1966 Osram NA/T 200W-3

The NA/T 200W-3 is one of the few rare linear sodium lamps fully conceived and manufactured in Germany. This is the third model of this kind developed by Osram, who was the originator of this particular lamp technology in 1931. The lamp featured here is an advanced development model that was used for testing and characterization purposes at Osram’s labs in West Berlin. Development work began in 1965 in order to further improve the lamp’s performances and to enable a wider use of the technology in outdoor lighting applications. The Germans eventually released the upgraded lamp on the market in 1967.

Linear sodium lamps are characterized by a unique high-efficiency discharge tube design that features a non-circular cross section whose purpose is to enhance the diffusion of sodium ions towards the wall. Beside facilitating the generation and extraction of sodium’s resonant D lines, this diffusion also causes the discharge’s electric field to increase in order to maintain a stable ionization balance in the plasma. This characteristic leads to high operating and re-ignition voltages, especially in lamps of higher wattages, which made the use of bulky and power-wasting leakage-flux transformer ballasts a necessity for sodium lamps rated above 60 W. Osram first addressed this issue when it specified its 1.2 m-long Na 220W, introduced in 1962, for a use in combination with a series choke ballast. However, the high lamp voltage required a connection to 380 V mains, which limited the application of this lamp.

The NA/T 200W-3 shown here was designed to solve this particular limitation while maximizing the lumen efficacy achievable with this sodium lamp technology. To this end Osram used GE’s crescent-shape discharge tube design (first applied to sodium lamps by BTH in 1958) with a shorter indentation space in order to increase the burner’s surface-to-volume ratio so as to enhance the optical extraction of sodium’s resonant light from the discharge. However, such intricate burner design is bound to only increase the sodium diffusion rate, while increasing the total discharge length. In the case of the present lamp, it is 44 % longer than the electrode gap distance (1000 mm), and this should normally lead to a particularly high discharge voltage. However, Osram optimized the gas fill not only for a high discharge efficiency, but for a low discharge electric field strength as well. The Germans devised an unusual neon-argon-krypton buffer mix in order to obtain an optimum combination of low electron elastic collision losses in the discharge (i.e., limited heat production) with a reduced diffusion rate of sodium ions to the wall (i.e., limited charge loss rate from the plasma). The resulting ionization balance leads to a low ohmic impedance of the sodium discharge, which enables the lamp voltage to be reduced to 88 V only, thus permitting an operation on 220 V mains with a simple series-connected choke ballast.

Incidentally, the ballast impedance required for the proper operation of the NA/T 200W-3 (i.e., at 2.33 A) is very close to that of the 150 W HPS lamp control gear that was released in Europe later in the 1970s. Because the system’s open circuit voltage is limited to the mains voltage level, the lamp ignition scheme is similar to that of switch-start fluorescent tube systems: a thermal-switch starter first brings the electrodes to incandescence, which enables the production of free electrons that lower the lamp’s ignition voltage requirements. Next, the starter cools down and opens, which then causes a current disruption that results in a voltage surge from the magnetic ballast, leading to the initiation of the discharge between the electrodes.

In order to withstand the high pre-heat and drive currents, the lamp is built with robust coiled coil tungsten electrodes coated with a mixture of alkali earth oxides. Each electrode is provided with a pair of side metal plates which act as electron collectors, a feature needed to prevent the overheating of the filament when the electrode is in the anode phase. This is a design which is commonly found in highly-loaded fluorescent tubes, albeit implemented in different ways, but which is not present in any other commercial linear sodium lamps. This can be understood by the fact that Osram’s NA/T 200W-3 has the highest drive current of all lamps of this kind.

The NA/T 200W-3 has the same 1200 mm length as that of standard 40 W fluorescent tubes, but the sodium version is slightly larger in diameter (45 mm or T14.7 in 8th-inch denomination) in order to accommodate the 38 mm-wide (T12) burner. This lamp could nevertheless fit in the standard fluorescent lamp luminaires that were commonly used in the 1960s for illuminating the streets and industrial yards of West Germany. The geometrical compatibility of the lamp and the use of a compact ballast meant that standard fluorescent installations could be retrofitted with Osram’s sodium lamp system in order to realize large increases in illumination level and in system efficiency.

A proper thermal insulation of the discharge tube is critical to the realization of high lamp performances. To this end, the NA/T 200W-3 is provided with two barium mirror getters that are evaporated at each extremity of the lamp in order to maintain a good vacuum level through life. To limit heat losses further the outer jacket is internally coated with a semi-conducting layer of tin oxide, a feature which was introduced by Philips in singled-ended SOX lamps in 1964. In order to ensure a good lumen maintenance of the NA/T 200W-3 and to prevent any risk of ignition failure, its tin-oxide film is cut in two places in front of each electrode so as to keep its electrical potential floating. This avoids any electrical contact between the two opposite terminals of the lamp, while preventing the electrolytic extraction of sodium from the discharge tube. The latter is a known issue that arises when the semi-conducting IRC film is electrically connected to one of the electrodes via the evaporated barium mirror getter. This inevitably results in the gradual darkening of the lamp as the released sodium reacts with the semi-conductor film. Osram’s design is the earliest known solution to this particular problem.

When Philips first introduced sodium lamps with a tin oxide infrared-reflecting coating (IRC), Osram began using bismuth for its Na 220W, probably in order to avoid paying licensing fees to the Dutch. However, Osram’s IRC mirror proved less effective, certainly because of lower electron density and mobility in its bismuth material. As a result the Germans eventually adopted Philips’s superior tin oxide material in the second half of the 1960s. In this process the input power of Osram’s 220 W linear sodium lamp was reduced to 200 W. The improved infrared reflectivity of the tin oxide film was also instrumental to the realization of an increased light output of 29.0 klm from the initial 26.5 klm. This way Osram managed to achieve a lamp efficacy of 145 lm/W, which was no small feat, especially for a sodium lamp operating at a significantly higher current than in the case of British-made counterparts (2.33 vs. 1.55–1.60 A). In comparison, the 200 W SLI lamps developed by Thorn and GEC in England had a lumen efficacy of 140 and 103 lm/W, respectively. Interestingly, the introduction of the still superior indium-tin-oxide IRC mirror coating in 1968 enabled Osram to increase the efficacy of its 200 W NA/T further by 5 lm/W, with an output eventually reaching 31.0 klm (155 lm/W) in the last variant produced by them in the 1970s and early 80s. The German production of linear sodium lamps eventually ended in 1984 as a result of a too small demand caused by the much greater popularity of the superior single-ended SOX lamps (1984 was the year that Philips introduced the SOX-E with a lumen efficacy reaching 200 lm/W).