Does the cooling fan protect slides in projection?

[P3D Gabriel Jacob <jacob@xxxxxxxxxxxxxx>]

Dr. George A. Themelis writes

>I made this statement and has not been challenged yet.  It appears to me
>that the cooling fan is there to protect the projector and not the slides. 
>This is a result of the observation that the temperature of the slide in
>the projection beam rises steadily as it absorbs radiation.  One would
>expect that the cooling stream of air will cause the temperature to reach
>an equilibrium point. However, my observation has been that the temperature
>is still rising in the short period of time that the slide remains in the
>beam's path.  The heat-absorbing filters or cold mirrors block the IR
>radiation from heating the slide and these are the ones that essentially 
>protect the slide.
>
>So:  - heat absorbing system (filter, mirror) protects slides
>     - cooling fan protects projector
>
>Is this roughly correct?  If it is, those who have problems with melting
>RBT mounts should check the fitlers.  If the projector feels very hot and
>parts of the projector are cracking from excessive heat then the fan is in
>fault.

I think you haven't been challenged yet because you are right. I came to
the same conclusion a few days ago. As to the equilbrium point, you should
have tried a type K (or any of the other less popular letters)
thermocouple. I think it would have stabilized after a while. But by then
things would have started smoking. I was planning to post something
similiar to your post but would have gone further in speculating that if
the lamp holder was ceramic and assuming the projector case is shielded
or in metal (and lets not forget the poor lamp in this discussion) that
the fans main function is to protect the heat absorbing filter!

One has to remember that the function of this filter is to stop the IR
radiation from passing thru the system. It does this of course by absorbing 
it. When it does this, the consequence is that it very rapidly rises in
temperature. Regular glass cannot handle a sudden rise of temperature and
this is especially true with HAG, since it is made specially to absorb
IR radiation as opposed to plain glass which isn't. Even if the after the
temperature slows down if there is no ventilation to draw away the excess
heat the HAG will overheat and crack. HAG is not well suited to absorbing
high intensity IR and rapid temperature increases are to be avoided. This
is why as others have mentioned a better solution is to use IR mirrors,
which can work by either transmitting the IR thru or reflecting it. These
mirrors since they transmit the IR radiation don't raise in temperature
very much and can handle much higher IR radiation. That's why they call
them COLD mirrors! I suspect they are not often used in consumer projectors
is because HAG is probably cheaper. Most of it is made from block glass.

Thus it seems that yes the fan is not there to protect the film or even
the lamp (if it has minimal ventilation holes which it does)and is not of
the older type Bob Howard describes, but to protect the projector and more
importantly the heat absorbing glass, the unsung hero.

P.S. I don't like the acronym for heat absorbing glass. How about IRAG.

Gabriel






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