1975 GEC MBTW/V 175W in operation

Shown here is a peculiar 175 W self-ballasted blacklight mercury lamp made by GEC of England. Typical of earlier mercury lamps of this kind, the MBWT/V 175W features a pear-shaped outer bulb that was eventually superseded by an elliptical one. Even more interesting is its unusual wattage rating, which appears to originate from a burner and ballasting filament combination that is identical to that used in standard 160 W white-light self-ballasted lamps. The different nature in glass material of the outer bulb has an impact on the lamp's operating temperature, which is higher in the black-light variant, resulting overall in a greater power dissipation. This characteristic is unique to British lamps of the kind shown here and resulted in a service life shortened to about 2 kh. Other lamp manufacturers adjust(ed) the filament parameters to keep the total dissipated power at the same standard levels regardless of the bulb used, thereby maintaining a consistent service life across self-ballasted lamp types.