Slide Overheating Experiments

[P3D Dr. George A. Themelis <fj834@xxxxxxxxxxxxxxxxxxxxx>]

I am in the process of conducting a series of experiments regarding the
danger of overheating of slides in projection.  Preliminary results show a
valid reason for concern.  Here is what I did (a formal report will follow
at a later time):

First I estimated the melting point of the RBT mounts.  I used a heat gun
and monitored the temperature of the air at different distances from the
gun.  I established that the RBT plastic mount starts flowing at about 230F
(110F).  Since there was not much radiation involved in this setup there
was no difference between the black and white mounts and time did not
matter.  I double-checked the results by putting the mounts in boiling
water 212F (100C).  The mounts do get a bit soft but do not flow.  I would
say that under 200F (90C) these mounts maintain their structural integrity.

At home I repeated the temperature measurements using this time a steel
probe.  I verified that the temperature in the film gate is about 110F
(43C) with the heat filters on and 750W bulbs in place.  Without the heat
filters the temperature (as measured by the probe) rose to 170F (77C).  
This temperature is still safe for the RBT mounts. Why is the mount melting
fast then? (I mean fast, smokin and everything!!!)

As you've guessed, the problem is radiation.  The temperature that the
probe feels is not the same as the temperature that the RBT black mount
feels.  Any experiment that measures the temperature in the path of the
projector beam is actually measuring the tempeature of the probe which is
different than the temperature of the mount or the temperature of the slide
itself.

Gabriel with his simple lens & sun-burning-black-paper experiments knows
that the black-colored objects absorb radiation like crazy and get hot like
hell!  I wrapped the steel probe with black insulating tape and the
temperature rose over 300F (150C - no heat filters) in a matter of a
minute!!!  I used a white tape and the temperature was at the same level
as with the bare probe (170F).  I then painted the tape with a black marker
and the temperature skyrocketed again!  Yes, per Jon Golden's cooking
ideas, you can roast marshmallows in the projector, but you have to color
them black first!

With the heat absorbing filters in place the temperature of the black
covered probe is also higher than the bare probe but not at dangerous
levels (I measued 110F with bare probe and 170F with black probe.)
Based on these results I conclude that the heat filters are effective in
cutting off the IR portion of the radiation which contributes to heat
built-up.  I am not sure that the fan is there to protect the slides.  I
think it is there to protect the projector.  I measured the temperature in
front of the lenses (outside the projector) where there is no air
circulation.  It is about the same as inside where the air circulates.  If
radiation does the damage to the slides, air circulation is not be of much
help.

To test the condition of your heat filters, just place an RBT black
mount in front of the lenses.  If it melts, you have a problem with the
filters.  If the filters are OK, the RBT mount should be OK when placed in
front of the lenses.  Air circulation is not a factor there.

Even with the heat absorption filters in place if an RBT black mount stays
in place for a long time it can get soft and the temperature could rise to
200F with 750W bulbs in the TDC.  Use the RBT mounts with the white side
facing the lamps.  Temperature from radiation absorption rises with time. 
Do not leave slides in the projector for a long time.  The metal mounts
will remain cool and have an advantage over black plastic, but the dark
emulsion of the slides absorbs heat too.  Project overexposed slides only!
(just kidding!)

I plan to repeat these measurements using 500W in my TDC projector to see
the effect of bulb wattage.  In addition to the danger of overheating,
Kodachrome is reported to fade fast in projection, faster than E6 films. 
These results concern me a bit but do not stop me from projecting slides. 
Most of my good slides in projection are not my most favorite slides.  My
most favorite slides usually do not look all that great in projection. 
They are viewer slides.  Even if some of my favorite projection slides that
circulate PSA, or get often projected at home, suffer some damage due to
heat, I frankly do not care!  The enjoyment of seeing them projected and
sharing them with others who let me see their slides projected is all I
need to make me happy!

Greetings!  -- George Themelis


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