Will UV Filter Stop Bullseye Glass From Starting Fires?

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Discussion Overview

The discussion revolves around whether placing a clear UV filter over a magnifying lens would prevent the lens from focusing sunlight to a degree that could start a fire, particularly in relation to "bullseye glass" panels in windows that have been observed to produce intense heat and smoke upholstery. The scope includes practical applications and experimental considerations.

Discussion Character

  • Practical question, Experimental/applied, Technical explanation

Main Points Raised

  • One participant questions if a clear UV filter will inhibit the lens's ability to focus sunlight sufficiently to cause fire, citing personal observations of fire hazards from sunlight passing through bullseye glass.
  • Another participant suggests that the UV filter may not help, proposing that the glass likely blocks much UV light, and recommends using a neutral density filter or a lens with a negative focal length instead.
  • A third participant emphasizes the importance of experiments and suggests using a magnifying glass and UV blocking film to test the idea outdoors.
  • Another participant clarifies that ordinary glass, including magnifying glasses, typically blocks UV light, indicating that the issue may not be related to UV but rather to the concentration of visible light leading to increased heat.
  • One participant summarizes that any type of light focused to a small spot can increase energy density and heat, recommending the use of shades or neutral density filters to reduce light intensity.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of UV filters in this context, with no consensus reached on the best solution to the fire hazard posed by focused sunlight.

Contextual Notes

Participants note that ordinary glass may block UV light, which could affect the relevance of the proposed UV filter solution. The discussion also highlights the need for practical experimentation to explore the claims made.

William203
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Practical question: Will placing a clear UV filter over a magnifying lens inhibit the ability of that lens to focus sunlight to the degree necessary to produce fire?
I have a window with several "bullseye glass" panels in my home and have noticed that when the bright winter sun passes through at just the right angle, these panels produce a beam hot enough to get the upholstery smoking! Seems like a fire hazard and I'm wondering if applying the UV blocking film sold at home centers will fix the problem.
Thanks for your insights...
 
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William203 said:
Practical question: Will placing a clear UV filter over a magnifying lens inhibit the ability of that lens to focus sunlight to the degree necessary to produce fire?
I have a window with several "bullseye glass" panels in my home and have noticed that when the bright winter sun passes through at just the right angle, these panels produce a beam hot enough to get the upholstery smoking! Seems like a fire hazard and I'm wondering if applying the UV blocking film sold at home centers will fix the problem.
Thanks for your insights...

I suspect that it will not help. The glass panel probably blocks much of the UV. You would probably have better success with either a neutral density filter or with something that acts like a lens with a negative focal length.

I believe that I burned paper with polycarbonate lenses when I was a child. Polycarbonate plastic blocks most short wavelength. So the heat must have come from the focused visible light.
Many of the people at PF were probably such children!:wink:
 
Experiments and observations are important parts of science. Example of cheap equipment needed to test your idea: a magnifying glass, paper and UV blocking film (or sunglasses with UV protection to try it initially). But try it outdoors on a sunny day :wink:. EDIT: I just saw that Quantum Defect managed to sneak in a reply above just before me :biggrin:.
 
William203 said:
Practical question: Will placing a clear UV filter over a magnifying lens inhibit the ability of that lens to focus sunlight to the degree necessary to produce fire?
I have a window with several "bullseye glass" panels in my home and have noticed that when the bright winter sun passes through at just the right angle, these panels produce a beam hot enough to get the upholstery smoking! Seems like a fire hazard and I'm wondering if applying the UV blocking film sold at home centers will fix the problem.
Thanks for your insights...

As has been stated, you have a slight misunderstanding here. UVs are normally blocked by many clear glasses. Ordinary glass windows are often opaque to UV. And so will an ordinary magnifying glass. So you have been focusing on the wrong "culprit" (no pun intended).

Any type of light, when there's a lot of it and it is focused to a small spot, will increase in its rate of energy density. At some point, such a high energy density rate being absorbed will cause a significant increase in heat. This is what is happening in your case. The easiest solution is simply to put a shade, or some neutral density filters to cut back on the amount of light coming in.

Zz.
 
Thanks all for the quick and helpful feedback