RE: filmscanners: Re: looking at the Sun

["Shough, Dean" <dean.shough@lmco.com>]

> Shough, Dean wrote:
> 
> >> From a prctical point of view, I seriously doubt that glass absorbs a
> heck
> >> of lot of UV, certainly not over a long term.
> >> 
> > 
> > 
> > Correct.  Most glass readily transmits near UV quite well.  For example,
> a
> > very common optical glass, BK7 transmits 80% of light at 340 nm and 5%
> at
> > 300 nm.  This is typical for many glasses.  It is mainly the heavy
> flints
> > and rare earth glasses that tend to absorb the near UV.
> 
> Hi Dean,
> 
> I realize that your reference to optical glass is due to the discussion 
> of camera lenses and the risk of looking through the viewfinder at the 
> sun, but since this seems to be an area you have explored in some depth, 
> would you know how regular window glass or picture frame glass do in the 
> UV screening process?  Also, I imagine certain wavelengths of UV are 
> more damaging to things like ink dyes than others, so does the 
> "bandwidth" of wavelengths the glass screens make a big difference in 
> things like inkjet prints fading?
> 
> Art
>

Actually, I wrote about BK7 glass because I had the information readily
available.  All glasses strongly absorb UV radiation.  Ordinary crown
glasses, e.g. BK7, pass near UV.  The flints and especially the heavy flints
do not pass the near UV and may even appear slightly yellowish. 

Ordinary window glass is similar to BK7 in composition and properties with
the addition of contaminants due to the inferior manufacturing process.
These contaminants induce scattering centers that increase absorption in the
blue and red, giving window glass its characteristic green cast.  I have no
information about what happens to the near UV absorption.  Higher quality
glass ("white window glass") is processed without the contaminants and
should have optical properties similar to BK7.