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  #1  
Old 27 July 2009, 06:27 AM
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United Kingdom Blue flashlights and WWII blackouts

Comment: I was told that during World War II in London, England they issued
flashlights (torches the English would call them)with blue filters to use
during a blackout. The idea was that air raid wardens (or whoever was out
during a blackout) would use these to make some light to guide them
without alerting enemy planes. But (according to the story) blue light
travels farther than white light. Pilots flying high over London during a
blackout could see nothing except these blue arcs as the air raid wardens
shined their flashlights along the darkened streets. I don't know the
physics of light and I don't know if this makes any sense.
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  #2  
Old 27 July 2009, 06:30 AM
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How can a component of white light "travel father" than white light?
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Old 27 July 2009, 08:03 AM
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Different wavelengths get scattered at different rates by the atmosphere. But blue light gets scattered the most, so it should have been harder to see from farther away.

I know some torches use red filters as it is less harmful to your night vision - maybe this is just a confusion of that?
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Old 27 July 2009, 03:51 PM
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Quote:
Originally Posted by Squirt View Post
Different wavelengths get scattered at different rates by the atmosphere. But blue light gets scattered the most, so it should have been harder to see from farther away.

I know some torches use red filters as it is less harmful to your night vision - maybe this is just a confusion of that?
I like red filters for flashlights- great for games of flashlight tag, or as we called it, German Spotlight. Red light doesn't travel far or well, and blue should be even worse.
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  #5  
Old 27 July 2009, 04:28 PM
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Having read a lot of first hand accounts of the Blitz and seen photographs, films and documentaries I have never heard of ARP (Air Raid Precaution) wardens - or anyone else for that matter - having blue filters on their torches.

I had a quick look through Waiting for the All Clear: True Stories from Survivors of the Blitz by Ben Wicks. One story was from an ARP warden who was searching through a bombed out flat for survivors. He mentioned how everything was pitch black - wardens did not even seem to have torches of any kind. This is probably because they would need both hands to move rubble and rescue people.

Another story is from a warden who had to cycle home at around midnight once his duties were over. Following a raid one night he was sent to a paricular street. There were so many fires from buildings that he did not put his bicycle lamp on. Batteries were hard to obtain and he wanted to save his power. There was no mention of a blue filter on his bike lamp. If batteries were hard to obtain perhaps this was another reason why torches were not used.
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Old 31 July 2009, 05:22 PM
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The blue light can't travel any farther than the white light out of the same flashlight. The blue light is a component of the original unfiltered white light. Even assuming 100% transmisivity of blue light on the filter, the light in those frequencies would be no more intense with or without the filter.
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  #7  
Old 31 July 2009, 06:04 PM
jimmy101_again jimmy101_again is offline
 
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Not sure as to the validity of the "blue flashlight in a blackout" concept but US military flashlights from WWII to the present come with colored filters. Typically red, blue and diffuse white.

I know the red filter is to preserve a persons night vision, I wonder what the blue filter is for?
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  #8  
Old 31 July 2009, 06:50 PM
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Military

Quote:
Originally Posted by jimmy101_again View Post
Not sure as to the validity of the "blue flashlight in a blackout" concept but US military flashlights from WWII to the present come with colored filters. Typically red, blue and diffuse white.

I know the red filter is to preserve a persons night vision, I wonder what the blue filter is for?
Our flashlights have red, blue and green filters.

The reason for a blue filter is varied as the tasks that are required to use a flashlight. On the most basic, roads on most topographic maps are red and orange. Red filter makes them disappear.

When working on repairing equipment, blue is sometimes easier to work with when dealing with coloured wires and green nuts and bolts.

For my experience, different coloured filters allows for signalling different things. Moving at night in a brigade assembly area, it is nice to know that your entrance to your hide is the red-over-blue light arangement.

Plus, executing s survey scheme at night, it really helps to have your flag and plain banderoles marked in red and blue for night surveys.

There are plenty of good uses for differing filters.
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  #9  
Old 04 September 2009, 09:20 PM
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Quote:
Originally Posted by UEL View Post

On the most basic, roads on most topographic maps are red and orange. Red filter makes them disappear.
This is why military maps now use red-brown instead of red. When you use the red filter on your flashlight, the red-brown on the map looks brown.
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  #10  
Old 05 September 2009, 09:22 AM
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I believe that most naval signal lamps have a blue filter available as in peacetime a blue light would be easily distinguished from other lights at night, most of which would be white, red or green. Of course this would be true on land as well; there are few ble lights.
If you were searching for someone you would not use a filter as blue light would not be reflected from something red (like blood or even flesh); but it is possible that an existing stock of flashlights (which is the correct UK military term; torches/torchlights are for civilians) were issued which had the blue (and probably red and green) filters in the box, and their use was misunderstood (you have a blue filter, you must use it)
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