T3D Cooling lamps

[Virtual Imaging (John Vala) <vidi@xxxxxxxxxxxxx>]

Bob Maxey asks "Question: If you are not supposed to run air over a lamp, then why 
do manufacturers (Many, All or Most?) incorporate a blower of some kind."

   That is an easy one, and it is directly related to the coefficient of expansion of the lamp
 materials.  Quite simply, the lamp is made of glass and metal.  At room temperature 
and up to full operating temperature, the two pretty much expand and contract together 
enough to stay together.  They expand at different rates but not enough to sheer apart 
from each other.  Once the temperature gets too high, the metal has expanded more 
than what the glass can bear and they break loose from each other and CRACK!   I have 
had lamps that failed with no noticeable crack in the  glass but a micro fissure was 
created along the pin allowing the hot gas to escape (and air to come in) forming the 
silvery (?) coating on the inside of the envelope.

   So, you do want enough cooling to keep the lamp base from overheating, but not too 
much to cool the lamp envelope and keep the halogen cycle from happening.  The lamp
must get hot, but not too hot.


John Vala


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