||This lamp is typical of the German self ballasted mercury-blended products manufactured during the 1950's. The question is often asked as to why these lamps were offered in a different outer bulb shape than the standard elliptical bulb employed on regular mercury lamps.
There are two answers to this question. Firstly it must be remembered that the sole reason for the elliptical bulb shape is because it is designed such that all parts are spaced an equal distance from the arc tube, ensuring that the temperature is uniform around the outer bulb. These so-called isothermal bulb shapes are very important, because fluorescent phosphors do not work as efficiently if they are at a high temperature. Thus in any other bulb shape, the phosphor in some areas would be hotter than others, leading to a loss in efficacy and different apparent brightness in different parts of the lamp.
Mercury blended lamps, of course, contain a tungsten ballast filament in addition to the arc tube. The filament is physically compact because of its more uniform pattern of heat radiation, it is therefore found that a rounder bulb shape is better suited to maintaining uniform phosphor temperature in this style of lamp.
The second reason for a different bulb is quite simply because mercury blended lamps are traditionally used in incandescent luminaires. The long neck is required to reach the socket in many older fittings. However today, the round bulb has been completely abandoned. The reason is simply because mercury blended lamps are made in such small quantities that the special shape can no longer be justified.