||To coincide with the start of colour transmission by the BBC in 1967, a new twin-filament studio lamp was developed by Osram-GEC in response to a problem that had emerged with studio lighting in the early colour TV trials. Due to the poor spectral sensitivity of the first colour TV camera tubes, when the studio lights were dimmed so as to achieve the required illuminance levels, the change in lamp colour temperature resulted in poor colour balance of the electronic images.
The problem was most elegantly solved by GEC's invention of the twin-filament studio incandescent lamp. This reduced colour variation during dimming by switching on or off or dimming of one filament while the other was maintained at full power. The first lamp in the range was the non-halogen 2½/2½kW CP/20. In 1969 to compete with Thorn Lighting's development of the CP/32 quartz halogen equivalent, GEC upgraded its incandescent lamp to also feature a halogen fill, but still in a hard glass bulb.. Later as colour television camera sensitivity improved and less light was required, a 1¼/1¼kW version was developed. This proved to be insufficient for the independent TV stations, resulting in the intermediate 2½/1¼ kW CP/57 featured here.
The bulb is blown in A45 aluminosilicate hard glass, and sealed to a special wafer stem of borosilicate. That was made by pouring crushed glass into a carbon mould around the four tungsten contact pins, which had been pre-glazed in the area of the seal, and pressing the whole together while heating in an induction furnace. Special joints had to be developed between these heavy pins and the filament mount assembly, and consist of coils of tungsten to reinforce an arc welded joint. The bulb is filled with HBr and nitrogen.