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Re: How much light is reflected by a glass cover?




>I've been told plexiglass has
>better light transmission properties than soda-lime glass but I have never
>used it for optics. I'm sure there are plastic alternatives to glass that
>are probably better than our standard lids however you will always need to
>keep them nice and clean, I find my glass lids get dirty on a daily basis
>from evaporation and keeping them clean is a pain but important.

Most of the plastics will start to deform at *far lower* temperatures than 
the glass. If you can maintain a temperature below the maximum operating 
temperature of the plastic then something like Plexiglas or Acrylite (both 
trade names for Acrylic) will most likely work well. Acrylic is only 
useable to 180F, Polycarbonate sheet (Lexan) is good to 240F. There are 
exotics like optically clear FEP, which is essentially clear Teflon and can 
take much higher temperatures (to 400F), but it costs so much that just 
about anything would be more economical.  Any plastic will be easier to 
work with than glass if you need to cut or drill it though which is 
something to think about if you're building a DIY light setup.

Unfortunately the catalog I use as a reference for plastic properties for 
these sheets no longer lists the light transmittance information. I think 
Acrylic is generally "clearer" than glass, but it filters out different 
wavelengths of light than glass too, so it might be worse than glass for 
some applications and better than glass in others. I've used acrylic for 
all kinds of light setups myself and have never had a problem with it, even 
in an enclosed PCF setup where I thought the heat might be a problem (just 
holes for convection, no fan). Acrylic probably the best choice as a glass 
replacement for most light setups due to it's low cost -- just be mindful 
of the lower safe operating temperature.

>Perhaps
>finding a net or mesh as you said might be better, you can get one kind at
>fishing stores which is made of thin fishing line, shouldn't be toxic and
>will probably allow more light to pass than thicker or non-trasparent
>materials. Do they make trasparent fly screens?

There is a clear nylon mesh that exists, but I've never found a source for 
it in reasonable sizes. I haven't ever seen transparent window/bug screen 
though -- that stuff is always black or grey every time I see it.

         -Bill


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Waveform Technology
UNIX Systems Administrator