IR & UV

[P3D Elliot Burke <elliot@xxxxxxxxxx>]

photo-3d@xxxxxxxxxxxxxxx wrote:
> 

> 
> Date: Thu, 5 Dec 1996 23:09:21 -0500
> From: P3D Gabriel Jacob <jacob@xxxxxxxxxxxxxx>
> To: photo-3d@xxxxxxxxxxxxxxx
> Subject: Re: IR UV and other nasty stuff!
> Message-ID: <199612060409.XAA04206@xxxxxxxxxxxxxxxxxxx>
> 
> Dr. George A. Themelis writes
> 
> >This raises more questions:  How much UV is going through the glass lenses?
> >Is it true that a simple piece of glass blocks UV?  If yes, should an extra
> >piece of glass before the slide protect the slide from fading?  Are halogen
> >quartz bulbs more damaging to slides than regular glass tungsten bulbs?
> 
> >I suspect that the answer is not that simple. -- George
> 
> Actually it depends if it is the short or longer UV wavelengths we are
> talking about. The longer ones of course are closer to the visible and
> the shorter ones progressively farther out. Glass will protect to a degree
> against the more damaging shorter UV rays but not to the less but still
> damaging (and fading) longer UV rays. The ironic thing is that Halogen
> lamps use quartz in their glass envelope which allows alot of the shorter
> UV rays to transmit very well! Thats why they call them QTH or quartz-
> tungsten-halogen. Of course the tungsten and halolgen decribe the other
> elements of the lamp. The other lenses if made of glass will help protect
> against the UV but I think plastic, maybe some kind of plexi glass would
> help to absorb the damaging UV. Don't know if it could take the heat thou,
> unless it was strategically located before the film.
> 
> Gabriel
> 
This isn't quite true.  The envelopes of halogen bulbs have for many
years been made of Vycor or a similar glass.  This is a low expansion
glass with a higher than usual working temperature, although nowhere
near as high as fused quartz.  I invite you to try melting some fused
quartz, then compare with melting the envelope of a halogen bulb- there
is a huge difference!
UV transmittance of Vycor is less than that of fused silica.  Since most
bulbs are probably made from some non-Corning glass, it doesn't much
matter what the Vycor transmittance is if that isn't what used.  This
means I don't have to drag out my Corning catalog (yeah!) but before you
suggest I turn on my spectrophotometer, let me say the PMT's are in
another piece of equipment now.
BTW there is a large variation in the UV transmittance of fused quartz,
from <170 nm to about 280 nm, depending on level of impurities.
Quarz-halogen is an obselete term.

Elliot Burke
HighTide Instruments
elliot@xxxxxxxxxx


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