1965 Osram Spektral Tl

Thallium is a metal which was discovered in 1861 by spectroscopic methods and derives its name from the bright green light color emitted by its vapor when excited in flames. This element attracted some attention in the early days of discharge lamp development, during the first decades of the 20th century, because its spectral emission close to the eye’s maximum sensitivity could potentially enable high lumen efficacies. Although this metal has a low melting point of just 304 °C, it is also is characterized by a low vapor pressure (100 Pa at 1097 K) which requires high burner temperatures and made the design of even low-pressure lamps quite impractical. Mercury and sodium eventually prevailed and thallium was never used as a metal in commercial lamps, except for certain specialized applications like spectroscopy and interferometry.

Since the 1930s Osram markets a light source for lab use using this element, dosed in an argon-filled quartz burner where an electrical discharge is excited. The low metal vapor pressure plus the limited interactions between electrons and thallium atoms in the discharge result in a diffuse plasma characterized by a low impedance which limits the overall lamp power dissipation to 13 W only. Naturally, this contributes in the first place to the problem of bringing thallium into the vapor phase, which is complicated even further by the fact that the discharge is not inefficient enough to properly heat-up a bare burner to a level required for the proper emission of the element’s spectrum. The company solved the limited volatility of thallium with a thermal-insulating structure which consists in a cylindrical metal shell whose outer surface is coated with graphite so as to lower the thermal emissivity in the infrared and limit the extent of radiative cooling of the burner. A small opening on the side of the shell let some radiation escape from the confined argon-thallium discharge. A small quartz window attached to the shell’s opening limits the cooling of the burner there, thus preventing the condensation of thallium which otherwise would form a light-blocking mirror there.



The lamp featured here is a second-generation model, offered by the German company from the early 1950s to the late 1960s, which is fitted with a P28s prefocus base for the precise location of the light source in optical systems. The quartz burner enclosed in the heat-insulating shell is a standard type Osram employed in its HQL and HQA high-pressure mercury lamps made at the time. The discharge vessel is provided with two ignition probes connected to the resistors which are located at the side of the burner. These probes enable a reliable ignition below 200 V, which is crucial since this lamp was designed for an operation with an impedance connected in series to the 220 V mains.

The optical emission from the thallium discharge is nearly monochromatic, with one prominent spectral line in the green region at 535 nm which is usually isolated via the use of a filter or a monochromator for practical use. Moreover, there are two line clusters in the UVA region at 352–353 and 378 nm which are also useful to spectroscopy. Although not aimed at ultraviolet applications, the soft soda lime silicate glass jacket which encloses the inner components of the lamp has a reasonable optical transmittance in this spectral domain.