Whilst I was at a boot fair in Abergavenny, a dealer in militaria showed me what he said was an ARP miner's lamp made by Hailwood & Ackroyd that had blue glass fitted. He said that these were manufactured during the war and placed on gas leaks during air raids. The blue glass stopped them being seen from altitude.
"The Air Raids Precaution Department of the Home Office discovered in the tests carried out at Bedford that Aircraft flying at 2,000 feet cannot spot dime blue lights", Flyer Magazine.
I checked this up online and indeed a couple of sites mentioned this information. The site said that over 100,000 such lamps were offered for sale during the war at 3/6d. Some had red glass. I was initially sceptical of this until I found more about them online. There is allegedly an advertisement for this lamp from the war and I would be most interested to see it.
News about interesting insignia, ARP related info and period photos that turn up.