth) metals is used as a material.
. High-pressure sodium lamps.
. In addition to sodium gas, mercury vapor and low-voltage ignition( up to 2-4 kV) xenon are added to the gas mixture. The pressure in the flask is in the range of 4 to 14 kPa. It is easy to notice that, according to the general classification of discharge lamps, this range refers to low pressure. For sodium lamps above 14 kPa, this parameter does not rise. The range of 4 - 14 kPa is placed in the discharge of strong pressure.
The maximum efficiency is around 10 kPa. The partial pressure of sodium vapor is a tenth or twentieth of the total. Others account for mercury and xenon. The pressure of the latter( in cold form) is 2.6 kPa. If a mixture of neon and argon is used to reduce the ignition voltage, the light output of the sodium lamp is reduced by a quarter.
In the spectrum of high-pressure sodium lamps, in addition to the D lines, there is activity in the blue-green part of the spectrum. Due to which, the given shade is not yellow, but golden-white( the color temperature in the warm gap is 2000 K).The color rendering index( maximum at 2500 K) can be increased by increasing the partial pressure of sodium vapor and the diameter of the flask. At the same time, the light output is almost halved, and the service life is reduced. There is an increase in color temperature. In view of the negative results described above, such measures are rarely used.
Aluminum ceramics are used as the bulb material. Normal silicate glass is unsuitable, sodium vapors undergo a chemical reaction under the influence of considerable temperature. The compounds formed are stable, and the flask blackens noticeably within a few minutes after the start of operation of the product. Changes are irreversible, under the action of strong pressure there is a probability of complete destruction of the glass.
Polycrystalline ceramics and a tubular single crystal with wall thicknesses from 0.5 to 1 mm are equally resistant to the action of an aggressive medium up to a temperature of 1600 K, with some margin relative to the optimum point. Ceramics detects a decent transmittance of radiation in the visible range, which occupies 30% of the energy consumed by the sodium lamp.
Extreme temperatures require special input design. Made from niobium with a small( 1%) admixture of zirconium, they are sealed at the entrance to the flask with special glass cement( capable of withstanding the specified aggressive conditions).The alloy, so sophisticated in composition, was chosen for a reason. Designers have found a material whose thermal expansion coefficient is close to ceramics. As a result, it is possible to avoid deformations at the joints and seams. The same idea is used in metal window frames. It is known that the coefficient of thermal expansion of aluminum is close to the values of glass.
Sodium lamp pressure is inherent inertness. At the first ignition the light is yellow and monochromatic. Gradually, the product enters the mode with a simultaneous expansion of the emitted spectrum. To re-ignite the arc, the gas cools, taking 2-3 minutes. In order not to exceed the operating temperatures, it is required to exclude the reflection of radiation on the flask. Otherwise, the sodium lamp fails from overheating.