1977 Tesla RVIM 400W

Tesla of Czechoslovakia began experimenting with metal halides additives in mercury lamps around the mid 1960s. The state-owned company first focused its attention on relatively volatile, easy to use metal salts. following this strategy, the Czechs eventually developed and released their first RVI metal halide lamp in 1966, which consisted of a 400 W RV mercury lamp dosed with indium iodide. Because the emitted light color was strongly blueish, development work towards proper white-light lamps continued at Tesla. To this end, thallium and sodium iodides were added to the original RVI lamps, thereby replicating successful work done at GE (USA) and then at Philips (the Netherlands) a few years earlier on the so-called NTI-filled mercury lamps. Tesla released its improved, second-generation RVI lamp in 1968.

Although Tesla's original indium iodide lamp was a barely improved mercury lamp, the company kept on producing a limited number of those for decorative lighting purposes. The salt dosage was somewhat reduced in order to increase the light color saturation and the lamp reference was changed to RVIM (M for “modrý”, which means “blue” in Czech) so as to distinguish it from standard MH lamps intended for general lighting applications. During the 1970s the original bulged bulb was replaced by a tubular one in order to enable a usage in precision floodlighting luminaires (Tesla first introduced the tubular bulb in its HID lamps in 1969). The 1977 RVIM 400W shown here is in essence a direct descendant of Czechoslovakia’s first RVI lamps, and one of the earliest type of colored MH lamp built specifically for decorative floodlighting (GEC of England is the other company which also developed such lamp type in the late 1960s).

The present RVIM 400W was assembled using Tesla's standard white-light RVI lamp structure and burner, to which indium mono-iodide (InI) was dosed in place of the usual blend of indium, thallium and sodium iodides (i.e., the NTI mix). A feature which is typical of standard NTI-MH lamps but is useless in this RVIM is the steatite tube that sheathes the frame wire next to the burner. This photoelectron shield is normally used to limit the electrolytic loss of sodium from standard white-light burners. The rest of the lamp design is fairly typical of metal halide lamps produced by Tesla in the late 1970s. Two large barium-dispensing rings borrowed from the company’s radio tube division are placed near the lamp base so as to maintain a proper vacuum for the thermal insulation of the arc tube.

The quartz burner is of the 3-piece kind and consists of a central tubular body whose extremities are narrowed in a conical shape and are fused to narrower tube sections. These tubes are pressed onto a molybdenum foil which then forms the gas-tight end seals. The conical end-sections of the burner form the electrode chambers, and these parts are usually shaped with precision so as to ensure a well defined cold-spot temperature critical for the realization of consistent light technical properties among a large group of lamps. While those extremities arctube extremities are coated with a heat-conserving material in standard RVI lamps, such measure is not required in the present RVIM due to the high volatility of indium iodide and the need of a relatively low salt vapor pressure in order to attain the desired saturated light color.

Like its standard RVI counterpart, this blue-emitting lamp is also designed for an operation at 3.2 A on 400 W mercury lamp ballasts of the series choke type, for a usage on 220-V mains circuits. Discharge ignition is enabled by a switch ignitor which is connected in parallel to the lamp so as to generate high-voltage pulses by temporarily short-circuiting the ballast when mains power is first applied.

Tesla kept on producing RVIM lamps for more than two decades, until the dissolution of the Eastern Bloc and of Czechoslovakia in the early 1990s, which eventually led to a massive restructuring of this lamp manufacturer. Certain specialized discharge lamps, such as this RVIM, were then phased out in the process.