Mid-1960s Philips Spectraal RbII

The lamp featured here differs markedly from other models in Philips’s LL spectral source family in the fact that its rubidium vapor discharge radiates mostly ionic lines rather than the usual atomic ones. This ionic emission is obtained by particular plasma regime which is optimized to enhance the ionization of the metal vapor, a situation which is normally avoided in normal discharge lamps. The argon fill pressure is higher than in standard rubidium spectral lamps, which results in a higher dissipated power and an increased operating temperature. Both factors increase the metal vapor density and its ionization, which result in much higher excitation rates of rubidium ions. Incidentally, the higher argon pressure presents the benefit of lengthening the electrode lifetime because of the reduced sputtering and evaporation rates. However, the diffusion of rubidium atoms inside the burner is more difficult and can lead to heterogeneities in the alkali vapor density.

Like all spectral lamps made by Philips, this special model features several standard components which were used in the manufacture of mass-produced general lighting lamps. For instance, the triple-coiled triple-oxide electrodes and the alkali-resistant two-ply glass discharge tube came from SO-I 140W low-pressure sodium lamps. While the discharge tubes of mass-produced lamps were fabricated using mechanized methods, the burner inside this Spectraal RbII was entirely worked by hands due to the very small production volume of this model. The burner construction is of the first-generation type implemented by Philips in these alkali vapor lamps. The sealing technique consisted first in collapsing the burner's end onto a glass-sheathed fernico seal wire, which was then pushed inward so as to give the seal a flat end-profile. These feedthrough wires were welded to a transversal piece of molybdenum wire which holds the triple-coiled electrodes. The vessel is provided with a dimple the middle, which acts as a cold spot where the metal vapor condenses. This prevents the formation of a light-blocking rubidium mirror, and regulates the vapor pressure of this element. There are two exhaust tubes at each end of the discharge vessel, which enabled a continuous argon flush before seal off in order to remove impurities out of the burner.

This lamp, like all others from the Philips LL spectral family, is designed to operate on commercial 0.9 A auto-leak transformers used for SO-I 140W lamps. The 470 V open voltage of the ballast is high enough to ensure a reliable discharge ignition without the need for electrode pre-heating. The spectral emission from the ionized rubidium vapor features more lines than standard atomic line sources, which makes the Spetraal RbII more useful for applications such as spectroscopy and refractometry. Interestingly, the characteristics of this lamp were never published in the Philips literature and a quick reference to the emitted lines is given in the form of a photographic recording of the spectrum taped directly onto the lamp, near its base.

- While sodium vapor lamps require an excellent thermal insulation in order to reach a high-enough operating temperature, this lamp operates properly with a nitrogen-filled outer jacket where the discharge tube is cooled by heat conduction and convection. This stark difference in lamp design is explained by the high volatility of rubidium, which thus requires a lower temperature for an optimal discharge operation. The cold-spot temperature needed to reach a vapor pressure of 10 Pa is 485.7 K for this metal, which is 131.7 degrees lower than that of sodium. Moreover, discharges in rubidium are less efficient than in sodium vapor, which results in higher thermal losses that compensates for the cooling action of the nitrogen atmosphere around the burner.

- The burner design found in the RbII is of the first-generation type implemented by Philips for their alkali vapor spectral sources. The sealing technique consisted first in collapsing the glass tube's extremities onto glass-sheathed fernico seal wires, which were then pushed inward so as to give the seal a flat end-profile. Inside the vessel these feedthrough wires are welded to a transversal piece of molybdenum wire which holds a beehive electrode. In order to control the condensation of the metal fill and to prevent the formation of a light-blocking rubidium mirror, a dimple was formed in the middle of the burner. This cold spot is also instrumental for the control of the rubidium vapor pressure, which must not be too high in order to maintain a high mean energy of the electron gas in the discharge. Another interesting feature of the discharge vessel is the present of two exhaust tips, one at each extremity. While extremely labor intensive, the dual exhaust method enabled a continuous argon flush through a heated vessel before it was sealed off. This was a very effective way of removing impurities from the burner, as needed for a spectral source of proper quality.