Classic half-coated East German mercury lamp in operation

Shown here is a 1979 Narva NF125-21 that was designed for an operation in basic streetlight luminaires without reflector. The homogeneous illumination of the road surface is realized by a batwing-like light distribution of the lamp which is achieved thanks to a bulb half coated with a reflecting layer of titanium dioxide. Color correction of the mercury light is done by a thin layer of red-emitting yttrium vanadate which covers the entire bulb. That phosphor has a very low content of diffusing material so as to limit light scattering, which otherwise would homogenize the lamp's light distribution. Because the NF125-21 was intended for an operation exposed to weather elements, its bulb is made of a hard borosilicate glass that can withstand thermal shocks.