Late-1970s Philips GR37 B/2

Neon glow lamps are extremely robust light sources which are impervious to vibrations and can operate reliably over extended periods of time. These qualities were put to use in indicator lighting where these lamps proved highly popular before the advent of high-brightness LEDs. One limitation of the discharge lamps is their relatively low brightness which was sometimes compensated with of a front lens, integrated or not to the lamp. The obvious solution of increasing the lamp current was not implemented because this systematically leads to a sharp decrease in service life as a result of electrode degradation via ion sputtering. This phenomenon which happens at the plasma-cathode interface leads to bulb darkening and to the absorption and trapping of the fill gas, resulting in a gradual decrease of pressure over time. This change causes a progressive drop of the light output until the lamp eventually ceases its operation due to the impossibility of maintaining a self-sustained discharge in the tenuous atmosphere of the lamp. A higher driving current level is thus undesirable because it always leads to a greater sputtering rate of the cathodes, which invariably results in a faster rate of light output decrease over time and in a shorter service life.

A solution to this particular problem was found in the 1950s in the form of a mercury dose added to the Ne-Ar Penning fill of the discharge lamp, resulting in a reduction by a factor of 25 of the sputtering rate of nickel-iron cathodes at a given drive current. Mercury has a lower ionization potential than any of the noble gases used in glow lamps and this has the effect of reducing the strength of the electric field in the plasma sheath that covers the cathode, leading to less energetic ions which in turn causes less damage to the electrode. The discharge current and brightness can then be increased without compromising the life expectancy and the flux maintenance of the lamp. This solution was first implemented in numerical indicator tubes (i.e., Nixie tubes) where cathodic sputtering was a particularly important issue. The use of mercury proved only partially satisfactory in this application because the formation of a bluish haze around the edges of the negative glow made characters less readable. These tubes were therefore systematically coated with a red lacquer so as to filter out mercury’s emission and restore a proper visibility of the character-shaped cathodes.

The addition of mercury to the neon-argon mix in indicator lamps was implemented only at the beginning of the 1970s, when these sources started to replace incandescent lamps in large indicator lights of control panels. Mercury enabled the design of larger lamps with enhanced light output as needed for the application, and the GR37 B/2 presented here is one of the first high-brightness models offered by Philips (the Netherlands). Its asymmetric electrode construction is quite intriguing in view of the fact that the lamp was specified for an operation on AC circuits only, which usually calls for electrodes of equal surface areas. An interesting consequence is a glow discharge color which depends on the voltage polarity applied to the electrodes, and this difference becomes clearly visible at lower current levels. The wide grid electrode develops a mercury-dominated discharge with a characteristic blue color, while the higher current density and electric field strength at the ring electrode results in a neon-dominated plasma with a red-orange glow. The difference in spectral output arises from the mean energy of electrons in the discharge, which depends on the geometry of the cathode, a phenomenon that causes the selective excitation of elements in a mixture of gases.

The physical size of the GR37 B/2 is quite large compared to that of standard indicator lamps. Its B22 bayonet end cap houses a series resistor which stabilizes the discharge current to 7 mA, an electric intensity nearly twice as high as that of the largest standard glow lamps. Philips offered this particular model only briefly, from the late 1970s until the early 1980s, and production ended because of a lack of demand. Miniature incandescent lamps thus remained the source of choice in the applications targeted by the GR37 B/2, until the advent of high-brightness LED lamps in the early 2000s put a definitive end to filament lamps in panel indicators.