FIRE SERVICE AND GENERAL FIRE SAFETY TOPICS > Fire Investigation

SUNS LIGHT / SOLAR RAYS CAUSING FIRES

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Guest:
IHi folks

Was intrested by a serious broadcast last year on the BBC - it was a documentary about LFBs FITs

They mentioned some fires can be caused by sunlight focussed through windows igniting materials inside buildings.

How many cases have you come across where the fire was started by this?

And what kind of glass is more prone to allow this to happen - ie; stained glass, bowed glass etc etc :idea:

colin todd:
There are certainly cases where the rays were focussed by glass jugs of water or similar.

Guest:
Also shaving mirros are good for focusing energy. The Isle of White Fire Service have a picture of the effects of sunlight being concentrated, this picture is in Principles of Fire Investigation by Ide and Cooke published by the IFE. Consideration should also be given to the orientation of the room and the position of the sun in the day in question. Bare in mind, if the room of origin is facing North then it can't be the sun and if the day was cloudy then it is also unlikely. Stained glass windows or windows with patterns in the glass which have a concave arrangement will provide focusing of light much like a magnifying glass. The sun would have to be of a good intensity and the material close too i would suggest. Any questions, please make out a self addressed envelope to the usual address.

p.b.morgan:
Bull's eye glass was a favourite at one time.  The Fire Research Station did a lot of work on this a very long time ago. Several sitting-room curtains and hall carpets were ignited this way. There was a BRE Report on the subject but can't find it on the website.

Glass items such as paperweights have been implicated from time to time as well; dressing tables with mirrors can be helpful here.

Jim Munday:
It can be a pretty tempting cause when you've run out of other ideas but be careful!  The essential thing is that the sunlight has to be focused on the fuel package (transmitted light by a convex surface and reflected light by a concave one - the other way round won't work!).  Also you need to confirm by experimental measurement or calculation (check out your GCSE physics books) that the focal length of the supposed focusing medium is appropriate when compared to the distance between it and the fuel.  I've seen some classic non-possibilities, including a bulls-eye window in a front door which had supposedly caused ignition of a hallway carpet 3 metres away and a plastic sucker on a car window which allegedly ignited a seat back over a metre away.  When the focal length was measured, it was about 10 cm.
Our old MPFIU boss Adair Lewis had one incident similar to the example in Cooke and Ide, involving a make-up mirror, and I think the Acton FIT of the FB also had a similar case some time back.  Now that I work here in Oz it is generally accepted that broken bottle bases can start bush fires this way but only if they come to rest in the right orientation.

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