re: IR & UV

[P3D Gabriel Jacob <jacob@xxxxxxxxxxxxxx>]

Elliot Burke writes
>>Gabriel Jacob writes
>>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.

>Elliot Burke writes
>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.

Hmmm might not be completely true, true! But what your mentioning is not 
quite true also! First of all the term Quartz-Tungsten-Halogen is not
obsolete at all. Fused quartz is still used as a envelope for tungsten
halogen lamps when required. In spectrophotometers where the long UV is
sometimes required one would use a tungsten halogen lamp that is made of
quartz to enhance transmission of the UV. Even if quartz was not used in
halogen lamps, they never the less radiate more UV than a regular tungsten
lamp. A halogen lamp is designed to be operated at higher temperatures
compared to a regular tungsten lamp. This results in high radiant emittance
which is great for 3D projection :-) but not so great because we get a
corresponding increase in UV :-(. For these reasons quartz is one of the
better suited materials to handle this requirement. Now regarding Vycor,
this material has to handle the same requirements as quartz, thus has
to have similiar properties. You mention in your post there is a huge
difference in melting, actually this is not true. Regular glass starts to
soften at 700 degrees C while Vycor at 1500 C and Fused Quartz at 1650 C.
Next point regarding UV transmittance of Vycor being less than that of
fused silica. This is true, but not appreciably. The 50% transmission 
point of quartz is roughly 200nm. Vycor 250nm. Regular glass above 300nm.
You mention that most bulbs are probably made from non-Corning glass.
>From this I take it you mean not Vycor. If they aren't it is probably
a regular tungsten lamp which operates at lower temperature (and lower
UV output) and doesn't require high temperature envelopes. 
Now finally if indeed Vycor is being used instead of fused quartz the
term QTH is still not a misnomer. Vycor consists of 96% fused quartz.
Now I turn the tables and invite you to get your Corning catalog and or
find your photomultiplier tubes to measure the transmission with your
spectrophotometer. ;-)

Gabriel 
  
  


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