1969 Tesla RVY 250W

In 1969 Tesla of Czechoslovakia introduced its RVY range of yellow-colored high-pressure mercury fluorescent lamps intended for dusty environments where visibility is affected by (blue) light scattering. Available from 80 to 400 W, these lamps were essentially standard RVL mercury lamps whose transparent outer bulb is replaced by a durable mass-tinted one made of cadmium silicate glass. Such simple change kept development and tooling costs to a minimum and was possible by the fact that Tesla used a soft soda-lime silicate glass for its RVL outer bulbs at the time, a material that has thermo-mechanical properties similar to those of cadmium glass.

Typical of Tesla’s mercury lamps made in the late 1960s, this RVY 250W features a 3-piece arc tube with narrow-pressed moly end seals and tungsten electrodes activated with thorium oxide. The arc tube is held into place by a “delta” frame consisting of two triangular pieces of stamped iron sheet metal, a unique design developed in the second half of the 1960s at the company’s plant in Holešovice, Prague, and put into regular production there in 1968. The outer bulb is coated with a thin layer of calcium-strontium-magnesium orthophosphate activated with tin and manganese, giving a broadband reddish-orange fluorescence. Because of the weak red output from this phosphor the overall light color has a greenish hue, reinforced by the strong green and yellow lines from the mercury arc. The filtering of mercury’s blue lines by the cadmium glass has a limited impact on the lamp’s flux output, which for the present 250 W model went from 10 to 9 klm (initial level). The fluorescent material was eventually replaced in 1970 by a more efficient orange-emitting zinc-strontium orthophosphate phosphor activated with divalent tin, applied to both RVY and RVL lamps.

Beside its low production cost, a key advantage of Tesla’s RVY lies in its compatibility with existing mercury lamp systems. Whenever needed, light could be changed from bluish white to yellow just by swapping lamps. Such convenience resulted also in their usage in streetlighting, to highlight hazardous road sections, which was not what Tesla intended them for. The problem that made them not well suited for that application is the fact that red emergency vehicles could not be clearly identified under their light. Tesla eventually discontinued the RVY in 1973 when the company upgraded the outer jacket of most of its mercury lamps to a hard borosilicate glass in order to better withstand the rigors of outdoor applications, which no longer permitted mass-tinted bulbs. Also, that year saw the commercial introduction of the first high-pressure sodium lamp in Czechoslovakia, the Tesla SHC, a far more efficacious source of orange light which does not lack in red compared to high-intensity mercury lamps produced at the time.