Late-1940s Elektropodnik SKTB 9W

Work on fluorescent light sources began in 1859 when E. Becquerel (France) filled Geissler tubes (evacuated glass vessels excited with electrostatic generators) with crushed minerals in order to enhance the light emission and alter its color. The concept became a really practical one following the invention of the neon tube and its mercury-filled variant by G. Claude (France) in 1910. It took a couple more decades or research and development before the first practical fluorescent neon and mercury sign tubes (with argon as a buffer for the latter) could be released for commercial usage (A. Claude, 1933). The technology eventually evolved further for a usage in general lighting, first in luminaires with a set of R/G/B lamps and then with single lamps emitting white light with an acceptable color quality. This breakthrough was made possible following the development of the first synthetic phosphors, zinc beryllium silicate in particular.

The Czechoslovakian lamp shown here is one of those very early fluorescent sources derived from Claude's neon tubes. It is built with a set of robust cold hollow cathodes made of soft iron, designed for an operation on high-OCV leakage-flux transformers. Such design allows an instant light output and an extremely long service life thanks to the electrode's excellent reliability and durability. The main penalties for such advantages are a limited system efficiency and the need for high voltages. Typical of very early white florescent tubes, the SKTB is coated with zinc beryllium silicate, a material that was discovered in 1935 by A.H. McKeag of the GEC laboratories and by J.T. Randall from the university of Birmingham, both from England). They found that replacing some of the zinc in willemite with beryllium (with a trace amount of manganese added as a dopant) changed the fluorescence color from saturated green to an acceptable shade of white.

While these lamps found a widespread use in indoors lighting applications, a number of them were also made for some special purposes, like instrument lighting in the case of the present model (intended for gas chromatographs). Its short length (about 25 cm) limits the running voltage to 311 V (a couple kVs are still needed for ignition) and the dissipated power to 9 W only, enough for its intended usage. Interestingly, despite its long service life the SKTB 9W was not permanently built into instruments, it is a standalone light source with its own primitive end caps which consist of embossed copper disk terminals. The electrodes are also of a very early and rudimentary design without any back-discharge shield. As a result, the negative glow spreads outside the electrode's cavity and causes sputtering in certain areas, like in the vicinity of the end seals (see the brownish-colored iron deposits on the glass tube there).