During the 1970s, sealed beam automotive headlamps began to fall out of fashion for a number of reasons. Firstly they were available only in round shapes and automotive designers were becoming more interested in exploring rectangular and other profiles for their headlamps. This hit the factory hard because the automatic production equipment could not realistically be adjusted to handle the wide variety of different shapes and sizes that car manufacturers desired. A sixth production group, Line F, was eventually set up to produce rectangular shape lamps, but the production of these types was technically challenging and only a limited range of shapes and sizes could be produced.
Secondly, great improvements had taken place in the design of the rubber sealing gaskets on conventional headlamp assemblies, to reduce the ingress of moisture and dirt, and this presented a lower cost solution than the flame-fused sealed beam lamps. Thirdly, tungsten halogen lamps of compact dimensions were beginning to take over the headlighting market, and these were introduced with various pre-focussed caps which also made headlamp alignment a thing of the past.
Thus the advantages of the sealed beam lamp, at least for automotive applications, had been entirely eliminated and it only brought drawbacks of inflexibility and relatively high cost. Some types were produced with tungsten halogen capsules inside, but those could be produced still more cost competively by glueing the reflector and lens together instead of the flame-sealing process. Demand for Corby's lamps rapidly declined, and the factory was closed in April 1976 with the loss of over 500 jobs. As the market continued to dwindle, the British manufacturers once again reverted to sourcing in their requirements from the Americans, who were able to sustain their own automotive sealed beam production for many years longer owing to the much larger size of that market.
Meanwhile, demand was still high and in fact growing for the general lighting PAR38 lamp, and the high speed automatic production line for these types was saved and relocated to Thorn's Leicester
factory, along with some smaller single-head type machines that produced PAR36 and PAR64 lamps for aircraft and stage/studio applications. Thorn later gave up the PAR38 operations and once again sourced in its requirements, primarily from GE's Canadian factory but occasionally also from Osram Germany and Philips of the Netherlands, which had become more cost-effective due to their faster equipment which operated with lower scrap levels. The PAR64 production still survives today, having been re-located once again to the GE-Tungsram operations in Hungary.
As of 2015 the building still exists, and is in use for the production of Weetabix breakfast cereals.