1961 Philips SOI 60W

The earliest sodium lamps, first introduced in 1932 by Philips (the Netherlands), featured a detachable discharge tube and a transparent Dewar jacket whose function is to limit heat losses. While this design proved effective and made the technology economically viable, its unsealed configuration made it prone to internal dirt accumulation as a result of thermal cycles. This problem was solved by Osram in Germany with the introduction of an hermetically sealed vacuum lamp design in 1936, the NA300 and 600. However, these were too costly to produce at the time and they were eventually phased out before just 1940. It is only in 1955 that integral-jacketed sodium lamps became commercially available on a wide scale (GEC of England).

These new SOI lamps were far more impervious to dirty atmospheres than earlier Dewar-jacketed SO lamps, but the lack of atmosphere around the discharge tube resulted in a new problem: the temperature was no longer homogeneously distributed, which caused a gradual migration of sodium away from the hot electrode regions. This distillation process leads to a progressive depletion of sodium that eventually results in an inefficient discharge burning in pure neon, the start/buffer gas. This process became a life-limiting factor in many lamps, which turned into a “red burner” before the expected end of service life. Philips addressed this specific problem with the introduction in 1955 of condensation dimples blown on the side of the discharge tube at regular intervals so as to keep an even sodium distribution throughout life.

The lamp shown here is of the second-generation SOI type introduced by Philips in 1959. This model features an additional glass sleeve around the burner, held into place by six support springs at each extremity. This sleeve absorbs most of the infrared radiation emitted by the discharge tube and re-emits half of this energy back towards the source. This decreases the infrared output of the lamp by 50 % and raises the efficacy by 23 % compared to the previous generation of SOI lamps. Another interesting design improvement is the optimized position of the dimples, which better compensates for the greater sodium distillation rate near the electrodes caused by the steeper temperature gradient there. This change brought the actual mean service life back to the 6,000 h of (some) Dewar-jacketed SO lamps.

The sodium reservoirs never solved the distillation problem entirely in the narrow-bore SOI discharge tubes. This transport process remained a persistent limiting factor that prevented the realization of longer lasting lamps. The problem was particularly acute in the smaller 45 and 60 W lamps due to the shorter distance between the hot electrode sections and the cold U bend of the discharge tube, resulting in a particularly strong temperature gradient overall. Moreover, this problem was exacerbated by the small diameter of the discharge tube, which is necessary for the lamp to reach its optimum regime given its thermal design. This narrow bore makes the burner prone to thermophoretic effects. As the tube section near the electrodes becomes depleted of sodium the discharge there becomes hotter due to a greater influence of the neon buffer on the plasma operation. This local temperature elevation will speed up the migration rate of sodium vapor, further depleting the discharge of that element, which inevitably increases the rate of discharge temperature growth. Eventually, this results in two whole sections of the discharge (one attached to each electrode) burning in pure neon, and the large temperature difference between the normal cold sodium-rich section and the hot depleted one effectively prevents sodium vapor from diffusing back, thus resulting in durable red burners that kill the lamp’s efficacy.

Philips tentatively addressed all these problems with the lamp shown here. While it seems like a normal SOI 60W on the outside, its design is special for the fact that it has a low-pressure nitrogen atmosphere whose role is to homogenize the burner temperature via heat conduction and convection processes (the latter being limited by the low fill pressure so as to prevent hot gas buildup). Since a high vacuum is no longer required, the four barium azide rings located near the base were not activated, hence the absence of the barium mirror getter that is usually found on the bulb neck of sodium lamps. Another interesting design feature unique to this SOI 60W is an extended sleeve that includes the discharge tube's U bend so as to increase its temperature and reduce the temperature difference with the electrode sections. This is a design not found in the commercial variant of the lamp, where the U bend lies just outside the volume enclosed by the sleeve.

While these changes proved effective in reducing the sodium migration rate, this came at the cost of a lower output flux caused by the higher thermal losses incurred by the inner lamp atmosphere. The glass sleeve limits this problem by confining the hottest gas around the burner while forming a thermal barrier with the outer jacket, but the effect of the gas fill on the lamp performance is still too significant, so the concept was never released on the market. Sodium lamps with an evacuated jacket thus remained the norm and the distillation problem was eventually mitigated in the next generation of lamps, the SO-X introduced by Philips in 1965, where a larger burner diameter and an optimized buffer gas fill greatly reduced the rate of sodium migration through life.