||Self-ballasted mercury lamps made by American firm Duro-Test marked a significant departure from the traditional design principles that had been applied to lamps of this type. The filament in early lamps was employed primarily to improve the colour properties of the mercury discharge. However the high power dissipation in the filament made these lamps inefficient, and limited their lifetime to around 3000 hours due to the fact that the filament was highly overloaded each time the lamp was lighted, when the volt drop across the arc was low and across the filament high.
Duro-Test took a different approach, and instead optimised its lamps for the best luminous efficacy, with only a modest degree of colour correction. The filament temperature was lowered to reduce its dissipated energy, and to permit an increase in life. The marketing strategy behind these lamps was primarily to reduce system cost by eliminating the ballast, while offering slightly improved colour. The first model was this 750W rating in R-57 bulb, launched in 1956. Later in 1959, a smaller 450W version and larger 1750W ratings were added.
The arc tube is essentially the same as from a 400W mercury lamp, and is ballasted by a 350W resistance. Unusually, the ballast takes the form of two tungsten filaments instead of the usuual one. These are mounted parallel to and alongside the discharge tube. Possibly this would have offered some advantages in avoiding spatial colour deviations in the light from this lamp, when used in reflector luminaires. It is not known why a clear reflector shaped bulb was adopted for the 750W rating - possibly a reflector coated version was also offered at one time.