Sodium-vapor lamp

A sodium vapor lamp, a gas discharge lamp. Wherein the gas discharge from vapor of the sodium element there is an emission of substantially monochromatic light.

The principle

The sodium vapor lamp belongs to the category of metal halide lighting. Unlike fluorescent lamps Sodium lamps require no fluorescent phosphor. In these lamps, the gas discharge itself already produces visible light, and it does not first have to be converted into this. Thus, the efficiency is significantly increased, since the human eye is particularly sensitive to this color area.

The right- monochromatic light (especially by low-pressure lamps) hardly allows color vision. However, the contrast sensitivity in this area of the color spectrum is high. Sodium vapor lamps are used for night lighting of roads, public places, industrial sites, including in the military area. The yellow light attracts insects and less is desirable for maintenance and environmental reasons.

Sodium vapor lamps require a ballast and ignitor. Specific sodium vapor lamps - which have been developed as a replacement for mercury vapor high- pressure lamps in street lighting - come from without ignitor. They have an internal ignitor, but are only available in smaller power ratings.

Cold sodium vapor emits almost exclusively light of wavelength 589.00 nm and 589.59 nm from ( sodium D -line, sodium double line ), so is almost monochromatic. Photons of this frequency are sent from or to the state of the sodium atom in the transition of electrons. The level is split by spin -orbit coupling into two sublevels.

During a cold start sodium lamps are lit mostly reddish. In this state dominates the light of the auxiliary gas neon, since the sodium is still largely fixed in cold air. The discharge of the neon gas heats the air and promotes the transfer of solid sodium in the gas phase, so that the light color is stabilized after a few minutes and dominated by sodium.

Hot sodium vapor emits due to random collisions of atoms broadband. From the perspective of an observer, the radiation from sodium atoms move according to their thermal motion randomly in all directions. When the atoms are just moving away from the observer, the emitted light appears slightly red-shifted by the Doppler effect and the atoms moving toward the observer, the emitted light appears slightly blue-shifted, so that in the spectrum of the emission lines of sodium ( and neon ) widen.

Related principles have the mercury vapor lamp and the xenon gas discharge lamp.

According to the conditions inside the light pipe, a distinction between sodium vapor lamps and low pressure sodium vapor high pressure lamps.

Sodium vapor high pressure lamp

The sodium vapor high pressure lamp (also called HS- lamp) has 150 lm / W ( lumens per watt ) an extremely high light output and is available in capacities of 35 watts to 1000 watts. Benefits from 50 to 150 watts are common for street lighting. These lamps use internally sodium amalgam and by the addition of certain noble gases (usually xenon) can be achieved, a certain amount of blue in this lamp type and thus improve the color reproduction.

Sodium vapor high pressure lamps consist of a tank filled with sodium and an inert gas discharge tube ( burner ). This is for thermal insulation in an evacuated glass bulb in tubes or ellipsoidal. The gas filling is in the operation of the air under high pressure. The burner is due to the high operating temperature ( about 1000 ° C) of high-purity transparent alumina ceramic. The electrode terminals are made of niobium. The burner is fixed by strong metal wires in a glass flask, which also serve as power supply. The contacting of the lamp with the lamp base is mostly done on a Edison screw in sizes: E27, E33 and E40. The protective glass in the piston reaches operating temperatures of over 300 ° C. For initiating a voltage of up to 5 kV is required.

After ignition, the high pressure sodium lamps are lit at first faintly. After four minutes enough sodium is in the gas phase and the maximum brightness is achieved. After switching off the lamps need to cool down until the internal pressure has dropped, so that they can be turned on with standard ignitors. However Sofortzündgeräte or hot restrike igniters are able to fire high pressure sodium lamps, which have just been turned off. The hot ignitor has ignition voltages of over 30 kV, ionize the discharge path of a hot sodium vapor high pressure lamp. This process is only possible with double-ended lamps.

The average service life is specified at 30,000 hours of operation. This corresponds to three and a half years of continuous operation. The manufacturers do at times also a service life of four years for a defined A / Ausschaltregime, such as when used as street lighting. Frequent switching on and off shortens the service life. The operating current must be maintained by the proper choice of the ballast.

