1962 Osram Spektral Ne

Electrical discharges in low-pressure neon are mostly known for their applications in commercial and signage lighting, where their bright red-orange light is most useful to draw attention and to create certain colored illuminations. This noble gas is also used to a lesser extent in light sources intended for scientific and educational purposes. The first of such lamps was the Geissler tube invented in 1857 and which proved invaluable for the spectroscopic characterization of gaseous elements during the second half of the 19th century. The discharge in these tubes is excited between a pair of cold plain-metal electrodes to which very high voltages were applied using electrostatic generators. High voltages in the order of several kilovolts and higher were required both to initiate the discharge and to pull enough current from the cold cathode to feed the plasma.

A more practical form of gas discharge light source emerged with the invention in 1904 of the oxide-coated cathode by A. Wehnelt, which enables much higher discharge currents at considerably lower applied voltages. Lamps built with this new electrode were not only more compact, they were also much brighter and could be driven with simple, more practical power supplies. The ignition part was solved around the 1920s with the introduction of an auxiliary probe near the main electrode so as to initiate a low-voltage glow discharge and prime the gas volume with free electrons. Such pre-ionization facilitates the creation of a plasma between the main electrodes and this lowers the lamp's breakdown voltage to less than a kV, often to less than 200 V for certain gases like neon.

Both developments critically enabled economical discharge lamps suitable for a wider range applications than those of Geissler tubes. This technological evolution was picked up rather quickly by lamp manufacturers in the first decades of the 20th century. Such development at Osram (Germany) began around the 1910s under E. Lax and M. Pirani, whose work led to breakthroughs such as the low-pressure sodium lamp, first tested at the company's Berlin factory in 1931. During that decade Osram was also one of the first lampmakers to release a range of compact spectral lamps built with oxide-coated electrodes and with internal means of discharge ignition at low voltage. Such means consisted either of electrode pre-heating, which results in the thermal emission of electrons, or via local low-voltage glows created with auxiliary electrodes. The method employed depends on the lamp fill and in both cases the discharge ignition was aided by the mild radioactivity of the thorium oxide compound used as an electrode emitter.

These spectral lamps are still being produced to this day, although in a different format. In the course of their long production run, the design of Osram's spectral lamps was updated three times so as to adapt to the conditions and needs of the times. The neon-filled model featured here is of the second-generation kind, produced between the early 1950s and the late 1960s and characterized mainly by a P28s prefocus end cap which allows a precise positioning of the light source in optical systems.

The Spektral Ne is built with a discharge tube made of a hard glass material, most likely aluminosilicate or Pyrex, provided with a pair of conical-tipped electrodes of the dispenser design which were normally employed in HQA/HQL high-pressure mercury lamps made by the Germans at the time. The electrical feedthroughs are representative of a relatively early handmade sealing technique which involves the collapse of the discharge tube extremities onto tungsten wires. The burner is provided with two ignition probes at each ends, connected to the opposite electrode via high-impedance carbon resistors. This ignition circuit enables a self-starting operation on a simple series-impedance circuit (an inductor or a resistor being used as the ballast) fed from 220 V mains.

An interesting design feature of the lamp lies in the middle part of its discharge vessel which was stretched during production so as to form a narrowed section. This particular profile raises the discharge power density and temperature between the electrodes, a step taken to prevent the condensation of metal vapor there. Such precaution is of course unnecessary with noble gases and this shows that Osram's production method consisted in using a given burner design for various lamp fills. Of particular interest here is the burner's refractory glass material, which is a relevant design feature for a neon lamp that dissipates 40 W across 4 cm of gas volume. It is in the nature of electrical discharges burning in light noble gases to generate a lot of heat, especially at high input power density.

The light emitted from the discharge is characterized by a bright red color which is typical to this element. The spectral output consists of a cluster of numerous spectral lines grouped between 590 and 750 nm, in the orange and red part of the visible spectrum. This kind of emission has a relatively limited use in science and is mostly applied to spectroscopy, calibration, and educational purposes.