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There are several distinctive characteristics, in the range of visible light, from nuclear explosions. One of these is a characteristic "dual flash" measured by a bhangmeter. This went into routine use on the advanced Vela nuclear detection satellites, first launched in 1967. The earlier Velas only detected X-rays, gamma rays, and neutrons.
The bhangmeter technique was used earlier, in 1961, aboard a modified US KC-135B aircraft monitoring the preannounced Soviet test of Tsar Bomba, the largest nuclear explosion ever detonated. The US test monitoring, which carried both broadband electromagnetic and optical sensors including a bhangmeter, was named SPEEDLIGHT.
As part of Operation BURNING LIGHT, one MASINT system photographed the nuclear clouds of French atmospheric nuclear tests to measure their density and opacity. This operation is borderline with Nuclear MASINT.
Bhangmeters on Advanced Vela satellites detected the what is variously called the Vela Incident or South Atlantic Incident, on 22 September 1979. Different reports have claimed that it was, or was not, a nuclear test, and, if it was, probably involved South Africa and possibly Israel. France and Taiwan have also been suggested. Only one bhangmeter detected the characteristic double-flash, although US Navy hydrophones suggest a low-yield blast. Other sensors were negative or equivocal, and no definitive explanation has yet been made public.