Tocsin, I didn´t realise that an observer would have to exit the bunker following a nuclear strike.
What was the one minute delay for? Was it to avoid the heat and blast effects of the strike, or to give time for the photographic papers to develop? Were the papers intended to give a picture of the airburst?
If there were subsequent strikes, would the No. 3 observer have to go outside again?
The process is shown in the film at the point linked below: