SUNS LIGHT / SOLAR RAYS CAUSING FIRES (Again)

[firstforensic]

Whooa; just a moment! Glass in vehicles is not polarised - its toughened. This causes internal stresses in the glass which can be OBSERVED with polarised glasses as a series of bands and zones.

The thickness of the glass, any tinting or inhomoegenity, imhogge, imhomogeo (bits in it) will result in greater internal refraction and hence a reduction in the suns strength through it.

[firstforensic]

PS

Windscreen glass, as we all know, is not toughened but laminated; ie two panes of glass with a thin plastic "sandwich" bewtween them. So a combination of thickness and plastic will greatly reduce the suns effects.

[Clevelandfire 3]

I thought some car manufacturers nowadays used polarising glass in their cars. Or is it a polarising film applied to the glass?

[SeaBass]

I've come across this phenomenon twice in my career.  Both instances were clearly caused by glass ornaments which were still in situ, and both resulted in an arc of charring on adjacent surfaces. In both case's the light passed through doubled glazed windows before being focussed by the ornaments, however,  one of windows had had a blast protective laminate applied to the outside of the glass.   

[SamFIRT]

Thanks Ruby.
 
Do you have any photographs of the incidents that you would be willing to share?

Also further to the vehicle windscreen/glazing debate

"A car windscreen is made up of two pieces of glass that are separated by a layer of 'PVB' (polyvinyl butyrate) that essentially 'glues' the two pieces together. This has several benefits, the most important being that it acts as a barrier in the event of a crash because the windscreen is held together when fractured. This layer can also help the windscreen block out harmful UV rays, can add some tinting, and can help reduce the amount of sound that comes through the windscreen. Modern, glued-in windscreens contribute to the vehicle's rigidity" from a glass replacement firm's website.

Now that obviously describes a laminated windscreen and there are still some toughened ones out there. Several manufacturers add tint to their vehicle glazing and some also add heater strips to de-mist /ice. This will obviously cut down the amount of EM energy entering the vehicle. I just wondered if anyone on this forum has knowledge of some research into this area. So as to save me having to repeat experimentation that may have been better carried out elsewhere.

[SeaBass]

I'm afraid that I've only been able to find two pictures of one of the incidents but can't load them on to firenet. Happy to e-mail them but thers not much to see.  Both pics show a small charred line (about 20mm long) in a hard wood window sill and the glass ornament that caused the burning. In this case the window was north westerly facing.

In the second incident, which occurred through a south facing window, the focal length was surprisingly long as the ornament was situated on a window sill which was about 400mm high and some 700 mm from the burn on the carpet tile floor covering, that is of course assuming that the ornament hadn’t been moved after the burning occurred.   The local smoke detection did not operate and the burning was only discovered when the occupant of the office returned from holiday.