||Dating back to the mid 1960's, mercury lamps such as this were still required in small quantities without the fluorescent phosphor coating which had become standard on all lamps many years earlier.
Principally they were employed in streetlighting applications which had been designed with a long spacing between each lighting column, frequently encountered in many continental European countries, and where the good optical control attainable with clear lamps was required to throw the light uniformly across a wide area. It had been found that retrofitting the later colour-corrected phosphor coated lamps into the same luminaires did not achieve such good optical control, and unlit patches of road could occur in between the very brightly lit areas under each lantern. Thus until new lanterns were also installed which were better suited to distributing the light of coated lamps, a small but significant requirement still remained for the earlier clear versions.
It is interesting to note the extensive whitening of the arc tube in this early lamp, which has completed many years of service. The rapid lumen depreciation of mercury lamps has been a drawback since their earliest days, but during the 1950's a tremendous amount of research went into preventing the arc tube blackening which caused this. GE of America was first to introduce a non-blackening arc tube, and this Osram lamp is made along the same lines. A physically larger electrode which runs cooler is found at each end of the arc tube, and in addition the chemistry of the electrode and its emitter coating has been modified such that deposits on the arc tube wall appear white instead of black, thus helping to avoid such rapid lumen depreciation.