The end of life of sodium vapor high- pressure lamps is evidenced by periodic extinction and re-ignition. This process is called " flashing " or " Cycling". After ignition of the lamp the lamp voltage rises slowly and reaches, due to the aged gas filling has a value in excess of the mains voltage. The lamp goes out. After cooling, the lamp re-ignites and the process starts again. The cycle time is typically in the range of 10 to 15 minutes. In order to suppress the resulting blinking mode, have special ignitors with automatic cycling detection. The automatic cut sets in and prevents further attempts when the lamp is extinguished within two hours with increasing operating voltage three times.

Sodium vapor low -pressure lamp

Sodium vapor low -pressure lamps (also LS, NA or called SOX lamp ) are among the most efficient electric light sources that are out there now. With up to 200 lm / W, they enable an efficient and energy -saving lighting that can be used due to its monochromatic character only there, however, where there is no color fidelity arrives ( streets, intersections, industrial plants ). The operating temperature is significantly lower than in the high-pressure lamp. To ignite tensions already rich in the range of 1 kV, depending on the model also satisfies the appropriate voltage. After a cooling time of 2 minutes can be ignited again. Sodium, low pressure lamps have a significantly larger discharge vessel as high pressure sodium lamps. Usually, it is designed as a U-shaped tube, which is located in the protective cover of a glass bulb. This outer bulb is often provided with an infrared reflective coating that keeps the operating mainly in the outdoor high. Well low pressure lamps sometimes found as street lighting, but mainly for the lighting of pedestrian crossings, and recognizes them on the yellow light color.

Today's standard high-pressure sodium, low pressure lamps of the type SOX are only available with a standard UK BY22d bayonet socket and not with the spread in Germany Edison socket ( screw mount). The bayonet mount ensures the correct position of the lamp in the luminaire. The smallest commercially available sodium vapor lamps are the NA18 from Osram and Philips 18W SOX (25-30 euros ). For proper operation current of about 330 mA with European mains voltage of 230 V/50 Hz two standard chokes should be ( for fluorescent lamps 58/65 W) connected in series.

Use in the darkroom

Sodium, low pressure lamps are suitable as safelight for black and white positive - processing, since the monochromatic yellow light matches the minimum sensitivity of the photographic layer of photo paper or print film and a particularly bright safelight is possible.

For processing color materials sodium low pressure lamps are also used, also taking advantage of sensitization gap. The alternative are specially selected amber LEDs.

Use in film editing

The Disney studios developed in the 1960s, a process for film editing, which exploits the insensitivity of film emulsions (also for the then color film ) and "Yellow Screen" or called " Sodium vapor process". So actors could act in a studio recording before an illuminated with sodium vapor lamps focusing screen and a special camera illuminated by a semitransparent mirror simultaneously two films. The film took the scene itself across the spectrum, the former color film could play on. The second film was sensitive only to the narrow spectral band of the sodium vapor lamp and produced as a so-called "Travelling Mat ," an animated mask that allows stood exact import a background image.

Compared to rear projection, this method had the advantage that the scene can be better illuminated without weakening the contrast of the rehearsed backdrop. The then also known blue screen technique produced at that time still annoying blue fringing at the edges between the scene and the background that do not occur in the Yellow screen process. With the improvement of the blue screen technique, this method lost due to higher expenses in importance. Notable films in which the sodium vapor was related process, Mary Poppins and Alfred Hitchcock are the birds.

Plant Lighting

High-pressure sodium vapor lamps are suited by their emission maximum well as additional exposure ( supplementary light ) in the cultivation of ornamental plants under glass. These lamps come in addition to a broadening of the Na emission lines in the yellow region and also a line in the red. Since photosynthesis ( depending on the type ) takes place mainly at 670 nm, is this light in the longer wavelength ( red ) area important to the application. Because of the pleasant light color to use for living room near the terrarium is possible. Sodium, low-pressure lamps, which emit at 590 nm only and engbandig, however, are insufficient in the color matching.

Use in the theater

The sodium vapor lamp is often used in theater as an effect light. The almost complete disappearance of all colors with exclusive lighting with sodium lamps is a deliberate stylistic device. Even the slow visual glow out of the darkness with its gradient is occasionally used as a mood. Since the sodium light is not dimmable, but adjustable with electronic ballasts, there are various attachments for the lights, which allow blades with a one- and fade-out effect.